Wire sizing question

rplarry · January 2015

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

Cariboocoot · January 2015

Re: Wire sizing question

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.

softdown · January 2015

Re: Wire sizing question

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

BB. · January 2015

Re: Wire sizing question

What 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

rplarry · January 2015

Re: Wire sizing question

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

mike95490 · January 2015

Re: Wire sizing question

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

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)

gww1 · January 2015

Re: Wire sizing question

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

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

Cariboocoot · January 2015

Re: Wire sizing question

rplarry wrote: »

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

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

This is very important: you do not connect one array to the inputs of two MPPT charge controllers.

Therefore your original premise is also wrong as the current from the total panels must be divided between the two controllers and carried on separate wires.

rplarry · January 2015

Re: Wire sizing question

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

Cariboocoot · January 2015

Re: Wire sizing question

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.

westbranch · January 2015

Re: Wire sizing question

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

Cariboocoot · January 2015

Re: Wire sizing question

westbranch wrote: »

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

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.

Cariboocoot · January 2015

Re: Wire sizing question

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.

westbranch · January 2015

Re: Wire sizing question

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

rplarry · January 2015

Re: Wire sizing question

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

Cariboocoot · January 2015

Re: Wire sizing question

rplarry wrote: »

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

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.

Wire sizing question

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