Need help to specify solar panel wires for new RV installation

Rodeworthy · May 2007

Hi, I am new to this board. I have limited knowledge of RV solar energy technology but I intend to learn. In the meantime, I do have a rather urgent need to determine the size and characteristic of the wire pair that should be used from the solar panels to the battery area.

I have on order a new fifth wheel trailer that will be equipped with an Xantrex RS3000 inverter and 4 Interstate U2200 Workaholic 6V batteries. The manufacturer as part of its standard package provides solar prep wiring to the roof from the battery box area. I have learned this wire pair is #10 wire. I do not know the designation of the wire other than the size. I questioned whether that gauge wire will be sufficient to handle my future needs when I eventually install solar panels and other related hardware. The manufacturer has agreed to install whatever wire pair I deem suitable and can supply. It would be very advantageous to have this wire pair installed during the construction of the trailer especially since the wires may be heavy and inflexible. The manufacturer has requested 30 feet of each colour.

I have not yet determined what my energy consumption will be and final details of my solar installation. I envision starting with two 120 or 130 Watt panels with the provision to add a third if it becomes necessary. A suitable controller will have to be selected, preferably a MPPT type. I have time to determine all the hardware required for this project including the panels but I need a suitable pair of wires routed now during the imminent construction phase.

Somewhere along the line #6 wire was suggested for the project.

First I need to know if #10 wire as specified by the manufacturer is indeed inadequate for the equipment stated above. If it is inadequate what gauge and wire type would you suggest would efficiently support up to three 130W panels?

I have been advised to use #6 THHN stranded cable for the project. Is this what you would recommend? Do you need to know any other aspects of the project to make this determination?

Any assistance to get me past this first hurdle will be appreciated.

mike95490 · May 2007

Re: Need help to specify solar panel wires for new RV installation

You must first know your power needs. Barring that, somewhere, there must be a way to calculate watts per Sq foot. Figure how many sq ft your roof is, and come up with the watts. Divide by 24V and you get amps, and use that for your PV panel to calculate losses. I suggest 24V system, has less voltage loss.

here's 2 calculators for wires:

http://www.electrician2.com/vd_calculator.htm
downloadable spreadsheet: http://www.uwgb.edu/nevermab/voltage_drop_calculator_.xls

Good luck

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

rodeworthy,
you are wise enough to recognize that #10 would pose a problem and indeed it would. with a 30ft run of wire going with 3 12v pv inputs would show a vd of 1.56v or about 13% using #10 using the current specs of kc130s of 7.39amps. the suggestion of #6 would give .61vd for 5.15%. now a #2 wire run would give .245vd for 2.04%. the advantage of the 12v arrangement is that if one pv shades then only one pv goes low, but you need heavy wire to carry it efficiently. now if you elect to go with 2 pvs in series, which would mean up to 4 pvs in the future, you could get away with a smaller wire, but the 4th pv presented with 2 strings represents 14.78amps flowing so the wire would still be somewhat large although smaller than the 12v arrangement. this is where #6 would work as it would give a drop at 24v of .412v for 1.72%. here if one pv becomes shaded the whole string downgrades and of course you weren't planning on 4 pvs, but a good mppt controller could do this for you if you decide to do it and you have the room. if you get a 4th pv and are using them in a 12v configuration this would give a current of 29.56amps on that #2 wire and would give a vd of .326vd at 2.72% that, although more lossy, is still acceptable. we have professed to keep losses to the controller under 3% and outback specifies under 2%. btw compromising the pv output by going with too small of wire because of the expense of the wire is assinine as the pvs themselves cost a fortune and you'd be throwing away much power to save a few $ in wire. for fire reasons and nec rules you don't want this to exceed 5% in losses from the pvs to the loads anyway. do not forget that you need to interconnect the pvs with wire to a combiner box before the 30ft homerun to the controller and that adds to the losses overall.
now if the length of wire you use increases or decreases, this changes your voltage drop and its percentage. this was calculated for a 30ft run at 65 degrees c and that represents 60ft of copper wire as there are 2 wires, being there are 2 polarities. as always, we recommend that you get a controller that uses a remote battery temperature sensor to monitor the battery temperature and that adjusts your charge voltage accordingly.

Rodeworthy · May 2007

Re: Need help to specify solar panel wires for new RV installation

Gentlemen,
Thank you for the time to respond to my plea for assistance.

Mike, it is not possible for me to know my power needs at this time. This will be a future determination as we evolve in the RV lifestyle. We can use that information in the future to help design the rest of the components of the auxilliary power system. What I believe to be achievable, knowing the inverter model, battery bank characteristics and a probable pv array is to select a suitable cable that will minimize losses between these devices.

