# what gauge wire to use for panels

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Im making my own panels and am finished with constructing the first two.Now figuring out the wiring that will come off the Jbox.Ive heard 16/18 AWG but want to be sure which.These are 82.8 W series wired panels.
Also had a question about what specifically defines the diff between a 12,24,36,48 V "system".
Have another question about diodes but that can wait and dont know if there are any here that have done DIY panels.....

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: what gauge wire to use for panels
Now figuring out the wiring that will come off the Jbox.Ive heard 16/18 AWG but want to be sure which.These are 82.8 W series wired panels.

Niel is our forum wiring shogun… He’ll need to know the expected voltage and current of your 82.8 W panels, how many are hooked in series and/or parallel, how far they are from the J-box, and how far it is from the J-box to the fuses, breakers and or controller.

Generally speaking, 18 AWG and 16 AWG is pretty small stuff, and 10 AWG or larger is typical for wiring PV modules. For example, I used 10 AWG THHN wire for the 15 foot run from a single 40 W STC PV module (16.8 Vmp x 2,38 A Imp).
Also had a question about what specifically defines the diff between a 12,24,36,48 V "system".

The basic definition is rooted in how many 6 V (nominal) or 12 V (nominal) batteries are wired in series in the battery bank. For example, if you have four Trojan T-106 6 V batteries wired in series, that would be a nominal 24 V system. The actual battery charging voltage would be around 29 V, and the PV array’s typical STC spec would be around 34 Vmp.
Have another question about diodes but that can wait and dont know if there are any here that have done DIY panels.....

HTH,
Jim / crewzer
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Re: what gauge wire to use for panels

solarmacintosh,
i am going to ask you some questions like how many cells per panel did you put into it? how were they connected to each other? etc. how you have this arranged is just as important as your other questions and gives us a better picture as to what we're dealing with. generally each 12v pv consists of 36 cells and multiples of this for 24v 48v etc., and sometimes uses a blocking diode to prevent reverse current flows at night. many controllers eliminate the need for those diodes, but in multi panel arrangements in the case of shading it's possible for the good pvs to dissipate power through the shaded pvs if no blocking diodes are used. there's also bypass diodes too that you could incorporate in the case of shading, but i don't see that as quite as important for most systems.
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Re: what gauge wire to use for panels

Ok ill just keep it to 1 deal for now as i need definitive answers for each question.Ill stick to the wire for now and just need to know what gauge to use for stringing from the + and - from the panel to the charge controller.Dont know how far....gonna be on the roof but for it to face south,it will have to be on the upper level,this increases distance from the battery bank in garage but to avoid trees,debris gotta be upper level and for best exposure.Maybe 35 yds? 105ft.? man that seems way too far....only way though.So yeah its a long string,guess a heavier gauge would help then?
BACKGROUND: These panels will be four altogether--82 W each using 36 cells 2.3W each.Will power all my electronics,computers--takes 2 rooms in this house offgrid(going room by room).To interconnect the cells? dont see relevance but used standard cell tabbing .005/.55 and standard bus ribbon.In series not parallel.Havent had them in the sun but 15V in lamp light but expect 18V --not sure on A maybe 3(this is for each panel).Fluctuates huge between sun and artificial so ill reserve those numbers but wht wire should i use just to go from the panels to the controller?
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Re: what gauge wire to use for panels

ok solarmac, in running the calculations #4 shows to be real close to 3% losses with 6amps of pvs for 12v. the run of 105ft becomes 210ft of wire because you have a negative and a positive wire to run that length. if it were my call i'd go #2 wire. this would put it at about 1.9% losses and is good for a bit of expansion(another pv) or using with an mppt controller. use #10 to the jbox for as short as possible if the jbox is right there with the pvs. if you run seperate wires to the controller for each pv then go with #6 as this will give a bit over 2.3%. to be sure you understand that means a #6 wire for the positive and a #6 for the negative of the pv and do this for the other one as well yielding 4 wires running to the jbox near the controller
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Re: what gauge wire to use for panels

