Wire Size Calculator
What gauge copper wire for that circuit — ampacity and voltage drop, both checked?
⚠️ Electricity can kill and fire is forever. This calculator is for planning and education, not a substitute for the NEC, local code, or a licensed electrician.
How this is calculated
- Ampacity: NEC 310.16 copper table (75°C), with the small-wire caps from 240.4(D): 14 AWG → 15 A, 12 AWG → 20 A, 10 AWG → 30 A
- Continuous loads are sized at 125% of rated current (NEC 210.20)
- Voltage drop % = (2 × K × I × L) ÷ circular mils ÷ volts × 100, with K ≈ 12.9 for copper
- If drop exceeds 3%, the recommendation upsizes until it clears — lights stop dimming, motors stop running hot
Common questions
What size wire for a 20-amp circuit?
12 AWG copper is the standard 20 A wire — and that is what you will get for a 20 A, 120 V circuit up to about 95 ft. Past that, voltage drop takes over: at 150 ft the drop is nearly 10% on 12 AWG, so the calculator recommends upsizing to 6 AWG.
Why does wire length matter so much?
Resistance adds up: at 150 ft, a 20 A load on 12 AWG loses nearly 10 volts of your 120 — the lights dim when the saw starts. The National Electrical Code treats 3% drop on a branch circuit (5% total including the feed) as the practical limit.
What about aluminum wire?
Aluminum needs roughly two sizes larger than copper for the same ampacity, and residential aluminum is now restricted to service entrances and certain 240 V appliance runs with proper connectors. This calculator is copper-only — if your project involves aluminum, size it separately.
Is a 15-amp breaker on 14 AWG wire always safe?
For the ampacity itself, yes — that is the classic residential lighting circuit. But distance still matters: a 15 A load at 120 ft of 14 AWG drops over 5%, so long runs to a detached garage or shed often warrant 12 AWG even on a 15 A breaker.
This is for planning — what does "code" actually require?
Your local jurisdiction's amendments to the NEC are the law, not this calculator: panel capacity, grounding, conduit fill, GFCI/AFCI requirements, and permitted DIY scope all vary. Permits and inspections exist because electricity does not forgive shortcuts. For anything beyond an extension cord, a licensed electrician is cheap insurance.