Niel,
You have vindicated me somewhat. When I visited the RV manufacturer in March I had done some preliminary work that indicated a #2 cable pair would be optimum for the installation. That was dismissed as preposterous by the factory representative who stated no one had ever installed a cable pair that big. He indicated another customer had researched this subject well and had specified #6 wire for his trailer. Being only marginally enlightened in this subject I said I would revisit the requirements. I telephoned NAWS and related my requirements to a gentleman who recommended the #6 THHN stranded. That is how we arrived at #6. There has been no attempt to economize here, rather I am trying to find the correctly engineered specification for the components at hand. The 2 to 3 PV panels are not written in stone. If 4 panels prove to be optimum then so be it. The number will be limited by available real estate on the roof. Weight plays a factor too.

With regard to length, the 30 feet is what the factory asked me to send. I have reason to believe the finished length will be considerably shorter than that. However, to be conservative I have stayed with the 30 foot number. Any miscalculation due to surplus length should work in my favour. It is notable that any reduction in linear length will be doubled due to the twin runs. I am curious to know what the numbers would say for a 25 foot (total 50 foot) run. I should learn how to produce these calculations myself.

I am somewhat miffed that the manufacturer has left it up to me to determine the design for this portion of a fairly expensive auxilliary power option. With your permission I would like to present your figures to the manufacturer's engineers to see if they will concur on the cable required and will instruct their factory installers to install the correct cable.

I do appreciate your specifying the acceptable loss limits and specing out the losses for the cables mentioned. This should make it easier to make the argument for the larger cables.

The controller or pv panels have not been selected yet but I do anticipate a MPPT controller and recognize the need for battery temperature monitoring. The combiner box is fuzzy to me. Do I have it right that this acts as a junction box and that it can be installed on the roof of the RV near the PV array?

Thanks again for your assistance.

LeighC · May 2007

Re: Need help to specify solar panel wires for new RV installation

Hello Rodeworthy,
If you download the MX60 MPPT installation & user manual from the Outback site it has good charts for different voltages & currents for a 2 wire run @ 1.5% Voltage drop.
The MX 60 which is probably overkill for your situation but very versatile, will accept a 2AWG cable.
Not trying to advise anything here, just adding my 2 cents worth. I have had a MX 60 on a motorhome for a few years & am very happy with it, I can use any solar panel or battery configuration I want, roof area dictates the power I can put out ( currently 660W STC ) but I bush camp for extended periods. If you keep wires on the larger end of the scale at time of manufacture you can do any thing you want later on, at the top of the efficiency chart.
Hope this helps. Leigh

K4GAA · May 2007

Re: Need help to specify solar panel wires for new RV installation

Rodeworthy,

Welcome to Solar RVing. This site certainly helped me when I was setting up our RV. I have 180 watts of panels and I had the manufacturer run #10 wire. I installed the panels and charge controller. #6 would probably give me at least 10% more charge current under optimum conditions. You will probably want to go with a 12 volt battery system since that's what most RVs are setup for. Depending on the time of year/temperature you're camping you might not see much benefit from an MPPT controller. Given the price of solar panels - you might want to consider only two and purchase a small generator to charge the batteries if needed. You might even be able to run the AC on a generator.

You might want to consider a monitor to keep track of the charge state of the batteries. This is helpful in avoiding discharging them below 50% capacity. It also helps to conserve capacity as well since everyone can see the impact of running extra lights or even the 12 volt coffee pot.

I really enjoy camping in the wild without the noise/bother of a generator. Good luck with your new rig. Ray

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

rodeworthy,
let me have you verify the total length of wire you say you will need. i will reitterate that a wire run is both wires going distance x. the length we are after is the total wire length or the length of the positive wire + the length of the negative wire and this is usually 2 times distance x. if the total wire you will need is only 30ft this is a 15ft run and your wire requirement would be cut in half. i still read this as a 30ft run and 60ft total, correct?
you can do calculations yourself by going to the top subject in this general solar topics area in a sticky for the voltage drop calculator. i, with the aid of another, came up with this excel calculator. i inputted the formuli and he implemented them into the excel program. if you don't have excel, you aren't alone as i used an excel clone that is specified in that sticky. i use 65 degrees c as the temperature needed for solar use, whereas the engineers are probably using 20 degrees c(room temp). solar applications are anything but room temp and higher temps increase the resistances of the wires. i also used the standard resistances at 20 degrees c as my starting point or base for copper wire and allowed for the temperature changes we could see with solar. it also allows for you to see the effect of having the wires cold too meaning it isn't just the pvs that do better in the cold. :-D
as to showing this to them you may do so if you would like. even show them what outback says about the wire length for the amount of current it has to pass. leighc, can you provide him with the link? remember, we are going for low voltage drop percentages to minimize wasted power that would show up in the form of heat in the wire.

LeighC · May 2007

Re: Need help to specify solar panel wires for new RV installation

The link to the MX 60 Manual is [url][/url]http://www.outbackpower.com/pdfs_manuals/MX60_Manual_6_2.pdf
Their cable size chart is on page 10. You may have to copy & paste this address because I can't get it highlighted.
Hope this helps -Leigh

Rodeworthy · May 2007

Re: Need help to specify solar panel wires for new RV installation

niel wrote:

rodeworthy,
let me have you verify the total length of wire you say you will need......

