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Breaker Size Calculator

By · Updated Aug 2026

Split continuous and noncontinuous current, apply the 125% treatment only where it belongs, and screen the next standard OCPD rating.

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How to Use

  1. Split the load correctly - Enter current expected for less than three hours in the noncontinuous field and current expected at maximum for three hours or more in the continuous field when the NEC definition applies.
  2. Let the calculator do the 125% step - Sizing current is noncontinuous amps + 125% of continuous amps. Do not multiply the continuous portion yourself before entering it.
  3. Read the result as a planning OCPD - The tool selects the next configured standard ampere rating at or above the calculated sizing current.
  4. Use the conductor screen carefully - It is only a basic ampacity screen. If the conductor carries the sizing current but is below the next standard OCPD, the result is marked for review rather than automatically passed or failed.
  5. Check equipment-specific rules - Motors, HVAC equipment, welders, transformers, appliances, generators, and other equipment can have dedicated OCPD rules or nameplate/listing instructions.
Quick answer

A 16 A continuous load plus an 8 A noncontinuous load produces a 28 A sizing current: 16 × 1.25 + 8 = 28 A. The next standard rating in the configured list is 30 A. That is a planning result, not permission to place a 30 A breaker on any conductor or equipment.

Keep the Two Load Buckets Separate

Load buckets100% noncontinuous125% continuousSizing currentadd the two partsStandard ratingthen verify limitsA standard rating is not a substitute for equipment-specific OCPD rules.
The 125% treatment belongs on the continuous portion, not automatically on the entire load.

Why This Is Better Than “Amps × 1.25”

A mixed load can contain both continuous and noncontinuous current. Applying 125% to the whole load overstates the sizing current. Applying it to none of the continuous portion understates it. This calculator keeps the two buckets separate.

After the sizing current is calculated, the next standard ampere rating is selected from the configured sequence. That is still only one part of overcurrent-protection design. Conductor protection rules, equipment articles, nameplates, listed instructions, panel/device ratings, and the adopted code can control the final result.

The optional conductor screen intentionally distinguishes three outcomes: the conductor does not even carry the sizing current; it carries the sizing current but is below the selected standard OCPD and needs next-size-rule review; or its basic ampacity is at least the selected standard rating.

Examples

ContinuousNoncontinuousSizing currentNext standard rating
0 A22 A22.0 A25 A
16 A8 A28.0 A30 A
24 A0 A30.0 A30 A
32 A10 A50.0 A50 A

Examples show the generic load arithmetic and standard-rating step only. They are not equipment-specific installation recommendations.

Planning Method

The calculator performs two transparent steps:

Sizing current = I_size = I_noncontinuous + (1.25 × I_continuous)
Planning OCPD = First configured standard rating ≥ I_size
Actual-load utilization = (I_noncontinuous + I_continuous) ÷ selected rating × 100

The conductor screen compares basic configured ampacity with both the sizing current and the selected standard rating so an in-between case is not mislabeled as a simple pass.

Code Sources

  • NEC 210.20(A) addresses branch-circuit overcurrent protection for continuous and noncontinuous loads using the noncontinuous load plus 125% of the continuous load, subject to the listed exceptions and equipment-specific provisions.NFPA 70 development record for 210.20(A)
  • NEC 240.6(A) lists standard ampere ratings for fuses and inverse-time circuit breakers. A standard rating is not automatically suitable for every conductor, load, or item of equipment.NFPA 70 development record for 240.6(A)

Next Steps

Complete the circuit checks rather than stopping at a breaker number:

  1. Size the conductorApply ampacity correction and adjustment, then check optional voltage drop.
  2. Check a long runA larger conductor may be needed for voltage drop without changing the OCPD rating.
  3. Convert equipment power to currentCombine several W or VA loads when current is not already known.

Related Calculators

Amp to Wire Size CalculatorQuick conductor lookup from current before detailed installation checks.Generator Size CalculatorBackup power for the panel.Box Fill CalculatorBox capacity for the circuit.Ohm's Law CalculatorVolts, amps, and resistance.

Overcurrent protection is one part of the circuit. Browse the electrical calculator collection.

FAQ

What counts as a continuous load?
The NEC definition generally describes a load where maximum current is expected to continue for three hours or more. Equipment-specific rules and exceptions still matter, so classify the load from the applicable code and equipment information rather than from this calculator.
Why not multiply the whole load by 125%?
Because the generic branch-circuit treatment applies 125% to the continuous portion and 100% to the noncontinuous portion. A mixed load should keep those parts separate.
What does “review next-size rule” mean?
It means the selected conductor carries the calculated sizing current in this basic screen but its ampacity is below the next standard OCPD rating. Next-size-up provisions are conditional, so CalcShed does not automatically call that arrangement compliant.
Can I use this for motors or HVAC equipment?
Not as the final sizing method. Motors, air-conditioning/refrigeration equipment, transformers, welders, appliances, and other equipment can have dedicated OCPD rules and nameplate or listed-instruction requirements.
Updated Aug 2026 · See our Methodology
Planning tool only. Final overcurrent protection can depend on the exact load classification, conductor protection, adjustment/correction factors, equipment-specific articles, nameplates and listed instructions, available fault current, device/panel ratings, and the locally adopted code. Use a qualified electrical professional for installed work.