Off-Grid Solar Calculators

Wiring and electrical safety

12V/24V/48V DC Wire Gauge (AWG) Calculator

Pick copper AWG for 12V, 24V or 48V DC from current, one-way length and a 3% drop budget. Voltage drop is VD = (2 × 12.9 × I × L) / CM (NEC Chapter 9 Table 8, copper 75 °C). Enter the one-way length; the formula already doubles the run. The size must also meet the copper ampacity limit (NEC Table 310.16, 90 °C column).

Inputs

Units
A
ft

Distance from source to load. The formula already doubles it for the return conductor.

System voltage of the battery or series string.

Terms of Service

Results

Please tick the box above to display the Result.

Engineering methodology

Imperial copper voltage drop VD = (2 × K × I × L) / CM, with K = 12.9 at 75 °C from NEC Chapter 9 Table 8. L is one-way feet; the 2 covers the round-trip conductor. Required circular mils = (2 × 12.9 × I × L) ÷ allowed volts, where allowed volts = system volts × (max % ÷ 100). Metric equivalent: resistivity ρ = 0.02145 Ω·mm²/m (same 75 °C copper as K = 12.9), so VD = (2 × ρ × I × L) / mm² and required mm² = (2 × ρ × I × L) ÷ allowed volts, with L in one-way metres. The chosen IEC 60228 size (or AWG) must also meet an ampacity limit — the NEC Table 310.16 copper 90 °C column for AWG, or IEC 60364-5-52 Table B.52.4 method C (70 °C PVC, 30 °C ambient) for mm². On 14, 12 and 10 AWG, NEC 240.4(D) also caps the current at 15, 20 and 30 A, because no larger fuse may protect them. Terminals listed at 75 °C can cap the circuit below the 90 °C column under NEC 110.14(C). 3% drop is a common DC performance target, not an ampacity limit. This tool is copper only; it does not apply aluminium, grouping or soil correction.

Why 12V DC needs thicker cable than 48V

For the same load watts, current quadruples when you go from 48 V to 12 V (I = P ÷ V): 1,200 W is 25 A at 48 V but 100 A at 12 V. Voltage drop is I × R (Ohm’s law), and a 3% budget at 12 V is only 0.36 V. NEC Chapter 9 Table 8 copper at 75 °C uses K = 12.9 in VD = (2 × K × I × L) / CM, with L one-way feet.

3% drop is not ampacity

Percent voltage drop is a performance target so inverters and LED drivers do not brown out. Ampacity is how much current the insulation can take without overheating. This tool enforces both a drop budget and the NEC Table 310.16 copper 90 °C ampacity column, which NEC 240.4(D) caps at 15, 20 and 30 A on 14, 12 and 10 AWG. Because most listed terminals are rated only 75 °C, NEC 110.14(C) can force a thicker size than the table used here — so can bundling or 60 °C insulation.

Frequently asked questions

Is the cable length one-way or round-trip for voltage drop?

Enter one-way feet. The formula already multiplies by 2 for the outgoing and return conductors. Using round-trip length would double the drop again and oversize the cable.

Why do 12 V systems need much thicker wire than 48 V?

For the same watts, current I = P ÷ V. Drop VD = I × R (Ohm’s law). Lower voltage means higher current and a larger fraction of system voltage for the same ohms. A 3% budget at 12 V is only 0.36 V.

Is 3% voltage drop a fire-safety ampacity limit?

No. Percent drop is a performance budget. Ampacity (heat in the insulation) is a separate table. This tool refuses a size below the ampacity in its table, but 75 °C terminals, bundling, engine rooms and 60 °C insulation can need a thicker cable than that table.

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