Niel, you are correct. I have been requested to provide 2 x 30 feet of wire or total of 60 feet. My feeling is there will be somewhat less than that in the final installation. I don't know where they route it or how much is used in horizontal runs. Excess will no doubt be coiled and left for my discretion. I am using the 60 feet number to arrive at a conservative result.

you can do calculations yourself by going to the top subject in this general solar topics area in a sticky for the voltage drop calculator. i, with the aid of another, came up with this excel calculator. i inputted the formuli and he implemented them into the excel program. if you don't have excel, you aren't alone as i used an excel clone that is specified in that sticky.

I actually downloaded that Excel v-d calculator several days ago and was working with it. Shortly after my Excel crashed and is unuseable (unrelated to the calculator). I must admit this computer is living on borrowed time and needs replacement. That will happen soon. I could attempt to revive Excel by using the Office repair but I am hesitant because for next few days I cannot afford to lose my connection to the outside world in case anything goes wrong with the repair. Once past this little crisis of mine I would love to talk to you about those formulas.

i use 65 degrees c as the temperature needed for solar use, whereas the engineers are probably using 20 degrees c(room temp). solar applications are anything but room temp and higher temps increase the resistances of the wires. i also used the standard resistances at 20 degrees c as my starting point or base for copper wire and allowed for the temperature changes we could see with solar. it also allows for you to see the effect of having the wires cold too meaning it isn't just the pvs that do better in the cold. :-D
as to showing this to them you may do so if you would like. even show them what outback says about the wire length for the amount of current it has to pass. leighc, can you provide him with the link? remember, we are going for low voltage drop percentages to minimize wasted power that would show up in the form of heat in the wire.

I agree with your assessment with the temperature compensation. An RV installation can and will be subjected to wide variations from room temps.

I did get leighc's message about the Outback MX60 controller. I downloaded that manual and actually read the whole 35 pages. (that's why I am a bit late replying :-)). That is a very capable device and the tables are indeed useful. I will use that information and your numbers in a communication with the manufacturer today. I will let you know what they say.

I am in a bit of a bind since I am leaving on a long trip on Monday and will be out of touch until the end of May. I would like to get this cleared up before I leave. If I do decide to send them #2 wire do you think the welding wire shown in NAWS online store is suitable?

Thanks again for you help.

Rodeworthy

Rodeworthy · May 2007

Re: Need help to specify solar panel wires for new RV installation

LeighC wrote:

Hello Rodeworthy,
If you download the MX60 MPPT installation & user manual from the Outback site it has good charts for different voltages & currents for a 2 wire run @ 1.5% Voltage drop.

Hi Leigh,
Thanks very much for the information on the MX60 and how it is working for you. That certainly is a very capable system you have there. There just may be an MX 60 in my future too but we will see. I read the entire manual last night and I was impressed. The charts are very useful and I thank you for pointing them out.

Regards,
Rodeworthy

Rodeworthy · May 2007

Re: Need help to specify solar panel wires for new RV installation

K4GAA wrote:

Rodeworthy,

Welcome to Solar RVing. ......
You might want to consider a monitor to keep track of the charge state of the batteries.......
I really enjoy camping in the wild without the noise/bother of a generator. Good luck with your new rig. Ray

Hi Ray,
Thanks for the welcome to solar Rving. Not there yet but soon. I definitely will be installing a battery monitor. I will augment this system with one or two Honda generators but my goal is to be a good RV citizen and utilize the silent power of the sun. Generators can then be used discretely.

Regards,
Rodeworthy VE3GP

crewzer · May 2007

Re: Need help to specify solar panel wires for new RV installation

First I need to know if #10 wire as specified by the manufacturer is indeed inadequate for the equipment stated above. If it is inadequate what gauge and wire type would you suggest would efficiently support up to three 130W panels?

I have been advised to use #6 THHN stranded cable for the project. Is this what you would recommend? Do you need to know any other aspects of the project to make this determination?

With regard to length, the 30 feet is what the factory asked me to send.

Rodeworthy,

I’d recommend #8 THHN wire for your application. Voltage drop in the wiring certainly needs to be considered, but it’s my view that it needs to be done so from an overall system perspective.

If you’re planning on using a PWM (non-MPPT) controller (i.e., a Morningstar TriStar, Prostar, or SunSaver Duo; or a Xantrex C-series), the PV array will operate at a voltage equal to the sum of the battery voltage plus the voltage drops in the controller (minimal) and the wiring (PV to controller, and controller to batteries). As long as the operating Vmp of the PV module can meet this requirement, then that’s all you need.

For example, if your battery voltage is 14.4 V, and the voltage drop in the controller and the wiring is, say, 1.6 V, then all you need is a PV module that can deliver ~16.0 V (measured at the module terminal) when it’s both hot and delivering full current at ~mid-day.

Since you’re planning on using 130 W modules, it looks like you might be considering Kyocera 130 modules. If so, their STC Vmp is 17.6 V. Even allowing for a hot-module voltage drop, they shouldn’t have any problem operating at ~16.0 V, or ~90% of their STC Vmp spec.

