NEC wire sizing rules
The National Electrical Code requires branch circuit conductors to be sized for 125% of continuous load plus 100% of non-continuous load.
NEC Informational Note 210.19(A) recommends limiting voltage drop to 3% on branch circuits and 5% total overall across feeder and branch circuits.
Worked example: 120V 20A branch circuit
A 120V 20A circuit running 50 feet requires 12 AWG copper THHN (rated 25A at 75°C).
For longer runs over 75 feet, up-sizing to 10 AWG prevents exceeding the recommended 3% voltage drop threshold.
NEC conductor sizing inputs
Design Current
Operating current in Amps before applying continuous duty factors.
CALCULATOR QUESTIONS
US NEC wire size & ampacity calculator questions
What wire gauge is needed for a 20-amp breaker in the US?
Under NEC rules, a standard 20-amp branch circuit requires minimum 12 AWG copper conductors.
What is the NEC rule on voltage drop?
NEC recommends that conductor size be selected so voltage drop does not exceed 3% at the farthest outlet, with 5% maximum total including the feeder.