you're right jon as i was seeing him say he just built his second one so i went by 2 pvs. i get a rough of about 19.3amps based on 17v vmp, but it will still be around the gage you state for it. solarmac if you could try and verify that distance you need it would help confirm the figures, but it's going to be big gage no matter what as i made 2 errors. one on the pv power and the other in the number of pvs. at this point it may be better for the bigger controller like the mx60. jon reconfirm that with 210ft of wire at 19.3amps and your 19.8amps to cover both bases. you know it's late, but it's bothering me so i'll run it. at 19.3amps for 12v it is #2/0 for 2.94% and at 19.8amps for 12v it is #2/0 at 3.02%. i don't believe your current to be 4.95amps, but something less. #2/0 leaves no room for expansion beyond the 4 pvs.
#4 comes up good at 48v for me at 2.346% going with 19.3amps. 19.8amps would be 2.406% with #4. i think #4 at 48v and an mx60 is the answer as #2/0 is very large and expensive. forgive all of the confussion solarmac as we are trying out a new program and i guess i didn't read you right on what you have too. i confirmed all these calculations by hand as well. losses at low voltage dc are appreciable and that's why edison's dc distribution systems didn't win out over tesla's higher voltage ac.
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Re: what gauge wire to use for panels

Yeah ive had them in the sun now and im a bit disappointed...2.2A per panel at 18.5V thats for my 32 cell panels though--prototypes i had to make smaller because of barstock miscalculation but i think the wire situation is handled..ive been told elsewhere (homepower magazine) that 10 AWG is fairly standard so i went ahead with that wire and  as long as yall were talking controllers i thought id add a question about that.I purchased a morningstar 30A prostar-in the mail anyway-(non-MPPT) because i read that MPPT controllers are only really effective in arrays that are identical in panel V and Wattage.If you have off size panels in the array,the effeciency goes down the drain because i guess it cant differentiate the differing currents?Dont know why that would be a problem as all the juice is macking down the same central lines and reaches the controller equally but is there validity to that claim?Dont know but i didnt see spending 300 on something that wouldnt be much better than what i bought for 170.The thing i need now is an inverter.For now i wont be getting above 500W here--i am already expanding(its a sickness!) to 4 panels 2  82W and 2 78W but thats all it will be for a bit until i can get more cells.500W will be the max at this house though,its all i really need besides a seperate cool little system i designed for hot water.Ideas on an inverter?
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Re: what gauge wire to use for panels

i can't seem to read past you're saying "for my 32 cell panels though" when you had told us before that they are "82 W each using 36 cells 2.3W each". so which is it?
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Re: what gauge wire to use for panels
arkiejon wrote:
I don't think Home Power would tell you to run 19 amps @ 12 volts on #10 wire 105 feet. I have three brands of panels two 115 watt evergreen, 2 Sunwize 90 watt and 2 shell 85 watt panels running through an MPPT controller. You total all this up at 12 volts you get 580 watts. At 17.2 volts from the panels is 33.72 amps. But at times I get as much as 45 amps to the battery bank from 33.72 amps in panels. in normal sun they are always putting out the rating. So that is a small bit of BS on the do not use odd brands and sizes.

Why I was telling you to use an mppt controller was not to get a gain but allow you to use a #10 wire. We were talking about sending 19 amps through the wires or using an mppt controller to up the voltage so you would be putting less amps and not have to use a #3 or larger wire. I use #10 wire from my Evergreen panels to the junction box which is only about 5 foot away. then after they are combined to the other panels I run #4 wire.

If you do end up running 19 amps at any voltage through those small wires you will have charging problems because of the voltage drop added to the low voltage of the panels.

