🔌Wire Size Calculator
Enter the load and distance to see the smallest wire that handles both the amps and the voltage drop.
Your circuit
Use at least
10 AWG copper
Ampacity alone: 12 AWG · voltage drop alone: 10 AWG · actual drop 2.5 V (2.1%)
for ampacity
12 AWG
for voltage drop
10 AWG
wire rated amps
30
volts dropped
2.5
Ampacities use the NEC 75°C column with small-conductor limits (14 AWG copper = 15 A, 12 = 20 A, 10 = 30 A). Bundling, heat and conduit fill can require larger wire.
How it works
We check two things and show the larger wire:
- Ampacity — from the NEC 75°C ampacity table (with the small-conductor limits: 14 AWG Cu = 15 A, 12 = 20 A, 10 = 30 A).
- Voltage drop — VD = 2 × K × amps × one-way feet ÷ circular mils, where K = 12.9 for copper and 21.2 for aluminum. Default limit 3%.
Worked example
20 A at 120 V, 100 ft one-way, copper, 3% max drop (3.6 V):
- Ampacity alone: 12 AWG
- 12 AWG drop = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.9 V — too much
- 8 AWG drop = 51,600 ÷ 16,510 = 3.1 V ✓ → use 8 AWG
For planning only. Electrical work must follow local code and is best done by a licensed electrician.
Questions people ask
12 AWG copper for short runs. Long runs may need 10 or 8 AWG to limit voltage drop.
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Last updated October 8, 2026. These results are friendly estimates to help you plan — always double-check before buying or making big decisions.