A 1.6 V drop (10% of a PV module operating at 16 V) in the wiring is not trivial. But, for this case with a PWM controller, it really doesn’t matter, as long as the system can achieve target battery voltage and the wire doesn’t overheat. You could use larger wire to realize a smaller voltage drop (i.e., 0.8 V), but the panels would then operate at 15.2 V instead of 16.0 V, and you’d still end up with 14.4 V at the batteries. In other words, any "excess" voltage available to a PWM controller is, in effect, "thrown away".

If you decide on an MPPT controller such as the OutBack MX60 (And you should if you’re considering an array of > ~400 W STC), then one goal is to reasonably minimize voltage drop in the array-to-controller wiring. However, the MX60 can be configured for 24 V in and 12 V out (both nominal),. This configuration would cut the array current in half and reduce the power loss in the array-to-controller wiring by ¾. (P=I2R)

Either way, I believe #8 THHN will work satisfactorily for your planned application, as long as the cable is shielded from direct sunlight. In fact, I used #8 THHN (bought at Home Depot) for my old RV solar electric system (300 W STC array, PWM controller, and ~25 feet wire (each way)], and it worked without any problems. You could go with #6, but I doubt you'd see any marked improvement in performance. #2 is an unnecessary expense, it's bulky and difficult to work, and it doesn't fit in some controllers' terminals.

If the cable might be exposed to direct sunlight, then you might want to consider tray cable like this: http://store.solar-electric.com/8-2-tc.html

HTH,
Jim / crewzer

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

what jim is suggesting is that it is alright to forego 2v if it is still above the charging volts needed and that may be fine for those using a cheap controller and one small used or questionable pv, but i'm surprised at him suggesting that you should do it. you will lose too much power if you do this especially on an mppt controller. you would also lose by having to wait longer before the charge starts in the morning and it'll quit on you sooner in the evening. what jim is suggesting imho is the equivalent of an epensive stereo with multi thousand dollar speakers being fed with a long run of #24 or worse wire. in fact the #24 will work better on the stereo system than #8 would on your pv system because the stereo is sending ac through the wires, not dc. this just doesn't make sense to me to have big bucks invested everywhere except for the wires that he says is ok to go cheap on. yes, it'll work going with smaller wire, but do you want to compromise your investment by doing this?

crewzer · May 2007

Re: Need help to specify solar panel wires for new RV installation

what jim is suggesting is that it is alright to forego 2v if it is still above the charging volts needed and that may be fine for those using a cheap controller and one small used or questionable pv, but i'm surprised at him suggesting that you should do it. you will lose too much power if you do this especially on an mppt controller.

I agree that using #8 wire with an MPPT controller in a 12 V / 12 V configuration would not be optimal. However, an objective review of my analysis and recommendation above will show that I addressed PWM and MPPT controller’s separately, and my MPPT controller recommendation was for a 24 V / 12 V configuration.

you would also lose by having to wait longer before the charge starts in the morning and it'll quit on you sooner in the evening.

I’d love to see any technical analysis to support this statement. Early morning- and late afternoon charge current from an RV’s stationary PV array is so low that there’s virtually no voltage drop in the size cabling we’re discussing here.

what jim is suggesting imho is the equivalent of an epensive stereo with multi thousand dollar speakers being fed with a long run of #24 or worse wire. in fact the #24 will work better on the stereo system than #8 would on your pv system because the stereo is sending ac through the wires, not dc. this just doesn't make sense to me to have big bucks invested everywhere except for the wires that he says is ok to go cheap on. yes, it'll work going with smaller wire, but do you want to compromise your investment by doing this?

I just don't understand this analogy. I have an expensive stereo and I connected correctly sized wire to my speakers. I agree that small speaker wire would be a mistake for this application.

Returning to the original issue, my position is that there’s no benefit to be gained from using unnecessarily large wire to minimize the voltage drop from a PV array to a PWM controller as long as the system can drive the batteries to their target voltage and the wire is sized to handle the currents involved. To be sure, larger wire will result in a smaller voltage drop from the PV array to the PWM controller, but the PV array will then just operate at a lower voltage due to the smaller drop in the wiring, and the net benefit will be zero.

This approach may appear to be be unorthodox, but it’s technically sound.

Regards,
Jim / crewzer

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

you would also lose by having to wait longer before the charge starts in the morning and it'll quit on you sooner in the evening.

"I’d love to see any technical analysis to support this statement. Early morning- and late afternoon charge current from an RV’s stationary PV array is so low that there’s virtually no voltage drop in the size cabling we’re discussing here."

