Sizing PV and Battery Conductors: Ampacity, Derating and Voltage Drop

Conductor sizing is not one calculation. It is three, applied in a fixed order: start from 1.25× the continuous current (Isc for PV source circuits, continuous output for battery circuits), apply the conditions-of-use correction factors for temperature and conduit fill, then check the voltage drop over the actual run. Skipping the derating step is the most common error - a conductor that is fine at 30°C in free air is not fine in a rooftop conduit at 60°C.

Battery circuits carry high continuous current at low voltage. At 48V, a 10kW discharge is roughly 210A - that is 4/0 copper territory, and the voltage drop over a 20-foot run is material. Higher-voltage batteries reduce the current and therefore the conductor, which is why architecture decisions change the wire bill more than the wire selection does.

1. 1

   Establish the design current

   PV source circuits: 1.25 × Isc. Battery circuits: 1.25 × continuous inverter input current at minimum battery voltage.

2. 2

   Apply temperature correction

   Use the ambient temperature of the raceway location, not the outdoor design temperature, then apply the conduit-fill factor.

3. 3

   Select the conductor

   Pick the smallest conductor whose derated ampacity still exceeds the design current - not the largest that fits the lug.

4. 4

   Compute voltage drop on the real run

   Use round-trip length. A 3% drop at 48V/200A costs real output and heat.

5. 5

   Verify termination and OCPD compatibility

   Confirm the breaker/fuse rating, the terminal temperature rating, and that the conductor fits the equipment lug.