AWG Wire Gauge & Ampacity Calculator
Enter your current draw and run length to find the minimum safe wire gauge — with voltage drop and power loss calculated in your browser. Works for 12 V van builds, marine wiring, and household AC. Nothing uploaded.
Recommended Gauge
SafeFull AWG reference table
| Gauge | mm² | Ampacity (A) | V-drop | Amp OK |
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Learn more: wire sizing, voltage drop, and ampacity
Why undersized wire is dangerous
Resistance rises as a wire gets thinner, and whatever the wire resists comes out as heat. That is why the calculator prints power loss in watts next to the recommendation: 20 A over a 3 m run wastes 7.9 W in 10 AWG copper against 4.9 W in 8 AWG.
In a van or boat build that heat usually sits inside a wall cavity or a sealed conduit with nowhere to go. It is a different problem from a warm extension cord lying on the floor.
Voltage drop, worked out
The calculator counts the run twice, out and back, and uses a copper resistivity of 0.017241 Ω·mm²/m. A 20 A load over a 3 m one-way run through 8 AWG (8.37 mm²) copper loses 0.247 V, which is 2.06% of a 12 V supply.
Go one size thinner to 10 AWG and the same run comes to 3.28%, just over the usual limit. That narrow gap is why the comparison table marks every gauge pass or fail instead of printing a single number.
The target itself is a choice. The presets are 2% for sensitive electronics, 3% for general use, and 5% for lighting and other loads that tolerate a slightly low supply.
Why 12 V circuits need so much copper
Voltage drop scales with current, so the same appliance needs far more copper on a low-voltage system. A 240 W load pulls 20 A at 12 V and needs 8 AWG over that 3 m run.
The same 240 W at 120 V pulls only 2 A, and 18 AWG, the thinnest wire in the table, clears the 3% target at 0.21%. Ten times the voltage means a tenth of the current.
Ampacity depends on how the wire is installed
Every gauge in the table carries two current ratings from the 60°C column, one for wire in free air and one for wire in conduit. 12 AWG is 25 A in free air and 20 A in conduit, and 8 AWG goes from 50 A down to 40 A. Wire bundled inside a pipe cannot shed its own heat.
That gap changes answers. A 22 A load on a 2 m 120 V run passes on 12 AWG in free air, but tick the conduit box and the tool moves you to 10 AWG, because 12 AWG no longer has the ampacity headroom.
Copper against aluminium
Aluminium's resistivity here is 0.028264 Ω·mm²/m against copper's 0.017241, so it resists 1.64 times as much for the same cross-section. Sizing has to make that back with area.
On the same 20 A, 3 m, 12 V run, copper clears 3% at 8 AWG while aluminium needs 6 AWG, two sizes up the table. Aluminium costs less per metre, so budget for the thicker cable and the larger lugs before deciding.
FAQ
What wire gauge do I need for a 20A 12V circuit?
Over a 3 m one-way run, which is 6 m of wire there and back, 8 AWG copper holds the drop to 2.06% and stays inside the usual 3% target. 10 AWG looks close enough but lands at 3.28%. Run length changes the answer, so enter your own.
Why does run length matter for wire sizing?
Resistance adds up with every metre, and the calculator counts the run twice because the current has to return. Stretch that same 20 A 12 V circuit from 3 m to 5 m and 8 AWG no longer holds 3%, so the recommendation moves to 6 AWG.
What is the difference between AWG and mm²?
AWG is the US scale, where a lower number means a thicker wire. mm² is the metric cross-sectional area used in Europe and most of the world. The calculator shows both columns: 10 AWG is 5.26 mm² and 8 AWG is 8.37 mm², so the nearest metric stock sizes above them are 6 mm² and 10 mm².