Regards,
Jim / crewzer

sorry jim, but when the voltage is 2 volts lower it takes longer before it reaches the point of charge for the battery and will lose it sooner under the same conditions of lowered sun or watts/meter squared. this is the same arguement YOU have used against pvs that have 32 cells as opposed to the standard 36 celled pvs as that gives the about same 2 volt loss. the technical analysis is already proven through ohms law and is reflected even in the voltage drop calculator. i'd like to see your technical analysis of saying it would only drop 2v when the sun is strong.

crewzer · May 2007

Re: Need help to specify solar panel wires for new RV installation

niel,

Good try, but no good. Since there's little output current during low insolation, there will be little voltage drop in the wires from the PV array to the controller. Also, available Vmp under low insolation is typically better due to lower cell temperature, so this really isn't an issue as far as I'm concerned.

You are correct that I generally prefer 36 cell modules over 32 cell models, and nothing that I've stated contradicts that position. The 36 cell module's higher voltage helps overcome both low voltage issues attributable to high cell temperatures and high output current in the wiring, but these are most problematic at mid-day under full insolation, rather than early morning or late afternoon.

It's my view that the technical analysis I presented earlier remains valid. However, lets examine this quandry from another perspective. Let's stipulate that there's a 2 V mid-day voltage drop in the wiring and PWM PV controller between a 390 W (~22 A at "12 V") PV array and the battery. What would the voltage drop be early in the morning and/or late in the afternoon when the charge current is, say, 2.2 A (10%)?

Regards,
Jim / crewzer

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

jim,
i'm trying to keep an open mind about what you're saying jim, but i'm not convinced that you aren't artificially lowering the current further with the use of smaller wire. in any case i believe he was leaning towards an mppt controller. do you actually have any studies or examples of a real circumstance with a side by side comparison with the standard(large) wiring? i do think i'm following the general trend of thought you are trying to convey to me. i will ponder this more, but in the meantime if you feel you can explain better to me, feel free.
rodeworthy,
it seems we both have that gut instinct now even though we're being told something else. it is your install though and you won't hurt anything going with large wire imo. also note that welding cable would work, but you may find it difficult to work with the wire due to the many fine strands of wire that make up the cable. they do this to keep the cable flexible as it is for constant moving in welding applications. see the picture and compare to other wires that use fewer strands of larger wire for the same gauge. http://store.solar-electric.com/wc--2.html

BB. · May 2007

Re: Need help to specify solar panel wires for new RV installation

Niel,

If I may--I believe one of the important caveats in Jim presentation is the use of a PWM controller (although later in this post, I also address the issue for MPPT controllers and I believe that oversizing wire gauge hardly helps increase power in low-light conditions either) and because of a major physical characteristics of the silicon solar panel...

1. With a PWM controller, the output voltage of a solar panel is only controlled by three things, the amount of sun, the battery voltage, and the wiring/controller voltage drop (ignoring secondary--but real--effects of panel, battery, and wiring temperatures Vbatt charge state).

Once the solar panel has sufficient sun for its voltage to rise above Vbatt+Virc drop, then no matter how much more sun the panel gets, the voltage remains the same. Since the basic solar silicon solar panel does not output any useful current until it almost reaches its rated output voltage (sufficient sun or watts/sq), the size of the wiring when the panel starts generating is meaningless (i.e., the wiring has no voltage drop when the current is near zero amps).

Once the panel begins to generate more current, yes, the Virc (current*resistance+Vdrop of controller) becomes more significant--however, if the systems is sufficiently able to support enough voltage drop (Virc) at maximum current, then at lower sun angles/the current voltage will be the same, and the Virc will only drop because of the reduced current--1/2 current output from the solar panel implies 1/2 Virc (I know, it also depends if the controller drop if fixed voltage or I*R dependent--or a combination of both) so that Virc+Vbatt drops, which simply pulls down the solar panel voltage to below Vmax or Vmppt.

2. This gets back to the solar panel's Voltage in a PWM setting--its output voltage, once it has sufficient watts/sq of sun, is never controlled by the sun, it is controlled by all of the other voltage drops in the system. For a panel that Wind-Sun sells, here is a link to the PDF spec:

http://www.gepower.com/prod_serv/products/solar/en/downloads/gepv_173_datasheet.pdf

Notice that the panel voltage, under fixed sun, is virtually flat until the Vmppt "knee is seen". The slight downward slope of the left side of the graph is probably the I*R losses if the panel's internal resistance.

3. Since wiring loss is Pwir=R*I^2 (I squared time R), wiring losses as a part of system design drop by the square... Examples; assuming that the system design supports Virc+Vbatt<Vmppt (at temperature/state of charge/etc.), then even when the panel is at 1/2 current, the wiring loss is at 1/4 of design, and when the system is at very poor illumination (mornings/evenings/clouds) --once Vmppt>Virc+Vbatt-- say at 10% of output, wiring losses would only be 1/100 of design requirements (3% of full power loss)... And in the end, wire loss does not matter in a PWM system at all--the difference in current to the battery between a small gauge wire and a large gauge wire in a PWM system is ZERO.

Once the system has been designed to operate at maximum (worst case) solar current, panel voltages, wire IR drop at maximum current, and battery voltages, then oversizing the wire in a PWM system buy nothing.

