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 one-way feet; the formula already doubles the run. The size must also meet chassis ampacity.

Inputs

A
ft

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

System voltage of the battery or series string.

Results

Suggested copper size
4 AWG
Voltage drop on that size
0.31V
Voltage drop
2.58%
Circular mils used
41,740

Recommended gear

Engineering methodology

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). The chosen AWG must also meet a conservative chassis ampacity. 3% drop is a common DC target for sensitive loads.

Why 12V DC needs thicker cable than 48V

For the same load watts, current triples when you go from 48 V to 12 V (I = P ÷ 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 a conservative chassis ampacity. Bundled or 60 °C wire needs a thicker size than the table used here.

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 its conservative ampacity, but bundling, engine rooms and 60 °C insulation need a thicker cable than the table used here.

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