Acceptable line loss

idiggplants

OK, so quick background... I have a 100w renogy panel and a 20a mppt controller.  they are charging a battery at my remote pavilion.  will be powering a car radio and a handful of led lights, summertime only, for about 4 hours a night.  3 nights a month, or thereabouts.  i will also be substituting power with a jump pack on the shady days.

I had planned to put the panel on the roof, which is pretty shaded.  see what that got me, and then add a 2nd or 3rd panel.  

someone mentioned to me that i might be better off putting the panel(s) 125' away in a sunny field.  What is the actual acceptable line los % and end of run voltage?  this is a budget build, so i was hoping to use some of the endless 12-2 uf wire that i have lying around.  The way i see it, even at 50% loss, id still be getting the same amount of energy as i would on the roof.  I know, however, that i cant let the voltage drop too low, or the CC wont be able to use the energy.


im using this calculator, and im unsure of what amperage and voltage to put in for the panels.  

Estragon

Any shade, even dappled shade through nearby tree leaves, wires, poles, etc will cut output drastically. The panel(s) really need to be in full sun.

A second problem is the voltage from a single panel (or multiple panels in parallel) may not put out high enough voltage for an mppt type controller, especially when the panel heats up in the sun. IMHO, a pwm type controller would be a better choice for a single nominal 12v panel.

If you have a sunny spot 125' away, and are willing to add a second/third panel, you could wire in series to minimize voltage drop and make better use of the mppt controller. 3 panels in series would be ~54v and 5a. At 125' using 12ga wire would be ~5% voltage drop, which isn't terrible.

BB.

Voltage drop is not a simple x% answer... If you are using a MPPT (maximum power point tracking) charge controller (which is more expensive than a PWM pulse width modulation controller), The line loss from the solar array to the charge controller is just an operational question.

For example, if you have a 12 volt battery bank--You need ~15.0 volts minimum for charging (depending on battery type, temperature, etc.). And a minimum of 2 volt drop across the MPPT charge controller (reality, you probably want >10 volt drop--But that is another story).

So, for a typical higher end charge controller, you need ~17 volts minimum at the charge controller... And the maximum recommended input voltage Vmp-array is ~100 VDC (again, depends on local temperatures and specific controller model). But, roughly you are looking at an "allowable" drop of 100-17=83 volts from array to charge controller.

Realistically, there is almost never any reason to design for that high of drop. Typically 1% to 3% drop is a good design range. If you try to go below 1%, you are just throwing lots of money at copper for little gain (1% to 0% loss). 3% drop maximum is a typical number for functional loads without problems (i.e, 120 volt load * 3% = 3.6 volt drop for your AC loads).

Specifically, you have to look at your loads with solar... For example a 12 volt lead acid battery bank runs from ~10.5 volts dead to ~16.0 volts (equalization charge). Say you pick 11.5 volts as your minimum battery voltage (don't want to damage battery)... And you have a 12 volt AC inverter with 10.5 cutoff voltage... That gives you 1.0 allowable drop, but since a typical inverter can draw 2x current (for starting loads), you really only want a maximum voltage drop of 0.5 volts for wiring+breaker+fuses/etc. to load... 0.5 volts / 12 volts = 4% drop...

I recommend that you design two different systems (at least) on paper and see what works best for you... With the standard warning the solar PV panels only generate "useful" power in full sun, ANY SHADE will dramatically reduce output (by 50% to 100% typically). People do argue that parallel vs series connections of panels can reduce shading power reduction--Just try for no-shade between 9am and 3pm at least.

Let us try a couple very quick rule of thumb designs... One with a PWM controller and another with MPPT. Say you have 2x 6 volt @ 200 AH golf cart batteries (very nice for a starter/training/learning system--Decent power and cheap if you make any mistakes and murder the batteries--very common for first time systems).

Rule of thumb charging is 5% minimum (weekend/sunny season systems) and 10% to 13% rate of charge for full time off grid systems. For your needs, 5% minimum rate of charge should work:

• 200 AH battery bank * 14.5 volts charging * 1/0.77 solar panel+controller derating * 0.05 rate of charge = 188 Watt array

Lets pick two configurations. 2x 100 watt panels in parallel for PWM controller and 2x 100 watt panel in series for MPPT controller

Lets say you want a PWM controller and 125 feet of "one way" wire run (for the following voltage drop calculator)...

