Safety warning. This is an estimate for planning, not an electrical design. Wire that is too thin can overheat and start a fire. A licensed electrician must confirm the wire, fuses or breakers and installation under the National Electrical Code (NEC) and your local code, especially for solar panel circuits and battery cables.
How the size is chosen
The calculator gives the larger of two minimum sizes for copper wire:
- For the voltage drop: the thinnest wire whose voltage drop, over the full circuit length (out and back), stays under your limit. Copper resistance comes from the National Institute of Standards and Technology's copper wire tables, 1/58 ohm·mm² per meter at 20 °C with a coefficient of 0.00393 per °C (NIST Handbook 100), taken at 75 °C. Wire areas follow the AWG definition.
- For the current: the current × 125%, as for a continuous load, against the 75 °C copper ampacity in NEC Table 310.16, with the NEC 240.4(D) limits of 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG.
What it does not cover
- Higher ambient temperatures, wires bundled together, conduit fill and wire insulation types, which lower the allowed current.
- The extra NEC 690.8 factors for circuits from solar panels, which are based on the panels' short-circuit current.
- Aluminum wire.
- Fuse and breaker sizing.
Choosing a voltage drop
The NEC's informational notes suggest about 3% for a branch circuit and 5% in total. Many low-voltage systems use 2% or less for battery cables, where a small drop is a large share of 12 V. A higher system voltage needs thinner wire for the same power.
To size the battery bank itself, use the battery bank size calculator.