What size wire for 12 V at 30 amps?

It depends on length. At 10 ft one-way with 3 % drop, 6 AWG (0.29 V); at 25 ft, 2 AWG; at 40 ft, 1/0. The calculator shows every slot.

Why does 12 V need such thick wire?

The allowed drop is a percentage of a small number: 3 % of 12 V is 0.36 V, while 3 % of 120 V is 3.6 V. The same current over the same wire loses the same volts, so at 12 V it costs ten times the percentage.

One-way or total length?

One-way, from battery or panel to load. The formula doubles it for the return.

Does the wire still need to carry the current?

Yes. The drop rule usually forces a size well above the ampacity minimum at 12 V, but for short, heavy runs check the wire size calculator or the cable’s own rating.

12 V wire size calculator

Minimum size for the drop limit

Minimum copper size · 3 % drop6AWGFITS20 A over 15 ft one-way at 12 V; 0.36 V of drop allowed

System voltage

Current

Continuous load, or the fuse rating for a feed

Length, one-way

Battery to load; the return wire is counted in the formula

Drop on 6 AWG2 × 20 A × 0.491 Ω/kFT × 15 ft (NEC 2017 Chapter 9 Table 8 DC resistance at 75 °C, unchanged since 2005): 0.29 V · 2.5 %
Voltage at the load: 11.71 V
Power lost in the wiredrop × current: 5.9 W
Circular mils needed, exact2 × K × I × L ÷ VD, K = 12.9 (Cu) / 21.2 (Al): 21,500 cmil
Ampacity check still appliesThe drop rule sizes for efficiency; the conductor must also carry the current for its insulation and the run's ambient: see wire size

The 3 % limit is common practice for critical DC circuits (ABYC E-11 uses 3 % for panel-board, navigation and bilge-blower circuits and 10 % for others) and the usual solar and automotive rule of thumb. It is not an NEC requirement; NEC voltage-drop figures are informational notes. Low-voltage runs are dominated by resistance, so the DC resistance of Table 8 is the whole story.

Drop limit

Conductor

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Low-voltage runs are all about drop: at 12 V, three tenths of a volt is already 3 %. Type the current, the one-way length and the system voltage, choose the limit, and the rim marks the smallest copper or aluminum size that holds it, computed from the DC resistance of NEC Chapter 9 Table 8. Tap another slot to compare. The 3 % figure is common practice (ABYC E-11 uses 3 % for critical circuits and 10 % for the rest; solar installers use 2–3 %), not a code requirement.

Worked examples

Inverter feed: 20 A over 15 ft at 12 V, 3 %: 0.36 V allowed. 6 AWG copper: 2 × 20 × 0.491 Ω/kFT × 0.015 = 0.295 V, 2.5 %. 8 AWG gives 0.467 V, 3.9 %, too much. The exact need is 21,500 cmil, between 8 AWG (16,510) and 6 AWG (26,240). Open this run
Winch or fridge: 30 A over 25 ft at 12 V, 3 %: 2 AWG copper: 0.291 V, 2.4 %. Doubling the voltage halves the current for the same power and quarters the loss: at 24 V the same 30 A run needs only 4 AWG (0.462 V, 1.9 %). Open this run
LED strip: 5 A over 40 ft at 12 V, 10 % non-critical: 12 AWG: 0.792 V, 6.6 %. At the 3 % limit the same run wants 8 AWG. The limit you pick is the answer. Open this run
48 V battery bank: 50 A over 10 ft, 2 %: 8 AWG: 0.78 V of 0.96 allowed, 1.6 %. The ampacity check still applies: 8 AWG at 50 A needs 75 °C or 90 °C insulation and free air or a short raceway. Open this run

How it is worked out

Drop = 2 × I × R × L ÷ 1000, with R the Table 8 DC resistance at 75 °C in Ω/kFT and L the one-way length in feet. The factor 2 is the return conductor. Percent = drop ÷ system voltage × 100.

The minimum size is the first on the rim whose drop is within the limit. Circular mils needed exactly: CM = 2 × K × I × L ÷ VDmax with K = 12.9 (copper) or 21.2 (aluminum).

Power lost in the wire = drop × current. At 12 V a 3 % drop is also 3 % of the load’s power turned into heat in the cable.

The figures behind it

NEC Chapter 9 Table 8, Conductor Properties: Circular-mil areas and DC resistance at 75 °C (ohm/kFT) for 18 AWG to 1000 kcmil, copper uncoated and aluminum. Read from the NFPA-typeset 2017 text and cross-checked cell by cell with a 2014 reproduction; the table is unchanged since at least 2005, so the 2023 values are inferred from that history rather than read from a 2023 copy. Checked 2026-09-05. Source
K-factor 12.9 (copper) and 21.2 (aluminum): Ohm·cmil/ft at 75 °C, the Table 8 resistance per foot multiplied by the area; VD = 2·K·I·L/CM single-phase, √3·K·I·L/CM three-phase. IAEI Magazine, “Voltage Drop Formulas”, March 2017. Source
3 % / 10 % practice: ABYC E-11 (AC and DC electrical systems on boats) uses 3 % for panel-board, navigation-light and bilge-blower circuits and 10 % for other circuits; solar and automotive guides use 2–3 %. None of this is in the NEC; the NEC’s own 3 % is an informational note. Source

Questions people ask

What size wire for 12 V at 30 amps?: It depends on length. At 10 ft one-way with 3 % drop, 6 AWG (0.29 V); at 25 ft, 2 AWG; at 40 ft, 1/0. The calculator shows every slot.
Why does 12 V need such thick wire?: The allowed drop is a percentage of a small number: 3 % of 12 V is 0.36 V, while 3 % of 120 V is 3.6 V. The same current over the same wire loses the same volts, so at 12 V it costs ten times the percentage.
One-way or total length?: One-way, from battery or panel to load. The formula doubles it for the return.
Does the wire still need to carry the current?: Yes. The drop rule usually forces a size well above the ampacity minimum at 12 V, but for short, heavy runs check the wire size calculator or the cable’s own rating.

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