Volt & Grain

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

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

%
Conductor
Installation
Resultdefault case
Minimum conductor 9.74 mm² for 0.36 V of allowed drop
North America / AWG
6 AWG
true conductor 13.3 mm²
OK
Voltage drop0.264 V
Drop2.20 %
Rated, bundled52 A
UK / EU / AU stocked
10 mm²
stocked size, 10 mm² nominal
OK
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) / Vdrop

# 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.

AWGTrue areaNearest stockedDifferenceAmpacity, bundled
180.823 mm²0.75 mm²−0.07 mm² less copper7 A
161.31 mm²1.5 mm²+0.19 mm²10 A
142.08 mm²2.5 mm²+0.42 mm²15 A
123.31 mm²4 mm²+0.69 mm²25 A
105.26 mm²6 mm²+0.74 mm²35 A
88.37 mm²10 mm²+1.63 mm²50 A
613.3 mm²16 mm²+2.70 mm²65 A
421.2 mm²25 mm²+3.80 mm²85 A
233.6 mm²35 mm²+1.40 mm²110 A
142.4 mm²35 mm²−7.40 mm² less coppernot published
1/053.5 mm²50 mm²−3.50 mm² less copper135 A
2/067.4 mm²70 mm²+2.60 mm²not published
3/085 mm²95 mm²+10.00 mm²not published
4/0107.2 mm²95 mm²−12.20 mm² less coppernot 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.

Common questions

What size wire do I need for a 12V system?
It depends on the current and the run length, not on the voltage alone. At 12 volts a 20 amp load over a 3 metre one-way run needs about 6 mm² (roughly 9 AWG, so 8 AWG in practice) to stay inside a 3% drop. Double the run and you need double the area. Twelve volt systems need much heavier cable than mains work for the same power, because the same watts arrive as roughly twenty times the current.
Is 4 AWG the same as 25 mm²?
No. 4 AWG is 21.2 mm², so 25 mm² is the next size up rather than an equivalent. That direction is safe. The trap runs the other way: for four common AWG sizes the nearest metric cable is smaller, and substituting it leaves you undersized. Those four are flagged in amber in the table on this page.
What voltage drop is acceptable?
3% is the usual target for circuits where performance matters, such as lighting, pumps, fridges and any charging run, and 10% is tolerated for non-critical loads. Those two conventions come from ABYC E-11. A 10% drop on a 12 volt system means the load sees 10.8 volts, which many appliances will not run on properly, so treat 10% as a limit rather than a design goal.
Why does the calculator double my run length?
Because current has to return. A DC circuit carries the load out along the positive and back along the negative, so a 3 metre run is 6 metres of conductor and the resistance is doubled. Enter the one-way distance from the source to the load; the doubling is applied for you.
Does temperature change the answer?
Yes, and by more than people expect. Copper resistance rises about 0.39% per degree, so a cable at 60 °C in an engine bay carries roughly 16% more resistance than the same cable at 20 °C. The calculator corrects for the ambient temperature you enter rather than assuming a bench.
Does this size my fuse as well?
No. This tool sizes the conductor for voltage drop and checks it against a current rating. Fusing protects the cable rather than the load and is a separate calculation, which is why it is a separate tool rather than a number quietly bolted onto this one.