# Wire sizing question

rplarry
Solar Expert Posts:

**203**✭✭✭✭✭
I'm helping a neighbor install a solar array. It consists of 21 280 watt panels. I've wired them into 7 strings of 3 to the Midnight combiner box. Now I need to run a wire to the Outback load center. It is 35' one way. Each string of panels will produce 15 amps at about 110v. for a total of 105amps. The combiner box is set up to handle #2 wire but I was afraid that might not be large enough. What size wire would be appropriate for that amp load and wire length?

Thanks

Larry

Thanks

Larry

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## Comments

17,615✭✭Hit the brakes.

21 * 280 = 5880 Watts * 0.77 / 48 Volts nominal = 94 Amps.

Not even a MidNite Classic can handle that. Time for redesign.

3,150✭✭✭✭Check into virtual tracking. Pointing some panels to the west and to the east will reduce the overload to the charge controller. That is the key benefit of virtual tracking.

http://forums.energymatters.com.au/solar-wind-gear/topic5064.html

30,520adminWhat is the Vmp of the panels/array? 110 VDC might be a bit high if high desert/cold nights (Voc-cold-array might be over 140 VDC). Have you used the Manufacturer's string calculator to double check?

-Bill

203✭✭✭✭✭That wasn't the question, but since you are curious, there are 2 fm 80's for charge control.

8,766✭✭✭✭✭Well, it will be a problem if the sun comes out from behind heavy clouds at noon, and trys to slam 5.8Kw into the batteries, but normally, the morning current rise will slowly bulk up the batteries, and they may not demand the full 94 amps..

And I think you can program the classic to limit the input and output amps, so you can over panel a charger, and not cook it. (instantly)

|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar

gen: http://tinyurl.com/LMR-Lister ,

963✭✭The way you phrase the question in your first post, that was the question. You didn't say you were running two arays you ask about over 100 amps on the run.

gww

17,615✭✭May not have been your question, but it is a problem.

This is

veryimportant: you donotconnectonearray to the inputs oftwoMPPT charge controllers.Therefore your original premise is also wrong as the current from the total panels

mustbe divided between the two controllers and carried onseparatewires.203✭✭✭✭✭Oops, now that I have recovered from my brain fart, let me try again. 7 strings of 3 panels, I will run 4 strings on one leg in the combiner box and 3 strings on the other leg so I will have a potential of 60 amps on one leg and 45 on the other. Is #2 wire large enough to handle 60 amps on a 35' one way run?

Thanks for rattling my cage enough to wake me up.

Larry

17,615✭✭2 AWG will handle 60 Amps no problem.

Now about that Voltage ... 110? Is that possible Voc * 3? Because you should be calculating using Vmp, and on a 280 Watt panel that's usually around 30 so X3 would be around 90.

In any case the V-drop on 35' of 2 AWG even at 60 Amps and 90 or 110 Volts is going to be less than 2%. You could probably use 6 AWG in fact.

5,183✭✭✭✭I see another anomaly, Panel amps...... 280 W / 30v= ~9.3A not 15 as stated in post #1. Amps are not cumulative in a series configuration only Volts

KID #51B 4s 140W to 24V 900Ah C&D AGM

CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM

Cotek ST1500W 24V Inverter,OmniCharge 3024,

2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,

Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep

West Chilcotin, BC, Canada

17,615✭✭He is probably going by the maximum series fuse rating, which is of course not right. That rating does not enter in to figuring V-drop (and therefor power loss).

If you take the maximums given and multiply them out you get 110 Volts * 60 Amps = 6600 Watts, which is well above 21 panels * 280 Watts each - 5880 Watts.

You have to use the right numbers to begin with or else the calculations all fall down.

17,615✭✭Oh and if the panels really are 36 Vmp (110/3) then the current is <8 Amps which times four strings brings it down to 32 Amps of current from the combiner to the controller and that's even easier to handle.

But it then brings up a new problem of Voc being about 44 per panel and X3 gives 132 per string and you're now in the realm where a bit of cold will push the Voc up to where the FM60 will shut down.

There's always something looking to trip you up.

5,183✭✭✭✭Ya, the CC specific string sizing calculators sure make it easy, and avoid mistakes:roll:

KID #51B 4s 140W to 24V 900Ah C&D AGM

CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM

Cotek ST1500W 24V Inverter,OmniCharge 3024,

2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,

Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep

West Chilcotin, BC, Canada

203✭✭✭✭✭Yes, I am using the max rated amperage of the panel that is how I came up with a 15 amp potential in the string of three. I would rather over wire it than under wire it. also my neighbor happens to have some #2 wire left over from another project so Looks like we can use that. Thanks for your patience with me.

Larry

17,615✭✭This is unnecessary.

The series fuse rating has nothing to do with the maximum output of the array. It's about protecting the array (and your house) in case something goes wrong and one string shorts, thus becoming a conductor. At that point it would be subject to the combined Isc of the other strings. In this case you could be looking at 9 Amps Isc * 3 'good' strings dumping a total of 27 Amps to one 'shorted' string which is only capable of handling 15 Amps. Thus the circuit breaker trips and protects against the shorted string from heating up to the point of ignition.

A panel or string of panels with an Isc of 9 Amps is never going to produce 15, even under super-optimum conditions.

So in reality you have 280 Watt panels with a Vmp of ~30 and an Imp of ~9.3 configured as four parallel strings of three in series giving an array with Vmp 90 and Imp 37.2 which are the numbers used when calculating wire size and Voltage drop. V-drop matters most at this maximum power point, and all wires/conductors can take momentary surges above their continuous rating. As such using the 110 Voc is not right, nor is using the maximum series fuse rating X4.

But the 2 AWG wire is going to handle anything that array can produce. The downside is having to work with the heavy wire, especially as the charge controller won't accept it on its input terminals. You will probably need to transition from 2 AWG down to 6 AWG using crimped lugs & bolts or saddled bus bars.