Wire size .

I'm new to this site and also new to solar power. I have three solar panels a 70 , 80 , & 85 watt . I have them hooked up
with 8 ft of #10 wire on each one going to a buss bar. I was wondering what size wire should I be using from my buss bar
to my controller and from my controller to the batteries. I have six 2 volt batteries 360 amp hrs @8 hrs. I have this at a cabin.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,477 admin
    Re: Wire size .

    You need to know the current supplied/required to calculate wire sizes... And, especially, for solar, you need to know how long the wires will be for each application (for example, solar panels 3 feet from the battery can be sized just on the amount of current to be safely carried. However, if the panels are 100' (200' of wire run) away, you will need to make the wire larger to prevent excessive voltage drop from robbing power or even preventing charging of your batteries).

    V=I*R
    P=I*V
    P=I^2 * R
    P=V^2 / R

    So, if the system is 12 volts, then:

    I = P / V = 70w+80w+85w / 12 volt = 19.58 amps

    You can use this Excel spread sheet and enter your current, length, and temperature to get the minimum recommended wire gauge to use:

    http://www.wind-sun.com/smf/index.php?topic=1477.0

    You would do the same thing with your load... If it is a 500 watt inverter, the maximum current you would expect would be based on the lowest battery voltage before the inverter turns off (by the way, most inverters turn off right at the point they seriously damage your batteries by deep discharging them). Say, in this case it is 10.5 volts (for a 12 VDC inverter) and the inverter is 85% efficient:

    I=P/V= (500 watts * 1/0.85) / 10.5 volts = 56 amps

    Also, especially with batteries, you need a DC rated circuit breaker or fuse to prevent fires from short circuits. Generally, a fuse or breaker should be set at 125% - 150% of your maximum expected load (to prevent false tripping). Lead Acid storage batteries, even small ones, can supply an amazing amount of current and wiring/devices need to be protected from shorts.

    By the way, you should also take the maximum currents calculated above and multiply them by 1.25 to to allow for a margin of safety.

    If this is going to be installed in a home/business or other area, you should use the National Electric Code (NEC) or other applicable regulations for your area (NEC in the US overrides anything that the Excel spread sheet or posts on a website may tell you). It is very possible for fires to be caused by improperly wired/designed solar power systems and you want to make sure you are safe and you don't have problems with insurance (in case the worst happens).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Wire size .

    According to the calculator if I use all # 8 from my buss bar to the batteries I would have a voltage drop of 1.47% from
    array to batteries .Those wires would only be 1.5 ft [ 3 ft ]. I have lots of #8 wire . My inverter is a 2000 watt Cobra
    cutout volts is 10.5. I have 100 amp fuses on that. The buss bar for the panels is 8 ft away [16 ft of wire ].
  • System2System2 Posts: 6,290 admin
    Re: Wire size .

    The internal links don't seem to work for me on this forum, but if it's just a matter of dividing the watts by the volts, I have 160/12= 13.33 amps.

    Now in reading the installation instructions for my Morningstar TS-45, I see that they say that for 15 amps, 10 awg wire (what I have) has a max permissible length of 16.7'. So let's say ~20' for 13.33 amps to stretch it a little.

    This is nuts, and way different from what I read years ago when I first put up my system. I have maybe about 70' of 10 awg going to the cabin. Had to put the panels where the sun shines. According to this chart, I should be using 2 awg!
    Yet the existing wire was good enough to top off 6 Trojan batteries quickly when the sun hit. I never thought to check if it was heating up. Never noticed anything odd.
  • BB.BB. Super Moderators, Administrators Posts: 30,477 admin
    Re: Wire size .

    With long wire runs--it is not so much over heating you have to worry about--it is voltage drop.

    For example, if your panels are 17 VDC on a hot day, your battery needs to equalize at 15.5 VDC, that gives you 1.5 volts from solar panels to batteries for voltage drop--anymore drop than that, the current to your batteries will be reduced (because of V=I*R--where I has to drop because of the fixed R value).