• 200 Watt array * 1/17.5 volts Vmp = 11.4 amps Imp-array
• 17.5 Vmp * 0.03 drop = 0.525 volt drop

Voltage drop calculator for 125 feet one way run:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.5127&voltage=17.5&phase=dc&noofconductor=1&distance=125&distanceunit=feet&amperes=11.4&x=56&y=20
>2 AWG:
Voltage drop: 0.45
Voltage drop percentage: 2.57%
Voltage at the end: 17.05

Now, same thing with MPPT charge controller but two panels in series:

• 200 Watts * 1/35 volts Vmp-array = 5.7 amps Imp-array
• 35 volts * 0.03 drop = 1.05 volt drop

Voltage drop calculator for 125 feet:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=35&phase=dc&noofconductor=1&distance=125&distanceunit=feet&amperes=5.7&x=67&y=21

>8 AWG
Voltage drop: 0.90
Voltage drop percentage: 2.57%
Voltage at the end: 34.1

And if you allowed >3% drop, you could even use 14 AWG cable:
Voltage drop: 3.60
Voltage drop percentage: 10.29%
Voltage at the end: 31.4

That would cost your ~20 Watts from your array (in full sun/maximum current)--But it would still work fine with MPPT controller and only a little bit less than the 5% rate of charge recommended for 2x golf cart batteries for charging current.

The higher voltage you go on the solar array (with MPPT controllers), the smaller copper wire you need from array to charge controller.

It all depends on how much energy you need (Watts*Hours or Amp*Hours*Volts defines size voltage & AH of battery bank).. And the battery bank plus hours of sun per day define the size of solar array... Then you start picking equipment and configurations that will meet your needs.

The above is not a complete design discussion--But trying to show your options for a 125 foot feed from array to point of use for a smallish 12 volt battery system.

Your thoughts?

-Bill

westbranch

http://forum.solar-electric.com/discussion/60/voltage-drop-calculator#latest

Use this calculator,it is a bit easier. Be sure to use the 2 way wire length PV > battery > PV.... you can fill in any BLUE value, and it will populate where needed

littleharbor2

Panels out in a sunny field might sprout legs while you're away. If you're only using this system periodically you might get away with the rooftop install as you will be using up the stored power from the battery and as long as you don't use so much that you deplete the battery it will eventually recharge while away. If you add more panels you should keep them in parallel so even one shadow on one panel doesn't kill the output of the whole bank of panels as happens with series wired panels
. 
 125 feet away in a open field could work if you plan to deploy the panels only while you are there. Hopefully you wouldn't have to deal with gusty winds which will require a solid mounting That distance would require a higher voltage series wired array. The higher the better and also some larger gauge wire to limit voltage drop to an acceptable level. This type of setup will definitely require an MPPT controller and up to 6 series wired 12 volt panels, depending of the Voc. limit of the controller.

Figure which controller you will use and the highest voltage allowed then you can figure out how many series wired panels you can use and come up with your voltage and current. Current of a series wired string is the same as a single panel. the voltage is what adds.

littleharbor2

That particular controller  will allow you to wire up to 3 of those panels in series, max.

706jim

You mentioned "unlimited 12-2 wire". Regardless of which controller or panels you end up using, if this wire is free, simply double or triple it up (in parallel) to reduce voltage drop accordingly. I'm planning on doing something similar by removing panels from my roof and putting them on the ground facing east to capture a longer period of full sun. And to transmit the approximately 12 amps a distance of about 80', I'm just going to use #14 wire probably doubled. I would parallel black white and ground conductors together for + feed and the same on a second feed for the - feed. I think this would give the equivalent to about #8 wire at less expense.

idiggplants

thanks for all the replies so quickly! I don't really have time to add the reply I would like, but I would like to add a couple more notes just so I don't waste anyone's time. the battery will be a 50 amp hour 12v deep cycle marine battery. I could have a bigger one if you folks deem it necessary. That just happens to be what I have on hand. The Pavilion is about 125 ft away from our cabin which is powered by two 100w panels. The existing panels are on a 20 foot pole. Not too concerned with security. I'd be putting the pavilion panels on the same pole.

The big question I have is.. If I run double 12/2 uf wiring, and one 100w panel... Will it put any charge on the battery? Or will I absolutely need 2 100w panels to run the array at 24v?

Also still not sure what volt and amp numbers to put into the calculations.

littleharbor2

idiggplants said:

Also still not sure what volt and amp numbers to put into the calculations.

 The info should be on the back of the panels. If not look up your panels online.

BB.

Note, when you run two cables I  parallel, that drops the AWG number by 3.

2x 12 awg becomes 9 awg. 4x 12 awg becomes 6 awg.