If, this was a system with a MPPT controller--your wire size does matter a bit more--but again only near maximum current. If, one has decided that 3% maximum power loss in the wire at rated system current is "good enough" (and all other requirements for proper design of voltage drops, temperatures, battery chemistry, controller minimums, etc. have been met), then in all other conditions the amount of loss due to wiring Pwir losses will be less than 3%.

For example, as the controller/panel start to receive sun/energy, there is very little voltage drop due to Wire I*R losses and the power losses as a percentage of current are even less (R*I^2) because of the square of the current law. The controller is going to find Vmppt of the solar panel with little sun (which is pretty near Vmppt of optimum watts/sq anyway) and start conversion.

Yes, there will be a little Pwir loss, but the system will start with the same amount of sun in any case (as Vwir=I*R is very small) and if 1% current/power is used as a useful starting point, the losses from 1% current in the wiring will only be 1/10,000 of the Pmppt losses at noon... Doubling the wire gauge, again for a MPPT controller, will not start the charging process/conversion any sooner and, in any case, would only reduce the Pwir losses to 1/20,000 of those at maximum sun anyway... Of course, doubling the wire cross sectional area would change the 3% loss to 1.5% loss at maximum power--but this way swamps any "power gain at low sun" in the design.

So, even for a MPPT controller, my conclusion would be once you have set your design requirements and calculated those for worst case, maximum power, system, there is no economic/engineering justification for oversizing the cables based on the theory of collection economically useful power in low light conditions.

-Bill

crewzer · May 2007

Re: Need help to specify solar panel wires for new RV installation

niel,

I think we’re making some progress here. Also, I’d like to thank Bill for weighing in with his perspectives.

I think part of the problem is that we’ve been shooting at a bit of a moving target. In his original post, Rodeworthy indicated that he was looking at “..starting with two 120 or 130 Watt panels with the provision to add a third if it becomes necessary. A suitable controller will have to be selected, preferably a MPPT type.”, and that “The manufacturer has requested 30 feet of each colour (wire)”. Later, he added that “The 2 to 3 PV panels are not written in stone. If 4 panels prove to be optimum then so be it”.

As has been discussed several times, ~300W - 400 W STC of PV array is ~ the crossover point for upgrading from a PWM controller to an MPPT controller, due primarily to the MPPT controller’s internal tare losses as well as its cost. For an RV used primarily in warmer months, I suspect the crossover value is even higher as the cold ambient temperature benefits aren’t realized.

See: http://www.wind-sun.com/smf/index.php?topic=1960.0
And: http://www.wind-sun.com/smf/index.php?topic=1034.0

For example, I measured my MX60’s efficiency and found it to be 92% efficient when operating in a “36 V / 12 V” configuration and ~591 W in. It’s less efficient at lower power levels due to the internal tare loss. See: http://www.outbackpower.com/forum/viewtopic.php?t=952

I believe that another part of the problem is that we – me included – have failed to consider Rodeworthy’s requirement from a broader system perspective.

Since the array could range anywhere from two modules (~260 W STC total) to perhaps four modules (520 W STC total), one solution would be to use a PWM controller such as the Moriningstar TriStar 45 in a 12V / 12 V configuration. This way he could start with two modules in parallel and add one or two more fairly easily. Even with an ~7.4% voltage drop in the wiring at mid-day in bulk stage (PV current = ~30 A from a 520 W array under “ideal” conditions), I still think that #8 wire would be sufficient for this system.

I agree that larger wire wouldn’t hurt, but it wouldn’t help either,as the PV array would just operate at a lower voltage (as measured at the module terminals). And, as we know, typical charge currents will be lower than 30 A, so the voltage drop will typically be less. Futhermore, Rodeworthy's location (Canada) and his travel destinations may influence our calculations even further.

Another approach would be to consider a Blue Sky Energy SB3024i MPPT controller. This controller can handle up to 30 A out, and the input can be configured for either “12 V” or “24 V”. This flexibility would allow Rodeworthy to wire his array in either a 12 V or a 24 V configuration, depending on the number of PV modules.

In a 12 V (~30 A) array configuration, he’d probably need #1 wire to keep the maximum power loss / voltage drop down to 1.5%, and #4 would keep it down to 3%. However, wired for a 24 V array, #8 would keep the maximum loss down to 1.9%.

I own an MX60, and, as my posts over on the OutBack forum will attest, I’m a huge fan of this controller. However, I don’t believe that its combination of cost and low-power inefficiency make it a good choice for “small” systems.

As for any other examples, I can only offer up my old RV system. It had 305 W of PV on the roof wired for 12 V. Its STC Imp spec was ~18 A, and the connection from the array to the MorningStar PS-30M controller was about 21 feet of wire (each way; ~42 feet total). It was ~another 4 feet (each way; 8 feet total) from the controller to the batteries. I used #8 THHN wire, and, even with a relatively low module Vmp spec of 16.9 V, I was able to reach 14.4 V and 15.1 V (both ref 77 F) for bulk, absorption and EQ charging of my flooded-cell batteries – with the sense lines connected, of course. Accordingly, larger wire would not have improved my system performance, and it would have been a waste of both material and money.