It is all in fun so do as you feel but be carful and not over heat some wires and burn your home down..
Well i took the advice of a local installer and used 10awg, morningstar controller and 1500w inverter with bank of 6V batts and it MACKS.Thanks to those who tried to help.
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Re: what gauge wire to use for panels

i think if you doubt us here then you could measure your voltage drop by seeing the voltage at the pvs and then measure it at the controller input at the same time or under the same load conditions and sun conditions. if you did this at the distance you stated and the current we hypothesized then the dealer gave you wrong info. if on the other hand you failed to properly inform us as to the conditions or specs that would be your own fault. i suspect the dealer isn't getting the whole or correct picture from you either. we tried.
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Re: what gauge wire to use for panels
arkiejon wrote:
I have three brands of panels two 115 watt evergreen, 2 Sunwize 90 watt and 2 shell 85 watt panels running through an MPPT controller. You total all this up at 12 volts you get 580 watts. At 17.2 volts from the panels is 33.72 amps. But at times I get as much as 45 amps to the battery bank from 33.72 amps in panels. in normal sun they are always putting out the rating. So that is a small bit of BS on the do not use odd brands and sizes.

It is all in fun so do as you feel but be carful and not over heat some wires and burn your home down..

Each panel has a unique impedance or resistance (and this can change under sun conditions and temperature).  For max power transfer the impedance of the source (PV) should match or equal the load impedance(bat or Inv).  I think thats what a MPPT controller does, matches the load and source impedance.  If the panels are all similar the changes it (controller) makes work, I would think with different panels the controller couldn't match them all so it compromises.  just my thought...
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Re: what gauge wire to use for panels

patman,
you are on the right track with your thoughts, but you're not wording it right. firstly i have to say impedance is ac resistance, not dc. the resistance you are talking about is called the internal resistance. now mppt doesn't truelly match the source and load, but it does bring it more towards optimum. remember max power occurs when the source and load are of the same resistance. you will not find the internal resistance of a battery matching up with the calculated resistance from the output of an mppt controller.
this internal resistance does react with the resistances of other pvs that are of a different internal resistance and how it is affected depends on how it is placed in relation to the others(series/parallel). you are basically correct in saying it is better to get the same pv than mix them just as with batteries, but unlike batteries the pvs will not shorten their lives because of it. with the cost of pvs this high it is definetely better to get the same rather than have even a small amount of power being wasted due to mismatching of pvs. under some circumstances they can be intermixed, but you've got to know what you're doing.
• Solar Expert Posts: 62 ✭✭✭✭
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Re: what gauge wire to use for panels

you are right Niel thanks for clearing that up.  I forgot about the DC world, its so different, but "transmission line" is important.  I can't believe the wire size required.  I found this neat chart/calculator page
http://www.powerstream.com/Wire_Size.htm
With copper prices any spool of 6 or better wire should be a real deal.
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Re: what gauge wire to use for panels

patman3,
try this calculator as it was a joint effort between myself and arkiejon. it makes usage of various wire temperatures and other nice little features. it is freeware.
http://www.wind-sun.com/smf/index.php?topic=1477.0
• Solar Expert Posts: 62 ✭✭✭✭
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Re: what gauge wire to use for panels

thanks, spreadsheet worked and gave me a better idea overall of system
Is there any difference between STRANDED and SOLID copper wire? I would think solid has a larger cross section area (thus more current/ less loss compared to stranded same guage) but is harder to bend and work with. Stranded has tiny spaces between the individual circles, and some strands could act as fuses during high currents if away from the bunch?
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Re: what gauge wire to use for panels

For DC, solid and stranded wire (of the same gauge) will carry the same amount of current (as the contain the same amount of copper). If you will need to move something fairly often (like changing a panel tilt), then go with stranded. Solid copper will work harden and can eventually fail. The "finer" the individual strands, the more flexible it is (i.e., welding cable strands are very fine).

The only issue with solid and stranded cables that I have found is that for a given gauge, stranded cable is slightly larger in diameter than solid. For high strand count wires (like welding cable), it can be difficult to get all the strands into a crimp connectors (and some wiring terminals in fuse/terminal/equipment connects).

If you were working with AC (and particularly, high frequency AC), current tends to flow along the surface of the wire because of the magnetic fields (skin effect). In these applications, using stranded wire is better.

When purchasing the wire, watch the insulation type. Some are OK for use in the weather/sun (i.e., UV resistant), others are not and need to be protected on external runs.

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