What size breaker do I need for my circuit?
Two steps, in order: first, size the breaker to match the wire's ampacity (a 15A breaker for 14 AWG copper, 20A for 12 AWG, 30A for 10 AWG). Then, if the load runs continuously — 3 hours or more — keep the actual load at no more than 80% of the breaker's rating, per NEC 210.20(A).
Checking a specific wire and breaker combination? Use the wire size calculator → — it runs both checks (ampacity match + the continuous-load rule) at once.
1. The breaker protects the wire, not the appliance
A breaker's job is to trip before the wire behind it overheats — which only works if the breaker's rating matches what that wire can actually carry. That's why breaker sizing starts with the wire gauge, not the load: pick the wire for the job, then the breaker that protects it.
| Copper wire | Matching breaker |
|---|---|
| 14 AWG | 15 A |
| 12 AWG | 20 A |
| 10 AWG | 30 A |
| 8 AWG | 40 A |
| 6 AWG | 50–60 A |
Full sizing details and how the numbers were sourced (NEC Table 310.16) are in the wire gauge guide.
2. The 80% rule for continuous loads
Once the wire and breaker match, there's a second check for loads that run a long time — NEC Article 100 defines a continuous load as one expected to run 3 hours or more (think: a space heater, a water heater element, or lighting left on all evening). NEC 210.20(A) requires the breaker to be rated for at least 125% of that continuous load — which, worked the other way, means the load itself shouldn't exceed 80% of the breaker's rating.
Worked example: a 20A breaker (12 AWG wire) has a continuous-load limit of 20 × 0.8 = 16 amps. If the actual continuous draw is higher than that — say a 18A heater running for hours — the circuit needs to move up to a 25A or 30A breaker (with wire upsized to match), even though 18A is technically under the 20A breaker's trip point.
This rule doesn't apply to short, intermittent loads — a drill, a vacuum cleaner, a microwave running for a few minutes. It specifically targets things that run for hours at a stretch, because sustained near-maximum current is what generates the most heat over time.
3. Common mismatches to check for
The most common breaker-sizing mistake isn't a math error — it's swapping in a bigger breaker to stop nuisance tripping, without upsizing the wire behind it. If a breaker keeps tripping, the fix is a bigger wire and matching breaker (or reducing the load), never a bigger breaker alone on the same wire — that removes the exact protection the breaker exists to provide. An electrician or your local Authority Having Jurisdiction (AHJ) can confirm the right combination for your specific panel and wiring.
Common questions
- What size breaker do I need for my circuit?
- Match the breaker to the wire's ampacity first (15A breaker for 14 AWG, 20A for 12 AWG, 30A for 10 AWG). Then, if the load runs 3 hours or more (continuous), keep the actual load at no more than 80% of that breaker's rating, per NEC 210.20(A).
- What is the 80% rule for breakers?
- NEC 210.20(A) requires a breaker on a continuous load to be rated for at least 125% of that load — which is the same as saying the load should not exceed 80% of the breaker's rating. A 20A breaker should carry no more than 16A of continuous load; a 15A breaker, no more than 12A.
- Can I use a 20 amp breaker with 14 gauge wire?
- No. 14 AWG copper is rated 15 amps, and the breaker's job is to protect the wire — a 20A breaker would let more current flow than the wire can safely carry. Use a 15A breaker with 14 AWG, or upsize the wire to 12 AWG for a 20A breaker.
- What happens if a breaker is too big for the wire?
- The breaker won't trip until its own rated current is reached, by which point the undersized wire may already be overheating. This is the single most common wire/breaker mismatch and a common reason older wiring fails an inspection.
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-07.