HTH,
Jim / crewzer

Rodeworthy · May 2007

Re: Need help to specify solar panel wires for new RV installation

Wow....what have I done. I must say that every part of this RV purchase has been a struggle and it appears the solar power implementation will be no less so. Some time ago I had looked into the design aspects of adding solar battery charging capability to the optional Xantrex RS3000 inverter offered by the vendor. I knew at the time it was complex and would require further research on my part but I had to focus on other aspects of the whole project. I felt I had the beginnings of a good system but knew I should deal with the wiring from roof to controller before the trailer was built. Now it would appear that I did not leave enough time to deal with this detail and it may even be better to struggle with installing the wire after the full system design has been worked out. I came to this site as an admitted novice and being naive I thought it would be a simpler decision that it appears to be. I did receive and accept a good design criteria to keep the wiring losses under 3%. The v-d calculator that is routinely promoted on this site indicated that #2 stranded copper wire would satisfy that criteria. The ensuing debate has put doubt on that specification. While I endorse and promote healthy debate on all manner of subjects I cannot join in since I am already in over my head. I was left to ponder what to do.

Friday, was my deadline for action. No one had made a case that said #2 wire would be detrimental to the installation. I had been in contact with the manufacturer and they were willing to put a pair of #2 wires into place and even install a dedicated mushroom cap to the roof for the purpose. They were none to thrilled about it but would do the job. I am almost ashamed to say I took the route that many people do when they are not sure what they are doing -I threw money at it. I ordered the #2 welding wire from NAWS for direct shipment to the manufacturer. In my haste to meet the deadline I did not wait to find out that, even if #2 wire was to be installed, welding wire is not optimal. What I don't know - is the welding wire unworkable or just difficult? The fine strands appear to be like battery cable and we used to solve that by immersion in a liquid pool of molten solder in an appropriate connector. Is tinning of the leads to a controller considered bad practice? What would have been a better wire choice from this vendor?

It is most likely that wiring order has not been filled but it will be close. They do require some process time for cutting. Perhaps I should just try to cancel the order and deal with all this later.

It would appear my desire to use a MPPT controller has been called into question as much of the debate has dealt with design criteria for other controllers. I will look forward to learning more about this in the future and without the pressure of deadlines. The subject is fascinating.

This may be my last post for a while since I am leaving on a trip tomorrow and I have no guaranteed access to the internet while away. I will check back in here whenever the opprotunity presents itself. I will be home May 29. I will check back here later today (Sunday).

I do thank all of you who have participated and continue to do so. I am learning but I think you can understand why I might still be confused.

Rodeworthy

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

rodeworthy,
it's not always a good thing to jump into action until the dust settles, but i believe you'll be fine with your decision. tinning the wire is ok to do and i assume you received my pm as i had mentioned the many fine wires that make up the welding wire. what i said was basically in answer to your questioning its use, that if you can work with it it is still whatever designated gauge of copper wire. the efficiency factors jim mentions carry some merit as this considers costs as was the whole intent of his postings on the subject to you as he feels it could've been done cheaper. he is correct about using the mx60 on a small system as it is more designed for a larger setup, but you may wish to expand to that 3-4 pv range making it well worthwhile to have now ahead of time. the designer admitted to me the crossover for the mx60 is between 200-300w so you are fine to use it with 2 kc130s as that is in that grey area if the cost is ok by you. you do still gain over a straight pwm with some of the other mppt controllers as well as jim pointed out. your losses would've been worse than that which jim listed as you need other wires to go from the pvs to your 30ft run.
though you are in canada it isn't like you're in the arctic as you are only 200 miles or so north of me and that wayne or even brock here on the boards are farther north than you are. do not feel bad that you threw money at it as you won't have to upgrade it later for any expansions to your system and you need not worry about if you went too small with it. you don't get into solar electric generating without throwing money into it anyway and this is why jim tried to go as cheaply as is possible in his responce to you because we do throw lots of money into it as it is. know that i do not condemn jim for showing you another possible alternative and that we are not feuding or fighting. ultimately, it is up to the individual to make the decision as to what they feel most comfortable with as we only give advice and the differences that were given on that advice is only a good thing as it makes you more aware that the answer isn't always black and white. though the posts confused you somewhat, you made the decision that you are comfortable with. for the record i think you made the right decision as i am the one here on the boards that says why go cheap on the wire as it is not as expensive as the rest of the system, but it is every bit as difficult to change should it be wrong so be on the safer side of things and you won't regret it.
also, if you got my pm would you like to try a qso some time in the future with me as you did not reply to my pm?

BB. · May 2007

Re: Need help to specify solar panel wires for new RV installation

Oversizing the wiring is not going to hurt you... And welding cable is a very good choice when used in places where vibration is present (thinner strands of wire don't work harden near as easily when exposed to vibration--like on a moving vehicle).