    If you have good, high voltage panels and cool weather, you could be getting 21 VDC from your panels, and have a 5.5 volt drop overhead and can run the wiring 3x longer (assuming no other losses) and have everything work just fine.

    If you have a MPPT (Maximum Power Point Tracking) solar controller--the difference between 21 VDC and 15.5 VDC can be converted into useful power to charge the battery--basically, as an example:

    21 vdc * 30 amps = 630 watts from solar panels
    630 watts / 15.5 volts = 40.65 amps charging the battery...

    So, with a standard controller, 30 amps from the panels would have given 30 amps to the battery (as long as voltage drop is not too great). With an MPPT controller that "extra voltage drop" is converted into useful work to charge the battery (about 200 watts extra in this example).

    So, with an MPPT controller, using heavier gauge wire means less energy loss to heat and more energy available for charging the batteries.

    Lastly, with a good MPPT controller--you can set the solar panel voltage so something very high (say, again for example) like 120 VDC--The current in the wires from the solar panel to the charge controller is now 1/10 that from the charge controller to the battery bank--so if you make the Panel to Controller run the long one, your wire gauge only has to be 1/10 as large (~6 awg smaller) for the same voltage drop/power loss (and you can use either less wire, or get more efficiency from the same wire).

    Of course, there is efficiency issues with all controllers and systems--so the details do matter and limit the actual efficiency gains you get with an MPPT controller. Design, installation, weather and such, all matter.

    This help any? Or am I just repeating what you probably already know?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Wire size .
    BB wrote: »
    You can use this Excel spread sheet and enter your current, length, and temperature to get the minimum recommended wire gauge to use:

    http://www.wind-sun.com/smf/index.php?topic=1477.0


    -Bill

    http://forum.solar-electric.com/showthread.php?t=29&highlight=excel+wire

    link corrected
  • System2System2 Posts: 6,290 admin
    Re: Wire size .

    Thanks, Bill.

    Actually, I don't have great comprehension of this stuff. I do have a MPPT-200 controller which I'm fixin' to replace with a Morningstar TS-45 because the MPPT does not equalize, and the float voltage adjustment drifts a fair bit. I don't think it allows me to change PV voltage.

    I have one 18V panel and 1 14V panel, both 80W.
    Say I had 2 Trojan 6V batteries, as topped off as I can get them, that I wanted to equalize with the TS-45. Could I unhook the 14V panel on a sunny day and do it with just the 18V?

    Also, is there any advantage to mounting the TS-45 near the panels? Or in a shady spot on a tree somewhere in between? In a suitable box, of course. It would be hotter in the summer and cold in winter.
    I haven't quite figured it out yet, but there may be a way to run the remote sensing info thru wires to my cabin so I can see what's going on.
  • BB.BB. Super Moderators, Administrators Posts: 30,477 admin
    Re: Wire size .

    I am not the charge controller expert here--but the TS family claims to have a 30 meter remote monitor kit, and a 10 meter remote battery temp sensor.

    Keeping the TS-xx cool and dry is #1. Keeping it closer to the battery so that you keep the extra wires (remote temp and battery voltage sense leads short) is not a bad idea.

    I don't think keeping a 14 volt and an 18 volt panel in parallel is going to hurt anything--just when the voltage gets above 14 volts, the 14 volt panel's contribution to the current will simply drop off as the 18 volt panel raises the voltage...

    Depending on the real Vmp an Imp (maximum power) ratings for the panels--and the actual temperature of the panels and battery equalization voltage at your operating temperature (try to keep batteries close to 77F and solar panels coo)--you could do better by placing the two solar panels in series (assuming current ratings are roughly equal) and adding the voltages together to 32 vdc and putting it through a good MPPT controller. You will get maximum power and reach equalization voltage under all conditions (assuming you meet the controller's minimum/maximum parameters).

