Introduction
Breaker sizing looks like a single division problem — watts divided by volts — but the number that comes out of that division is not your breaker size. Between the load’s current and the breaker’s rating sit three more steps: the continuous-load multiplier, the round-up to a standard size, and the wire check. The Breaker Size Calculator runs all four steps automatically; this guide opens the hood and shows every step with the math worked out, so you can verify any result by hand.
Each formula below includes a worked calculation you can follow with a pocket calculator. The examples use the domain-standard figures: NEC 210.20(A), the 125% continuous-load rule, and the NEC 240.4(D) wire protection limits.
Formula 1: Load Current (Watts to Amps)
Most loads are rated in watts, but breakers are rated in amps. The conversion for a simple resistive or single-phase load is:
Current (A) = Power (W) ÷ Voltage (V)
A 1,500 W space heater on a 120 V circuit: 1,500 ÷ 120 = 12.5 A. An 8,000 W range on a 240 V circuit: 8,000 ÷ 240 = 33.3 A. For quick conversions, the Amperage Calculator handles this step directly. Note the voltage matters enormously — the same 1,500 W at 240 V draws only 6.25 A, which is why heavy appliances use 240 V circuits.
Some nameplates list volt-amperes (VA) instead of watts. For sizing purposes, treat VA like watts: Current = VA ÷ V. Motor nameplates list full-load current directly — use that figure rather than converting.
Formula 2: The 125% Continuous-Load Rule
NEC 210.20(A) requires that where a branch circuit supplies a continuous load — defined as a load expected to continue for three hours or more — the breaker must be rated at no less than 125% of the continuous current:
Minimum breaker rating = 1.25 × Continuous current (A)
Why 125%? Breakers and conductors heat up under sustained current, and their trip characteristics are calibrated with a safety margin for heat buildup over hours. A breaker loaded to 100% continuously runs hot; the 25% headroom keeps operating temperatures within the breaker’s tested range.
Worked example — 16 A continuous: 16 × 1.25 = 20 A. The result lands exactly on a standard size, so a 20 A breaker is correct.
Worked example — 1,500 W heater (12.5 A) run all evening: 12.5 × 1.25 = 15.6 A. This does not land on a standard size, so it rounds up to a 20 A breaker (see Formula 3).
Worked example — 9,600 W EV charger at 240 V: 9,600 ÷ 240 = 40 A; EV charging is continuous, so 40 × 1.25 = 50 A → 50 A breaker.
Formula 3: The 80% Rule (Same Rule, Other Direction)
The 125% rule has an equivalent form that starts from the breaker instead of the load:
Maximum continuous load = 0.80 × Breaker rating
1 ÷ 1.25 = 0.80, so the two formulas are mathematically identical. Electricians often use the 80% form as a quick check: a 20 A breaker can serve at most 16 A of continuous load (20 × 0.80); a 30 A breaker at most 24 A. If your continuous load exceeds 80% of the candidate breaker, the breaker is undersized.
| Breaker rating | Max continuous load (80%) | Max noncontinuous load (100%) |
|---|---|---|
| 15 A | 12 A | 15 A |
| 20 A | 16 A | 20 A |
| 30 A | 24 A | 30 A |
| 40 A | 32 A | 40 A |
| 50 A | 40 A | 50 A |
| 60 A | 48 A | 60 A |
Formula 4: Mixed Loads
Real circuits often serve both continuous and noncontinuous loads. The Code formula handles them separately:
Minimum breaker rating = (1.25 × Continuous current) + (1.00 × Noncontinuous current)
Worked example: a workshop circuit with a 10 A dust collector that runs all day (continuous) plus 8 A of task lighting used intermittently (noncontinuous): (1.25 × 10) + (1.00 × 8) = 12.5 + 8 = 20.5 A → round up to a 30 A breaker on 10 AWG copper.
Formula 5: Round Up to the Next Standard Size
Breakers are manufactured in standard ratings — commonly 15, 20, 30, 40, 50, 60, and 100 A for residential work. A calculated requirement of 22 A cannot buy a 22 A breaker; you select the next size up (30 A). Rounding is always up, never down — a 20.5 A requirement on a 20 A breaker is an undersized breaker.
