Automotive wire gauge calculator
The smallest wire that keeps voltage drop in check for a 12-volt circuit, based on how much current it carries and how far it runs.
Wire gauge
| Gauge | Voltage drop | Percent |
|---|
This checks voltage drop only. Also confirm the wire's ampacity for its insulation and where it runs, and fuse the circuit to protect the wire.
How the gauge is chosen
Copper wire resistance is standard for each AWG size. The length is doubled because current travels out to the load and back through the ground. The calculator then picks the smallest gauge that keeps the drop under your limit.
A 3% limit is the usual target for anything where voltage matters, like lighting, ignition and fuel pumps. Noncritical accessories can tolerate up to about 10%.
Resistance values are standard AWG copper figures at room temperature.
Frequently asked questions
Why does voltage drop matter more in a 12-volt system than household wiring?
A given voltage drop is a much larger percentage of a 12-volt supply than of a 120-volt household circuit, so automotive wiring is far more sensitive to undersized wire — a drop that would be negligible in a house can meaningfully dim lights or weaken a motor in a car.
What voltage drop is considered acceptable?
A commonly cited guideline is keeping voltage drop under about 3% for non-critical circuits and under about 1-2% for sensitive circuits like ECU power or lighting, though the right target can vary by what the circuit is actually powering.
Why does wire length count both directions?
Current has to travel out to the load and back to the source to complete the circuit, so the actual resistance path is twice the one-way distance — leaving out the return path would understate voltage drop by half.
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