Wire Size Calculator
Determine the correct wire size for a circuit.
How It Works
Required cmil = (2 × 12.9 × Current × Distance) ÷ Voltage drop. Pick the smallest AWG whose cmil meets it.
Example
A 20 A circuit, 100 ft run at 120 V with 3% drop: ≈14,333 cmil → 8 AWG copper.
Frequently Asked Questions
What is voltage drop?
The voltage lost along a conductor. Excess drop can damage equipment and is limited by code to ~3% for branch circuits.
Why does distance matter?
Longer runs have more resistance, so they need larger wire to keep drop in check.
Is this for copper or aluminum?
This calculator assumes copper (K = 12.9). Aluminum needs larger sizes.