When to use this: when you have a known load in amps and a one-way run length and need a starting copper conductor size that satisfies both NEC 310.16 ampacity (75°C column) and the 3% branch-circuit voltage-drop recommendation. For planning only – confirm conduit fill, ambient derating, and termination temperature with a licensed electrician.
NEC Wire Size Calculator
Estimates a copper conductor size from NEC 310.16 (75°C column) and computes one-way voltage drop. For planning only; verify with a licensed electrician.
Results
Recommended copper AWG—
Ampacity (75°C)—
Voltage drop—
Within 3% target?—
Enter values above to recalculate.
Last reviewed: April 2026 (NEC 2023)
Tools we used or recommend for this work: Klein Tools wire strippers (Klein 11055), a Klein digital multimeter for verifying voltage at the load end, and Southwire Romex (or THHN if you are pulling in conduit) in the gauge this calculator returned. Links may be affiliate-tagged where the brand has a program.
