The wire size for a breaker depends on its amperage, matched to the conductor's ampacity in NEC Table 310.16. Under normal conditions (no ambient-temperature or conduit-fill derating), the chart below gives the standard THHN/THWN copper and aluminum size for common residential and light-commercial breaker sizes.
| Breaker | Copper | Aluminum | Typical Use |
|---|---|---|---|
| 15A | 14 AWG | 12 AWG | Lighting, general receptacles |
| 20A | 12 AWG | 10 AWG | Kitchen/bathroom receptacles |
| 30A | 10 AWG | 8 AWG | Window AC, electric dryer |
| 40A | 8 AWG | 6 AWG | Electric range, small subpanel |
| 50A | 6 AWG | 4 AWG | EV charger, large range, hot tub |
| 60A | 4 AWG | 3 AWG | Subpanel feeder |
| 100A | 4 AWG | 2 AWG | Subpanel, small service |
| 200A | 2/0 AWG | 4/0 AWG | Main service (residential) |
⚠️ This chart assumes 60°C/75°C-rated conductors at a 30°C ambient with no more than three current-carrying conductors in the raceway. Longer runs, hot ambients, or bundled conductors require derating — use the full Wire Ampacity Calculator for those cases, and always verify against the locally adopted NEC edition.
A circuit breaker's job is to protect the wire behind it from overheating due to overcurrent — so the breaker's trip rating and the conductor's ampacity have to be coordinated. The wire must be rated to carry at least as much current as the breaker allows to flow, per NEC 240.4.
If a breaker is rated higher than the wire's ampacity, the wire can overheat and start a fire before the breaker ever senses a problem — the breaker only trips based on its own rating, not the wire's actual temperature. This is why NEC 240.4 requires overcurrent protection to match (or be sized down to) the conductor's ampacity, with a short list of specific exceptions for small conductors and motor circuits.
The quick chart assumes ideal conditions. Three things commonly push the required wire size larger than the chart shows: (1) high ambient temperature — attics, rooftops, and boiler rooms reduce ampacity and require a correction factor from Table 310.15(B)(1); (2) conduit fill — more than three current-carrying conductors in one raceway requires a derating factor from 310.15(C)(1); (3) long runs — voltage drop, not ampacity, often forces an upsized conductor on 100+ ft feeder runs even when the ampacity chart alone would allow a smaller wire.
Aluminum conductors need to be one to two AWG sizes larger than copper to carry the same current, because aluminum has higher resistance per cross-sectional area. Aluminum is common on larger feeders (100A+) for cost reasons, but requires terminations rated for aluminum and an approved antioxidant compound at connections to prevent oxidation-related failures.
A 50 amp breaker typically requires 6 AWG copper (or 4 AWG aluminum) at the 60°C or 75°C column of NEC Table 310.16, assuming no derating is needed. This is the most common answer for a straightforward single circuit — but ambient temperature, conduit fill, and termination temperature ratings can change the required size, so this is a starting point, not a substitute for a full calculation.
10 AWG copper (or 8 AWG aluminum) is the standard wire size for a 30 amp circuit under normal conditions. This is common for window AC units, electric dryers on some circuits, and RV shore power.
4 AWG copper (or 2 AWG aluminum) is typically used for a 100 amp circuit, such as a subpanel feeder. At this amperage, voltage drop over distance becomes a bigger factor — for feeder runs longer than about 75-100 ft, upsizing the conductor beyond the minimum ampacity is common practice even when not strictly required by ampacity alone.
12 AWG copper (or 10 AWG aluminum) is the standard wire size for a 20 amp circuit — the most common branch circuit size for kitchen counter receptacles, bathroom receptacles, and general-purpose 120V circuits in the NEC.
No. The conductor must be sized to protect the wire, not just the load. The breaker rating and wire ampacity must be coordinated per NEC 240.4 — using a wire smaller than what the breaker requires creates a fire hazard because the breaker won't trip before the undersized wire overheats.