    The bigger issue is that MPPT controllers consume more power than simple PWM type controllers, and usually don't make much sense on systems with much less than ~400 watts of solar panels (extra power needed to run the MPPT function eats up any extra power generated--one of those "details" that separate theory from reality).

    And, really what you need to monitor is the state of charge of your battery bank. Measuring the current going in and leaving (plus battery voltage) tells you how the battery is being treated... And you can remote mount the battery monitor in the cabin--and can see how much battery power you have available at anytime (sort of like a battery "fuel quantity" gauge--battery voltage is much less accurate, especially under load/charging, at estimating battery state of charge).

    Look at this area (battery meters and shunts) for some ideas. Probably more useful than monitoring the TS-xx meter itself.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Wire size .

    Thanks again, Bill.

    I bookmarked the link and will delve into it later.

    I have no plans to increase my PVs, so maybe I'm better off with the TS-45. Which, incidentally, is rated for 12, 24, & 48V systems, so I could hook up the panels in series. Which should help with the wire gauge issue.
    Also the PST-DC/2812-8 buck-booster I ordered http://www.powerstream.com/dcdc.htm is rated to take in "10V - 30V DC for both 12 volt and 24 volt" so probably that would be fine to convert it back down to 12V at the business end. I'll ask them.
  • BB.BB. Super Moderators, Administrators Posts: 30,477 admin
    Re: Wire size .

    If I understand the TS-xx product line correctly... They are not MPPT type controllers... Just PWM which cannot efficiently "transform" the voltage from series high voltage connected PV panels into lower voltage battery banks.

    Basically, if you put you two 80 watt panels in series, you will only get 40 watts in charge to the battery using a PWM controller. So--you do not want to do this unless you have so much voltage drop that you don't get enough voltage at the battery terminals any other way.

    The controller is rated to control 12, 24, 48 volt battery banks--but the optimum panel string voltage a few volts higher than the maximum equalize bank voltage... So a 12 volt bank should be, say 18 volts, and a 48 volt bank would be some 63 volts of solar panels.

    I just re-read your post, and while the above is correct--you are probably thinking of rewiring you battery bank to 24 VDC, not just rewiring the solar panels themselves... So:

    If you converted your bank to 24 volts, you 18+14 VDC panel is still very close to not making the maximum battery voltage required for equalization... You need to look at the highest temperatures of the panels (and therefore, their lowest voltage on hot days), and you battery bank's equalization requirements at their average temperature. For Trojan 24VDC bank, you will need 31 VDC to equalize at 80F... If the battery bank is warmer, you need to compensate:
    [FONT=Verdana, Arial, Helvetica, sans-serif]Correct the charging voltage to compensate for temperatures above and below 80o F. (Add .028 volt per cell for every 10o below 80o F and subtract .028 volt per cell for every 10o above 80o F)[/FONT]

    That is very close to the 18+14=32VDC volts of your panels. If those are STC (standard temperature and conditions) Vmp (voltage, maximum power) ratings for your panels--their true voltage will be less as STC ratings significantly over-rate solar panels (STC is way too cool of panel temperature for most people in warm climates). So, if you have the panel model numbers--or otherwise know their actual Vmp voltages--you will have to be very accurate because you are right on the edge of having enough voltage to equalize a 24 VDC bank.

    Lastly, you probably don't want to use the DC/DC converters because that just adds more power loss do to the switching converter--unless this is your emergency backup system or you have more than enough power to loose another 20% to converter losses...

    Hope I am making sense here and helping you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wire size .

