Use the lowest-rated component in the circuit. Most breakers and devices are rated 75°C; some residential equipment is 60°C.
The NEC recommends 3% on a branch circuit and 5% total. These are recommendations, not requirements.
Reference data and planning arithmetic. Does not apply ambient or bundling derating, motor rules, or local amendments. Not a substitute for the NEC edition your jurisdiction has adopted, an inspection, or a licensed electrician.
What size wire for a 20 amp circuit?
12 AWG copper for a 20 amp circuit — but only out to about 47 feet at 120 V. Past that, voltage drop pushes you to 10 AWG, and past roughly 74 feet to 8 AWG.
Ampacity alone says 12 AWG at any length: NEC Table 310.16 gives it 25 A at 75°C, and NEC 240.4(D) caps the breaker at 20 A. That is the number most calculators stop at, and it is why a long circuit can be entirely legal and still leave a table saw struggling at the far end.
At 240 V the same conductor reaches about 93 feet, because the allowable 3% drop in volts doubles while the resistance stays the same. Note that these figures apply before any ambient-temperature or conductor-bundling derating.
Two constraints, and the bigger one wins
Sizing a conductor means satisfying two independent requirements:
- Ampacity — the conductor must carry the current without exceeding its insulation temperature rating. NEC Table 310.16. This is a safety requirement and it is enforceable.
- Voltage drop — enough voltage must reach the load for it to work properly. Advisory, 3% recommended.
On a short run ampacity governs. On a long run voltage drop governs, often by two or three sizes. A calculator that only checks ampacity will happily give you a conductor that is entirely legal and still leaves your table saw struggling at the end of a 150 ft run.
This page shows both answers and names which one won.
The 240.4(D) rule that overrides the ampacity table
Table 310.16 says 12 AWG copper at 75°C carries 25 A. You cannot put it on a 25 A breaker for general use, because NEC 240.4(D) caps overcurrent protection for small conductors regardless of what the ampacity table says:
- 14 AWG copper — 15 A maximum
- 12 AWG copper — 20 A maximum
- 10 AWG copper — 30 A maximum
- 12 AWG aluminium — 15 A; 10 AWG aluminium — 25 A
This calculator applies those caps, which is why it will never tell you 12 AWG is fine for a 25 A load.
Termination temperature is the one people get wrong
You must use the ampacity column matching the lowest-rated component in the circuit, not the wire's own rating. THHN is a 90°C conductor, but if it lands on a breaker rated 75°C you use the 75°C column. Most breakers and devices are 75°C; some residential equipment and anything marked for 60°C use pulls you down to the 60°C column.
The 90°C column is mostly used for derating calculations rather than for sizing straight to a termination.
| Load | 25 ft | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 15 A at 120 V | 14 AWG | 12 AWG | 8 AWG | 8 AWG |
| 20 A at 120 V | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
| 30 A at 240 V | 10 AWG | 10 AWG | 8 AWG | 8 AWG |
| 50 A at 240 V | 8 AWG | 8 AWG | 6 AWG | 4 AWG |
| 100 A at 240 V | 3 AWG | 3 AWG | 3 AWG | 2 AWG |
What this does not do
This is a starting point for a straightforward branch circuit or feeder. It does not apply:
- Ambient temperature correction (NEC Table 310.15(B)(1)) for conduit in an attic or in direct sun.
- Conductor bundling adjustment (Table 310.15(C)(1)) for more than three current-carrying conductors in a raceway — four to six conductors derate to 80%, seven to nine to 70%.
- Motor and continuous-load rules, which have their own sizing multipliers in Articles 430 and 210.
- Equipment grounding conductor sizing, which comes from Table 250.122.
Any of those can push the answer up a size or more. Where derating applies, the result here is a floor, not the answer.
Common questions
What size wire do I need for a 20 amp circuit?
12 AWG copper for runs up to about 47 feet at 120 V. Beyond that, voltage drop pushes you to 10 AWG, and past roughly 75 feet to 8 AWG. Ampacity alone would say 12 AWG at any length.
What size wire for a 50 amp circuit?
8 AWG copper on a 75C termination satisfies ampacity at 50 A. On a run over about 60 feet at 240 V, check voltage drop — it may push you to 6 AWG.
Why does 12 AWG only get a 20 amp breaker if the table says 25 amps?
NEC 240.4(D) caps overcurrent protection for 14, 12 and 10 AWG conductors regardless of the ampacity table. 12 AWG copper is limited to 20 A for general applications.
Which ampacity column should I use?
The one matching the lowest temperature rating in the circuit, usually the breaker or device terminal rather than the wire. Most are 75C. Using the 90C column because the wire is THHN is a common and unsafe error.