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Breaker Sizing Calculator (NEC 240)

Quick answer

Size overcurrent protection per NEC 240.4: 125 percent for continuous loads, standard sizes from 240.6, conductor rules, and motor exceptions.

Figures are estimates from this calculator’s standard formula — adjust the inputs in the tool below for your exact number.

How Does the Breaker Sizing Calculator Work?

Breaker = 1.25 × I-load → next standard size

NEC 240.4 G: conductors and equipment are protected at their ampacity, with the 125 percent uplift for continuous loads (NEC 210.20(A)). The calculator computes the minimum, jumps to the next standard size from NEC 240.6, and cross-checks the conductor. Example: a 24 A continuous load computes to 30 A; a 10 AWG copper conductor (35 A at 75 C) is protected by the 30 A breaker - compliant, with the next-size-up rule applied correctly. Appliance circuits have their own rules: NEC 422 grants fixed water heaters 125 percent, and AC units follow nameplate MCA/MOPD figures - the manufacturer has already done the 240.4 math for you.

  1. 1

    Enter load current

    Continuous or non-continuous amps.

  2. 2

    Read minimum rating

    125 percent where required.

  3. 3

    Verify conductor

    Ampacity against the chosen breaker.

Use Cases

Passing inspection on residential panel upgrades

Sizing protection for workshop machinery circuits

Documenting code compliance for EV charger installs

Common Mistakes

  • Upsizing a breaker to stop trips without touching the conductor - a fire code violation

  • Applying the 125 percent factor twice

  • Using HACR breakers where the listing does not permit them

FAQs

Can a breaker be bigger than the wire ampacity?

Generally no - that is the core of NEC 240.4. Exceptions: motor circuits (430.52 allows up to 175-250 percent FLA for short-circuit protection) and the next-size-up rule for non-continuous loads where the next standard size still protects.

What is the next-size-up rule?

NEC 240.4(B): if the computed value falls between standard sizes, you may use the next standard size up for non-continuous loads, provided the conductor ampacity is at least the computed current. 26 A computed on 10 AWG (35 A): a 30 A breaker is legal.

Why do motor breakers seem oversized?

Inrush: motors draw 5-8x FLA at start, so instantaneous trip protection is allowed at 175-250 percent FLA (NEC 430.52) while separate overload protection at 115-125 percent guards the windings.

What about GFCI and AFCI breakers?

They add personnel or fire protection on top of overcurrent sizing: GFCI trips at 5 mA of ground leakage (bathrooms, kitchens per NEC 210.8), AFCI detects arc signatures in living areas - size them with the same 125 percent math.

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