After rounding up, the wire check still applies: the selected breaker must not exceed the conductor’s protection limit. A 22 A requirement rounds to 30 A, which demands at least 10 AWG copper — if the existing wire is 12 AWG, the wire (not the breaker) is what must change. The most dangerous sizing error is upsizing the breaker without the wire; it’s catalogued with fixes in the common mistakes guide.
Formula 6: The Wire Protection Check
NEC 240.4(D) caps overcurrent protection for small copper conductors regardless of the load calculation:
14 AWG copper → 15 A max · 12 AWG copper → 20 A max · 10 AWG copper → 30 A max
For larger conductors, the general rule is that protection must not exceed the conductor’s ampacity under its conditions of use (ambient temperature and conduit fill can reduce ampacity through derating — the limits above never increase). Practical pairings:
| Calculated requirement | Breaker selected | Minimum copper wire |
|---|---|---|
| 15.6 A | 20 A | 12 AWG |
| 20 A | 20 A | 12 AWG |
| 20.5 A | 30 A | 10 AWG |
| 33.3 A (8 kW range) | 40 A | 8 AWG |
| 50 A (EV charger) | 50 A | 6 AWG |
Note the 8,000 W range example: 33.3 A rounds to a 40 A breaker. Cooking appliances use demand factors (NEC 220.55) that can reduce the calculated load, but the worked figure shows the raw rounding logic.
Putting It All Together: The Full Sequence
- Convert watts (or VA) to amps: A = W ÷ V.
- Classify the load: continuous (3+ hours) or noncontinuous.
- Multiply: 1.25 × continuous amps, plus 1.00 × noncontinuous amps.
- Round up to the next standard breaker size.
- Check the wire: the breaker must not exceed the conductor’s protection limit (14 AWG → 15 A, 12 AWG → 20 A, 10 AWG → 30 A for copper).
For a plain-English walkthrough of what each result means, see the complete guide. To practice the sequence on a realistic load list, work through the estimating guide.
Related Guides
- Complete Breaker Sizing Guide — the plain-English companion to these formulas, with worked examples.
- Estimating Guide — how to build the load list these formulas start from.
- Common Breaker Sizing Mistakes — where the math most often goes wrong in practice.
- Breaker Sizing FAQ — quick answers to the most asked sizing questions.
- Illustrative Case Studies — the full sequence applied to realistic hypothetical scenarios.
Frequently Asked Questions
Is the 125% rule the same as the 80% rule?
Yes — they’re the same requirement stated two ways. Breaker ≥ 1.25 × continuous current is identical to continuous current ≤ 0.80 × breaker rating, since 1 ÷ 1.25 = 0.80.
What counts as a continuous load?
Any load expected to run at maximum current for three hours or more: space heaters, EV chargers, water heaters, and similar. When in doubt, treat it as continuous — the conservative assumption is the safe one.
Why do I round up instead of using the exact calculated size?
Breakers are only manufactured in standard ratings (15, 20, 30, 40, 50, 60, 100 A). A calculated 22 A requirement must use a 30 A breaker; a 20 A breaker would be undersized by definition.
Does the wire size ever change the answer the formulas give?
Yes — the wire check is a hard ceiling. If the calculation calls for a 30 A breaker but the existing wire is 12 AWG (20 A max), you cannot install the 30 A breaker; you must upgrade the wire or reduce the load.
Get Your Number
The full sequence — convert, classify, multiply by 1.25 for continuous loads, round up, check the wire — is exactly what the Breaker Size Calculator automates. Run your numbers through it, then hand-verify with the formulas above; if the two agree, you understand your circuit. If they don’t, the formulas will show you which input to recheck.
Disclaimer: Calculator results are planning estimates, not engineering advice. These formulas reflect NEC general rules; local amendments, derating conditions, and specific equipment requirements can change the answer. Verify your plan with a licensed electrician and your local authority having jurisdiction (AHJ) before purchasing materials or starting work.
