Wire Size Calculator

1. Enter the load current. This is the circuit amperage from the breaker or the connected load. For motors, use the nameplate FLA.

2. Set voltage and circuit type. Most residential is 120V or 240V single-phase. Commercial often uses 208V or 480V 3-phase.

3. Choose conductor material and insulation. Copper with THHN (90 degrees C) is the most common for commercial work. NM-B is standard for residential in-wall wiring.

4. Set the number of current-carrying conductors. If more than 3 CCC share a raceway, the calculator applies NEC derating factors automatically.

5. Enter the one-way run distance. The calculator checks voltage drop and flags if it exceeds your selected limit (3% for feeders, 5% for branch circuits).

The calculator starts with NEC Table 310.16 base ampacities for each wire gauge at the selected insulation temperature. For continuous loads, it multiplies the load current by 1.25 per NEC 210.20.

The conduit fill derating factor reduces ampacity when more than 3 current-carrying conductors share a raceway: 80% for 4-6 CCC, 70% for 7-9, 50% for 10-20, and 45% for 21-30.

After selecting the wire for ampacity, the calculator verifies voltage drop using NEC Chapter 9 Table 8 resistance values. If the voltage drop exceeds the limit, it recommends the next size up.