Conductor Sizing

[jpeck]

How to figure conductor size? 3 panels in series 5 parallel Isc 9.71 amps going 40 feet to charge controller.

9.71x5=48.55x1.25=60.68x1.25=75.8 looks like #2 awg
Does this sound right?

[Amy@altE]

There's more variables. Check out the video I did that includes NEC tables. Go to YouTube and search on "Wiring Your Solar Electric System".

[carultch]

How to figure conductor size? 3 panels in series 5 parallel Isc 9.71 amps going 40 feet to charge controller.

9.71x5=48.55x1.25=60.68x1.25=75.8 looks like #2 awg
Does this sound right?

Sizing conductors requires four separate calculations, and you pick the larger of the four.
1. The terminal condition
2. The local wire thermal environment, and conditions of use
3. The overcurrent protection device, if required, must "protect the wire" once sized
4. The total length of the full circuit, and Ohmic voltage drop. Note that this also applies to other feeders in the same system. Often you have to verify that they all add up to a certain limiting percentage.


For Calculation #1, the #2 gauge wire satisfies this. #2 wire copper is rated 95A, at 60 Celsius. The value of total ISC * 1.25 * 1.25 = 75.6A. Your wire needs to be at least 75.6 A, which #2 copper is. Most of the time in the field, terminals are rated for 75C. Officially, those that are 100A and less need to be listed and labeled for 75C, instead of the default 60C.

For Calculation #2, you'll need to also know the environment where this wire is routed. This considers "derate factors" associated with bundling more than 3 active wires in a conduit, and elevated air temperatures due to direct sunlight. You usually have a general derate factor associated with the high ambient temperature in your area, if above 86F [30C]. This is only 2 active wires, so you don't have a bundling derate. This condition requires you to consider total ISC * 1.25, and usually it will apply to the 90C column of the NEC for 90C wire (which most is).

For Calculation #3, you need to answer whether A) there is an overcurrent device required, and B) whether your resulting wire has an ampacity at conditions of use and terminations that rounds up to it. There is a "next size up" rule, 240.4(B). If one isn't required, you don't need this calculation. This calculation is commonly required when derate factors apply, because OCPD is sized by 1.56*total Isc, and derated wire is sized by 1.25*total Isc.

For Calculation #4, you'll need to know the operating voltage (total Vmp at STC), and the operating current (total Imp at STC). This is a straight forward Ohm's law. dV = total Imp * R, where R is the total round trip resistance of the the wire at 75C. You then compare this to total Vmp as a percentage. It is usually your goal to keep the total of all voltage drops on a system less than 3%, but there isn't an official requirement. It is a power quality and system efficiency issue, rather than an NEC safety requirement. The NEC has recommendations, but not requirements.

As a guess, I'll assume that Imp = 9A, and that means total Imp = 45A. For #2 wire with a DC circuit of 40 ft one-way, this has a voltage drop of 2*40ft*45A*(0.184 ohm/kft) / 1000 ft = 0.6624 Volts. If your total Vmp is 100V, then this is 0.66%, which is acceptable. Check the rest of the circuits in your system, and confirm they add up to not more than 3%.

[Mike90250]

there is a link in my .sig below, to a spreadsheet calculator file for array wire sizing and loss. It's very good and will work well.

[Living Large]

There's more variables. Check out the video I did that includes NEC tables. Go to YouTube and search on "Wiring Your Solar Electric System".

It's a very informative video, and you follow the rules you get to partake in the Dance of Joy - a win-win.