Wire Gauge Calculator
Finding the correct wire gauge requires checking two separate limits under NEC: the ampacity requirement (the wire must carry the full circuit current without overheating) and the 3% voltage drop rule (voltage drop over the wire run must not exceed 3% of supply voltage for branch circuits). These two requirements can pull in different directions — a short run might need only 14 AWG for 15A ampacity, but a long run might need 10 AWG to keep voltage drop below 3%. This calculator does both checks automatically: finds the minimum gauge by ampacity, calculates voltage drop over the actual run length, and upsizes the gauge if the drop exceeds 3%. Enter circuit amperage, one-way run distance, voltage (120V or 240V), and wire material (copper or aluminum) to get the final recommended gauge, actual voltage drop, and whether the gauge was upsized for the run.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
What gauge wire do I need for a 20-amp circuit?
A 20-amp circuit requires 12 AWG copper wire minimum per NEC code. If the run is longer than about 75 feet on 120V, check whether the voltage drop exceeds 3% — if so, upsize to 10 AWG.
Can I use aluminum wire for branch circuits?
Aluminum wire is not recommended for branch circuits under 60A because connections at receptacles and switches are rated for copper only. Aluminum is appropriate for service entrance conductors and larger feeders (60A+) where all terminations are rated for aluminum (AL/CU rated).
What is the 3% voltage drop rule?
The NEC recommends (but does not mandate) that voltage drop on branch circuit conductors not exceed 3% of nominal voltage. At 120V, that's 3.6V maximum drop. At 240V, that's 7.2V. Voltage drop causes lights to dim, motors to run hot, and electronics to receive insufficient power. The wire gauge calculator checks this automatically and upsizes if needed.
What is the difference between 12 AWG and 14 AWG wire?
12 AWG copper wire carries 20 amps and is used on 20-amp branch circuits (kitchen, bathroom, laundry). 14 AWG carries only 15 amps and is used on 15-amp circuits (lights, general receptacles in living areas). Never use 14 AWG on a 20-amp breaker — the wire will overheat before the breaker trips. When in doubt, use 12 AWG.