You just got yourself in a discussion on how to wring the last few percent of efficiency out of your system... And the details vary when compared against different types of controllers and panels. I am sure that HAMs never get into those types of discussions about their radio gear. :roll:

The only thing I would add it to ensure that you have the correct size fusing, as close as possible to the battery bus, to protect the wiring (and the solar panels themselves) against shorts.

-Bill

Rodeworthy · May 2007

Re: Need help to specify solar panel wires for new RV installation

Gentlemen,
I am very relieved to know that I did not go too far wrong with the hasty decision I had to make on the wire. Rest assurred, for the balance of this project I will not have to be I such a hurry and will take the time to weigh all the factors -and there are many. I just got myself between two conflicting schedules - the factory's need to proceed with construction and my pending trip. I will stay with the welding wire. Given more time I may have made a different decison but I reasoned that oversizing now was much more economical than having to work "through the roof" at a future date. Rv's are not the easiest things to route wires through and it is easy to compromise the roof and cause leaks. Not good -especially with a new unit under warranty. I am happy to have the manufacturer do this work and the cost is relatively small when considering the whole project.

Crewzer and Bill, your observations have not been lost on me. Particularly the point about the low-power efficiency of the MPPT controllers and the cross-over point to justify a move to this type controller frpm the PWM design.

I am still hopeful to lay the groundwork for a system that will grow with me as my needs (amps) become more defined. Thanks for all your input and I look forward to continuing with this discussion at the end of the month.

mike95490 · May 2007

Re: Need help to specify solar panel wires for new RV installation

not a plug, but for info, has anyone researched the odd/custom built PV panels at
http://www.amsolar.com/am100.html they have a 27V, 44cell, 100W panel, shaped to fit RV roofs.

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

interesting pv and that 27v is open circuit as the vpm(or vmp as some put it) is 21.5v. this would have to be used with an mppt controller like the mx60 as one from say blue sky tracks the voltage linearly in its stepdown. in other words if you're going from 24 to 12 and the input is actually 34v it would initially track to half of that or 17v which works fine on both manufacturer's controllers. now if the input was 27v the only one that will work is the mx60 as in the blue sky it will linearly translate to 13.5v and that's too low. this is true at least for my sb50, but i do not know if this holds true for all of their products that can downconvert or not, especially their newer models that i have not boned up on. this pv could not be used on my sb50.

crewzer · May 2007

Re: Need help to specify solar panel wires for new RV installation

Rodeworrthy,

I suspect that your cable challenges may not be over. It turns out that welding cable may not be such a good idea for your installation. John Wiles, the well-respected U.S. “guru” of PV installation safety and the applicable standards, regularly takes the use of welding cable in PV systems to task.

Here’s a link to a simple Google search string that turned up numerous articles from Wiles on this issue: http://www.google.com/search?hl=en&q=welding+wiles+UL&btnG=Google+Search

I once used fine-strand 2/0 welding cable for my battery-to-inverter connections. I found that cutting and attaching the cables to make intermediate connections to the fuse block (+ cable) and to the shunt (- cable) was a real pain. I suspect that I’ll never use this type of cable again.

For large gauge general-purpose wiring, I use UL-listed 19 strand #2 cable from Home Depot. For my present system’s battery-to-inverter connections, I bought very high-quality pre-made 2/0 cables that use UL-listed components and tools, and that are very flexible.

See: http://www.solarpanelstore.com/solar-power.wire.html
and: http://www.cobrawire.com/x-flex/

However, if it’s #2 welding wire that you’ve got, then let’s look at ways to make the best of it.

A major concern is that you probably won’t be able to connect #2 wire directly to the PV modules, as it’s typically much too big for the terminal connections. Accordingly, you’ll need something like a “splicer/distribution block” to transition between the big #2 wire and the smaller wire (#10?) that’s connected to the PV module terminals. Another benefit of these blocks is that they make it easy to connect multiple modules (or module strings) in parallel. Depending on which controller you buy, you may need a similar solution at that end of the PV cabling in order to connect smaller wire to the controller.

In addition to pondering which controller to buy, you’re also going to need to consider circuit breakers and/or fuses, batteries, temperature compensation and a remote battery temperature sensor, and a few other things. Perhaps we can continue this discussion when you return from your trip.

Regards,
Jim / crewzer

niel · May 2007

Re: Need help to specify solar panel wires for new RV installation

jim,
he is locked into the welding cable he bought and they probably already had put it in previously. he is off on his vacation this day so further posts will have to wait until he gets back. i did mention to him before of its difficulties to use and that he'll need a distribution box to connect all of his pvs with his #2 wire run, but he bought the cable before i had pmed him on it. that pm he also opened late as he hadn't noticed it right away.

crewzer · May 2007

Re: Need help to specify solar panel wires for new RV installation

niel,

Thanks for the info. I'm aware that Rodeworthy is on travel, but I wanted to post the additional comemnts on the wire and the blocks before I forgot :roll: , and I also hope he'll return to forum after his trip to discuss other system components. Additionally, I hope that these comments might be usful to others considering similar installations.

Thanks,
Jim / crewzer

Need help to specify solar panel wires for new RV installation

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