    "The bigger issue is that MPPT controllers consume more power than simple PWM type controllers, and usually don't make much sense on systems with much less than ~400 watts of solar panels (extra power needed to run the MPPT function eats up any extra power generated--one of those "details" that separate theory from reality)."

    though true, they draw a bit more, it is the mx60 (and those like it) you are thinking of with the 400w requirement (and it can be a bit lower as boB admitted that the breakeven point is between 200-300w, but better near 400w). the bluesky line of mppt controllers do operate with a smaller footprint than something like the mx60 and can be used at lower pv wattage levels. many of the bluesky line of mppt are made with smaller systems in mind.
    as to the voltage downconversion you would still have a problem using the bluesky line of mppt controllers as the downconversion is tracking linearly meaning, if it is too low for charging the 24v battery that it would downconvert too low for the 12v battery as well. you may have to rethink your system somewhat to make it work.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Wire size .
    I have no plans to increase my PVs, so maybe I'm better off with the TS-45. Which, incidentally, is rated for 12, 24, & 48V systems, so I could hook up the panels in series. Which should help with the wire gauge issue.

    Loohan,

    Careful here... the TriStar is rated for 12 V, 24 V and 48 V systems, but it does not down-convert from a high-vultage PV array to a lower voltage battery system. In other words, if you're still planning on using a "12 V" battery bank, then the PV array must still be wired for "12 V".

    HTH,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Wire size .

    Thanks again,all.

    Right, Bill, the TS is not MPPT. Also, come to think of it, if I do a 24V system, that means 4 Trojans, which, according to Crewzer, is way too much battery for my panels to feed.

    So I'm back to a 12V system and using the buck-booster just for delicate electronics.

    If conditions are right, I'll disconnect the wires from the PVs and measure what voltage I'm getting at the cabin end.

    Another possibility is that I could buy another higher-voltage panel for my cabin, and use the old 14V one for a little workshop bldg I'm making. Use the MPPT-200 and 2 old trojans on that. It only gets partial sun, and none after mid-PM, though, unless I ran a wire 25' or so, which I could do with a heavier gauge wire, to put it in more sun.
    Occasionally I might have to haul those batteries over to the home system for an equalization, though.

    I can't find the paperwork for the panel that I think is 14V, and the label on it is no longer legible.
    The other panel is alternatively called Matrix PW 750 or Photowatt M-PW750-80 (some specs at http://tinyurl.com/2cjnbn ).
    In fact, I do own a 2nd one of these I got free, never used, but damaged, that I might be able to fix up and mount. I'll start another thread on that. Then I could either keep the 14V'er, too, or move it to the other bldg.
  • System2System2 Posts: 6,290 admin
    Re: Wire size .
    Loohan wrote: »
    Another possibility is that I could buy another higher-voltage panel for my cabin, and use the old 14V one for a little workshop bldg I'm making. Use the MPPT-200 and 2 old trojans on that. It only gets partial sun, and none after mid-PM, though, unless I ran a wire 25' or so, which I could do with a heavier gauge wire, to put it in more sun..

    On 2nd thought, scratch that idea. It's more like a 45' run, and I don't need the juice in the other bldg, whereas I could use it in the cabin.
  • BB.BB. Super Moderators, Administrators Posts: 30,477 admin
    Re: Wire size .

    Remember Voc is a few volts higher than Vmp--and you are also missing the voltage drop from current flowing through the system...

    Your best bet, if you have an in-line amp meter from the solar panels/in solar charger, is just to watch the amps when max charging... If the maximum current roughly equals the panel(s) rating--then everything is just fine. Just running the panels in 12 volt parallel configuration will probably work just fine most of the time--the low voltage panel may drop out during hot weather and equalization, but overall you will get most of the energy it has to offer (and a good PWM controller is probably the best bet).

    Use the spreadsheet to calculate your wire losses--but even if they are higher than 3% (towards 5-7% or more), that is probably better than the 20-30%+ loss with the typical DC/DC converter (there are a few better, and many worse).

    If you are getting shading during what is otherwise good sun--then it might be worth moving your panels and adding the wire. Also, if you would need to re-rack, you could look for a good one axis tracker (if you don't have too much wind in your area). A good way to get some additional power--you will have to compare the costs of adding a tracker vs adding fixed panels as to what would work best for you (trackers add some power, but also usually require maintenance--I would tend towards a good fixed installation with fixed panels--but your situation may be different).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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