Solar DC and AC Cable Sizing per PEC and NEC 690.8

Solar cable sizing has an extra twist over ordinary circuits: PV modules can exceed their rated short-circuit current under bright, cold conditions, so the code multiplies it before you even start.

The 1.56 factor for DC source circuits

Select the conductor ampacity at this current, then apply corrections. On a rooftop, add a temperature adder for conduit in sun (often +20–30 °C) and derate for conduit fill — rooftop DC cable frequently ends up at 4 mm² or 6 mm² even for modest currents.

PV wire specifics

• Use listed PV wire / PV1-F for exposed module interconnections — UV, moisture, and 90 °C+ rated.
• Inside conduit, THHN/THWN-2 is acceptable for the string home runs.
• Keep positive and negative of a circuit together to minimise induced-voltage and loop area.

Voltage drop

Design targets are tighter than for AC branch circuits:

Every 1% of drop is 1% of revenue lost for 25 years — the economics justify upsizing.

AC side

Size the inverter output conductor at 1.25 × the inverter’s maximum continuous output current, then check voltage drop to the point of connection and coordinate with the backfed breaker rating.