Wire size calculator

Load current

The amps the equipment draws. Use the nameplate, or watts ÷ volts. For a circuit sized to a breaker, enter the breaker size.

Article 100

Wire type

The insulation's temperature rating. THHN/THWN-2 building wire is 90°C. NM (Romex) has 90°C conductors but is limited to the 60°C column.

Table 310.4(1), 334.80

Terminal rating

Breakers, lugs and equipment terminals have a temperature rating, and the wire can't be used above it. Most equipment today is marked 75°C. Unmarked equipment on circuits of 100 A or less (or wire 14–1 AWG) is limited to 60°C.

110.14(C)

Hottest air temperature along the run

Use the hottest place the wire passes through: an attic in July, a boiler room, outdoors in the sun. Hot air lowers how much current the wire can carry.

310.15(B)

Current-carrying wires together

Count the hots, plus neutrals that carry unbalanced current, in the same conduit or cable. Don't count ground wires. A single 120 V circuit has 2; a 240 V circuit has 2; a 3-phase circuit has 3. More than 3 together means the wires run hotter and must be derated.

310.15(C)(1), 310.15(E), (F)