CategoriesElectrical

Electrical Calculators

Electrical calculators for conduit fill, wire sizing, and load calculations. NEC code compliant tools for electricians and DIYers.

6 Calculators

NEC Compliant

Always Free

Wire size follows current, distance and temperature rating, in that order.

Conductor sizing starts with the load in amps and the NEC ampacity table, but the number you land on depends on which temperature column you read. Most residential wiring is NM-B cable, terminated on devices rated for 60°C, so the 60°C column governs even though the conductor itself may be rated higher. That is why 12 AWG is good for 20 amps in a house and not the 25 amps its 90°C rating suggests.

Distance is the second factor. Voltage drop should stay under 3% on a branch circuit, and on long runs that forces a size up regardless of ampacity. A 20 amp circuit at 100 feet is usually fine on 12 AWG; at 150 feet it often is not.

Settle this before you calculate

Continuous load in amps: Watts divided by volts, then add 25% for anything running more than three hours, which is what the code calls continuous.
One-way run length: Voltage drop uses the round trip, so measure the actual cable route rather than the straight line through the walls.
Termination temperature rating: Default to the 60°C column for standard residential devices unless every termination in the circuit is rated higher.

Where the estimate goes wrong

Sizing the breaker to the wire: It works the other way round. The breaker protects the conductor, so the wire is sized for the load and the breaker matched to the wire.
Ignoring derating in a full conduit: More than three current-carrying conductors in one raceway means an ampacity reduction. Count the conductors before trusting the table.
Treating an estimate as a design: These calculators size to published tables, but a permit and an inspection need a licensed electrician. Local amendments override the national code.