Tools / Electrical

DC wire size, both ways

Size 12, 24 or 48 volt cable by voltage drop, then check it against the current rating. You get the AWG size and the nearest stocked metric size as two separate answers, because they are two different cables.

MethodVoltage drop, then ampacity

SystemsAWG and mm²

TemperatureCorrected

ConductorsCopper, aluminium

InputsDC

System voltage

Continuous load

Run length, one way

Battery to load. The return path is added for you.

Acceptable voltage drop

Ambient temperature

Conductor

CopperAluminium

Installation

BundledFree air

Resultdefault case

Minimum conductor 9.74 mm² for 0.36 V of allowed drop

North America / AWG

6 AWG

true conductor 13.3 mm²

Voltage drop0.264 V

Drop2.20 %

Rated, bundled52 A

UK / EU / AU stocked

10 mm²

stocked size, 10 mm² nominal

Voltage drop0.351 V

Drop2.92 %

Rated, bundled40 A

Note Bundled and loomed runs are derated by 20% here. Cable inside an engine bay, or run against a hot surface, needs more derating again.

FYI 6 AWG is 13.3 mm² of copper, not 10 mm². AWG and metric sizes are different products, not two labels for the same cable. Buy by the column that matches your supplier.

How this was worked out

# DC carries current out and back,
# so the resistive path is twice the run.

  A  =  2 · I · L · ρ(T) / V_drop

# resistivity rises with temperature

  ρ(T) = ρ₂₀ · (1 + α · (T − 20))

Substituted

I, current20 A

L, one-way run5 m

V, system12 V

Drop budget3 % = 0.360 V

ConductorCopper

ρ at 20 °C0.0172 Ω·mm²/m

Ambient25 °C

ρ corrected0.01754 Ω·mm²/m

A required9.743 mm²

Why there are two answers

Ten AWG is a conductor of 5.26 mm². It is not 10 mm², and a supplier who describes it that way is out by roughly a factor of two. Cable sold as 10 mm² in the UK, Europe or Australia is nearly twice the copper of 10 AWG.

That gap matters on a DC circuit in a vehicle, where the whole point of sizing is keeping resistance low enough that the voltage arriving at the load is still useful and the cable never gets hot. So this calculator refuses to print an equals sign between the two systems. It gives you the American size, it gives you the metric size your supplier actually stocks, and it tells you the true cross-section of each so you can see for yourself that they are not the same thing.

Buy from whichever column matches the shop you are standing in. Do not translate one into the other.

AWG to mm² reference

True conductor area against the nearest stocked metric size. Ampacity is a continuous current limit for copper, bundled, at 12 volts and 20 °C. Free air runs take more; engine bays take less.

AWG	True area	Nearest stocked	Difference	Ampacity, bundled
18	0.823 mm²	0.75 mm²	−0.07 mm² less copper	7 A
16	1.31 mm²	1.5 mm²	+0.19 mm²	10 A
14	2.08 mm²	2.5 mm²	+0.42 mm²	15 A
12	3.31 mm²	4 mm²	+0.69 mm²	25 A
10	5.26 mm²	6 mm²	+0.74 mm²	35 A
8	8.37 mm²	10 mm²	+1.63 mm²	50 A
6	13.3 mm²	16 mm²	+2.70 mm²	65 A
4	21.2 mm²	25 mm²	+3.80 mm²	85 A
2	33.6 mm²	35 mm²	+1.40 mm²	110 A
1	42.4 mm²	35 mm²	−7.40 mm² less copper	not published
1/0	53.5 mm²	50 mm²	−3.50 mm² less copper	135 A
2/0	67.4 mm²	70 mm²	+2.60 mm²	not published
3/0	85 mm²	95 mm²	+10.00 mm²	not published
4/0	107.2 mm²	95 mm²	−12.20 mm² less copper	not published

Amber rowsThe closest stocked metric size is smaller than the American conductor. Reaching for it as a substitute leaves you undersized. Go up to the next metric size instead.

Where a size has no published 12 V bundled figure we leave it blank rather than inventing one.