Bill

Estragon

I'm guessing the panel is roughly Vmp 18v and Imp 5a. There should be a label on the panel with actual numbers. With warm panel and ~5% voltage drop it's maybe 17v or a bit less at the controller input. Not a lot of headroom for mppt controller - maybe enough to charge but probably not if battery is cold or for an equalize cycle.

Whether 55ah is enough depends on the car stereo - some take a fair bit of power.

idiggplants

My first post linked to the panel and the specs. I see what they are, but I get confused with which different numbers I'm supposed to use.. Optimum vs open circuit and short circuit.

Maximum Power: 100W

Maximum System Voltage: 600V DC (UL)

Optimum Operating Voltage (Vmp): 18.9V

Open-Circuit Voltage (Voc): 22.5V

Optimum Operating Current (Imp): 5.29A

Short-Circuit Current (Isc): 5.75A

westbranch

You need to use the  'X' mp numbers when calculating line loss ... Vmp is the potential  'voltage generated' by the panel in full sunlight when perpendicular to the sun, and similarly for  Imp, the amps produced under the same conditions... these values are measured in a temp controlled lab so are essentially unobtainable all the time.

ScoobyMike

Use Vmp=18.9 and Imp=5.29 in the voltage drop calculator on calculator.net.  It is very simple and allows you to put in the number of conductors.  At 125' it shows 2.1V drop (11.11%). Using 2 sets of conductors cuts the loss in half.

idiggplants

> @ScoobyMike said:
> Use Vmp=18.9 and Imp=5.29 in the voltage drop calculator on calculator.net.  It is very simple and allows you to put in the number of conductors.  At 125' it shows 2.1V drop (11.11%). Using 2 sets of conductors cuts the loss in half.


When you say 2 sets of conductors, does that include the ground wire of the 12-2? So 2 sets of 12-2 gives me a 6%-ish loss aka ~17.76 at the controller? And that should run the controller pretty well yes?


So many thanks folks. I don't want to seem like I'm asking to be spoon fed info I can find on my own via Google... I'm actually at a friend's cabin with very limited service to respond, and I can't get the calculations to go through.

ScoobyMike

Leave the bare copper wire out of this power loss discussion, 2 white wires twisted together and 2 black wires twisted together, then hook one up to the + and the other to the - output of the solar panel will result in a 1.05V drop over 125' (at 5.29A).

westbranch

Use the link I posted .... you download it to your machine and will have it as long as you don't delete the file...not a Cloud related item...

idiggplants

ok folks.  back in civilization again!  many thanks again for all the help!

so..  i have enough wire to run 2 sets of 12/2 wire.  there will have to be a junction box mid way between the panels and the pavilion though, as i have numerous 100' rolls, and a few 30' rolls.  not ideal, but i think it will be ok.  

looks like that will give me 17.85 volts at the controller.  which im thinking will be sufficient?  

then ill watch to see if it keeps the battery charged on the majority of days.  if for some reason it doesnt, ill add a 2nd panel and that should definitely do the trick.

many thanks again!!!!

BB.

Yep, for a single panel that will be fine... Also, that is ~10% rate of charge for a 55 amp @ 12 volt battery.

If you are happy with the support of your loads (from battery)--It should be perfect with a PWM charge controller.

-Bill

idiggplants

thanks.  It is an mppt controller actually.  Fingers crossed that this will be enough battery.  If not, ill probably spring for a 2nd panel and one of the 31 series batteries, which is what we use in the cabin.

Photowhit

idiggplants said:

thanks.  It is an mppt controller actually.  Fingers crossed that this will be enough battery.  If not, ill probably spring for a 2nd panel and one of the 31 series batteries, which is what we use in the cabin.

For an MPPT type controller, you really need higher voltage. They work best with more 'head room' above the charging voltage. Thought I had said that in a previous thread, but perhaps it wasn't your thread.

idiggplants

this entire thread is based off of me using an mppt controller.  Not sure why we are even talking about pwm.  

BB.

MPPT controllers "need a voltage drop" across the charge controller to work efficiently. They really do not do quite as good with "low" Vmp input voltages (vs battery bank voltage).

Try it and see how well it works. If you see ~77% of rated panel output power on a cool/clear day near noon--It is doing fine. If you see less than 50%, then may be some issues.

Note that charge controllers only output "full array power" to a battery (bank) that is less than ~80% full (this is "bulk" charging)... Batteries that are near full, will naturally have reduced charging current (this is "absorb" charging).

Or, if you have a good size load on the battery (charger should output maximum power when there are heavy/steady loads).

-Bill