Enter the load and the run length. The calculator picks the smallest wire that meets the ampacity table, then checks voltage drop on long runs.
Rough estimate only. Based on NEC Table 310.16 ampacities (30°C ambient, not more than three current-carrying conductors) and Chapter 9 Table 8 resistance for uncoated copper and aluminum. It does not apply ambient or bundling adjustments, conduit fill, motor or appliance rules, or local amendments. A licensed electrician sizes the actual circuit.
How does the wire size calculator work?
It first finds the smallest conductor whose Table 310.16 ampacity at the chosen temperature column meets the load (125% for continuous loads), and applies the small-conductor limits of 240.4(D) (14 AWG copper 15 A, 12 AWG 20 A, 10 AWG 30 A). It then checks voltage drop over the round trip and moves up a size until the drop is within your target.
See wire size and breaker size for the plain-English version.
Common questions
What size wire for a 50 amp circuit 100 feet away?
It depends on the wire type and temperature rating. Enter 50 A and 100 feet in the calculator for a rough answer, then have an electrician confirm the actual cable and terminations.
Why does distance change the wire size?
Longer wire has more resistance, so more voltage is lost along the way. On long runs a larger wire keeps the voltage drop within a reasonable target.
General information for California homes under the California Electrical Code (based on the NEC). Local rules and inspectors can vary. Not a substitute for an on-site inspection. Never work inside a panel, at the meter or on live circuits.