For determining how big the wire needs to be in order to carry the current in the circuit safely we take into account the following assumptions, and simply refer to the chart.
Assumptions:
- Outdoor design temperature of 47 degrees C or less (covers all U.S.A. locations)
- MAXIMUM of 9 current carrying conductors in conduit
– 4 source circuits maximum
(having more increases the temperature inside of the conduit) - The 1.25 irradiance factor is included
- The conduit is min of 7/8 inches above the roof
- Does NOT account for voltage drop (this is for Ampacity- how big the wire needs to be to carry the current in the circuit
- The terminal temperature limits of 75 degrees C; be able to connect most all terminals out there
this table account for all NEC correction factors – PV source circuits
| Module STC Isc <= (short circuit current) | Minimum Conductor Size |
| 18.37 A | 10 AWG |
| 13.78 A | 12 AWG |
| 11.48 A | 14 AWG |
Based on your PV module STC Isc value seen on the label of each module, when it’s less then or equal to 18.37 A you can use 10 AWG safely. Or 13.78 A you can use 12 AWG safely. Or 11.48 use wire size 14 AWG safely. These are minimums.
This is only for PV source circuits.
Once we connect PV source circuits in parallel then we have a PV output circuit we have more current, and it’s another table.