12 V wire gauge chart by amps and length for cars, RVs, boats and solar, at 3 % (critical circuits) and 10 % (lighting) voltage drop, the two ABYC E-11 limits. 10 AWG carries 20 A only 7 ft one way at 3 %, 24 ft at 10 %. Double the voltage and the distance doubles.
04 · Voltage drop · DC
UPSIZE TO:
6 AWG · 6.20% DROP · 7 FT MAX AT 3%
20 A over 15 ft of 10 AWG copper loses 0.74 V (6.20 %), leaving 11.3 V at the load and burning 15 W in the wire.
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Feet one way; the current goes out and back on two conductors (or the chassis). Fuse each run at the battery.
Feet one way; the current goes out and back on two conductors (or the chassis). Fuse each run at the battery.
Feet one way; the current goes out and back on two conductors (or the chassis). Fuse each run at the battery.
05 / Field guide
Low-voltage DC carries a lot of current for its power, so the drop decides the gauge long before heat does. ABYC E-11 sets the marine limits most installers borrow: 3 % for critical circuits (navigation lights, bilge blowers, electronics, panel feeders) and 10 % for lighting and other non-critical loads.
The current goes to the load and back, so a 15 ft run is 30 ft of conductor (or the chassis on a car). At 12 V, 3 % is just 0.36 V: 10 AWG carries 20 A only 7 ft, 6 AWG 18 ft. At 24 V the same wire goes twice as far.
max one-way ft = (pct ÷ 100 × V × 1000) ÷ (2 × I × R Ω/1000 ft)
The site checks the gauge against NEC Table 310.16 at 90 °C as a conservative ceiling; 105 °C automotive and marine cable carries more. Fuse every positive run at the battery.
Size a car amplifier’s power wire for its fuse rating, not its advertised watts. For solar, the panel-to-controller run is a continuous load: keep it at 3 % or less, or move to 24 or 48 V.
Fuse every positive conductor as close to the battery as practical, sized to protect the wire, not the load. In a car the return is the chassis: keep the ground strap short, to bare metal, and as heavy as the power wire. On a boat, ABYC E-11 calls for a dedicated return conductor rather than the hull.
Crimp terminals with the tool their maker specifies and seal them with adhesive-lined heat shrink where moisture can reach; a poor crimp adds more resistance than several feet of cable.
For solar PV the NEC sizes conductors at 125 % of the maximum circuit current, which is itself 125 % of the panel’s short-circuit current (690.8), so 156 % of Isc in all. The drop rules above still apply. Moving a system from 12 V to 24 V doubles the reach of every gauge: 10 AWG carries 20 A 7 ft at 12 V but 14 ft at 24 V at the same 3 %.
Questions
4 AWG copper keeps the drop under 3 % (critical); for lighting at 10 %, 10 AWG.
Match the wire to the amp’s fuse rating and the run from the battery: a 60 A fuse over 15 ft calls for about 2 AWG at 3 %.
Because 3 % of 12 V is only 0.36 V. The same power at 120 V draws a tenth of the current and tolerates ten times the volts.
No. ABYC E-11 calls for stranded marine cable (tinned copper is the usual choice against corrosion); solid household wire work-hardens and cracks under vibration.
Yes. Doubling the voltage halves the current for the same power and doubles the allowed drop in volts: the same wire reaches four times as far.
Start from 156 % of the panels’ short-circuit current (690.8), check the ampacity at the expected temperature, then upsize for a drop of 3 % or less between the array and the charge controller.
In a car, yes: the chassis is the return. Keep the ground connection short and to bare metal. On boats use a dedicated negative conductor.