CIRCUIT BREAKER SIZE
Breaker size chart — standard sizes and the load each one covers
The standard breaker sizes, and the largest load each one may protect under the three rules the calculator applies. A continuous load — anything running three hours or more, so lighting, electric heat and EV charging — is sized at 125%, which is why a 32 A charger needs a 40 A breaker rather than a 35. Motors use the 250% inverse-time rule instead, because there the breaker only handles short-circuit protection while a separate overload device covers the thermal side.
| Breaker | Max non-continuous load | Max continuous load | Max motor full-load amps |
|---|---|---|---|
| 15 A | 15 A | 12 A | 6 A |
| 20 A | 20 A | 16 A | 8 A |
| 25 A | 25 A | 20 A | 10 A |
| 30 A | 30 A | 24 A | 12 A |
| 35 A | 35 A | 28 A | 14 A |
| 40 A | 40 A | 32 A | 16 A |
| 45 A | 45 A | 36 A | 18 A |
| 50 A | 50 A | 40 A | 20 A |
| 60 A | 60 A | 48 A | 24 A |
| 70 A | 70 A | 56 A | 28 A |
| 80 A | 80 A | 64 A | 32 A |
| 90 A | 90 A | 72 A | 36 A |
| 100 A | 100 A | 80 A | 40 A |
| 110 A | 110 A | 88 A | 44 A |
| 125 A | 125 A | 100 A | 50 A |
| 150 A | 150 A | 120 A | 60 A |
| 175 A | 175 A | 140 A | 70 A |
| 200 A | 200 A | 160 A | 80 A |
| 225 A | 225 A | 180 A | 90 A |
| 250 A | 250 A | 200 A | 100 A |
Standard sizes per NEC 240.6(A), residential through light commercial. Continuous-load multiplier per NEC 210.20(A); motor inverse-time multiplier per NEC 430.52.
The breaker protects the conductor, not the load — pairing it with an undersized wire is the dangerous mistake. Size the wire on the wire gauge calculator, and check whether the circuit also needs AFCI or GFCI protection.
About this calculator
This circuit breaker calculator picks the next standard breaker size (NEC 240.6) for a given load. Enter wattage or amperage and the calculator converts watts to amps using your system voltage, applies the NEC 210.20(A) 125% multiplier when the load is continuous (anything operating ≥3 hours — lighting, EV chargers, electric heat), and rounds up to the nearest standard breaker (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250 A). Motor loads use NEC 430.52 sizing (250% inverse-time breaker for typical service factor 1.15 motors) and are flagged separately. ESTIMATE ONLY — final breaker selection must be verified by a licensed electrician and the local AHJ before any work goes in.
How to use this calculator
Pick whether you're entering the load as amps or watts. If watts, set the system voltage so the calculator can convert (120V for standard outlets, 240V for dryers/AC/EV chargers). Pick the load type: continuous (≥3 hr operation like lighting, EV chargers, electric heat) triggers the NEC 210.20(A) 125% multiplier; motor loads use NEC 430.52 with a 250% inverse-time breaker rating.
The calculator returns the next standard breaker size from NEC 240.6 (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250 A) plus a minimum copper wire AWG hint. Match wire size to the breaker; never install a breaker larger than the wire it protects.
Worked example
For a 1,500W microwave on a 120V circuit, non-continuous use:
Base amps: 1,500 ÷ 120 = 12.5 A. Multiplier: 1.0 (non-continuous). Design amps: 12.5 A. Next standard breaker: 15 A.
Min wire: 14 AWG copper. Wired correctly with a 14 AWG home run from the panel and a 15A breaker.
For a 7.7 kW EV charger (32A continuous at 240V):
Base amps: 32. Continuous multiplier: 1.25. Design amps: 40. Next standard breaker: 40 A. Min wire: 8 AWG copper.
For a 5 HP motor on 240V (about 28 A running):
Base: 28 A. Motor multiplier (NEC 430.52): 2.5×. Design: 70 A. Next standard breaker: 70 A. The motor itself is protected by a separate overload device — the breaker only handles short-circuit protection (which is why the inverse-time multiplier is so high).
Common mistakes & waste factors
Forgetting the 125% continuous-load factor. NEC 210.20(A) requires it for any load running 3+ hours. Skipping it means the breaker trips during normal operation.
Matching wire to load instead of to breaker. The breaker protects the wire — wire size must match breaker rating, not just the load. A 12 AWG wire on a 30A breaker is a fire hazard.
Using motor sizing for non-motor loads. The 250% inverse-time multiplier is for motor short-circuit protection only. Applying it to general loads gives wildly oversized breakers.
Ignoring AFCI/GFCI requirements. Some circuits (bedrooms, kitchens, baths, outdoor) require AFCI or GFCI breakers per NEC 210.12 / 210.8. Those breakers cost 3–5× more and have specific compatibility requirements.
Rules of thumb
NEC 240.6 standard sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250 A.
Wire to breaker (75°C copper): 14 AWG → 15A. 12 AWG → 20A. 10 AWG → 30A. 8 AWG → 40-50A. 6 AWG → 60-65A.
Continuous load (≥3 hr): multiply amps by 1.25 before sizing.
Motor circuits: NEC 430.52 inverse-time breaker at ~250% of running amps for short-circuit protection (separate overload device handles thermal).
Always match breaker to wire size, never to load alone. Oversized breaker on undersized wire = fire risk.