Wire Gauge Calculator
Tell us the current, the run length and how much voltage you can afford to lose. We'll tell you what gauge copper wire you need.
Your circuit
- Copper wire at 20C. Hot engine bay runs have higher resistance than this.
- Sized for voltage drop, which is almost always stricter than the ampacity limit in automotive work.
- Wire gauge does not protect the circuit. Fuse to the wire, not to the load.
Fuse every circuit you add. A wire sized correctly for voltage drop can still start a fire if it shorts. Fit a fuse or breaker rated for the wire at the power source end, within 18 inches of the battery on any run taken directly from it.
Why voltage drop decides the gauge
In house wiring you size wire so it does not overheat. In a vehicle you almost always size it so the thing on the end still gets enough voltage to work. At 12 volts you have no headroom. Lose a volt and a half in the wire and a fuel pump runs slow, a winch crawls, and an amplifier clips.
That is why a circuit that would be fine on 14 gauge in a house needs 10 gauge in a truck. Same current, completely different constraint.
How much drop is acceptable
3% is the number to work to for most things. Sensors, engine management and anything that measures a voltage want 2% or better. Lighting and general accessories survive 5%. Starter motors get a pass at 10% because the run is short and the draw is enormous.
The return path counts
Current has to get back. If you run a dedicated ground wire, the electrical length is double the physical run. If you ground to the chassis, the return is through a very large conductor and adds only a little, but only when the bonding point is clean bare metal with a proper ring terminal. A painted or corroded ground is the single most common cause of a circuit that measures fine on paper and misbehaves in the vehicle.
Frequently asked questions
What gauge wire for a 30 amp circuit?
It depends on the distance, which is why a single answer does not exist. At 12 volts with a 3% target, a 30 amp circuit needs about 10 gauge over a short run and 8 or 6 gauge once you are running the length of a vehicle. Enter your actual run length above.
Should I match the wire gauge to the fuse?
Backwards. The fuse protects the wire, so the fuse is chosen after the wire and rated at or below what the wire can carry. Sizing wire down to match a fuse you already bought is how wiring harnesses melt.
Can I use household wire in a vehicle?
No. Solid core building wire work-hardens and cracks under vibration, and its insulation is not rated for oil, fuel or the temperatures in an engine bay. Use stranded automotive or marine cable with the right jacket.
Does bigger wire ever hurt?
Electrically, no. Practically, oversized cable costs more, is harder to route, and puts strain on terminals and connectors that were never meant for it. Go one size up from the calculated minimum if you want margin, not four.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.