How do I size a circuit breaker?

The standard rule is: breaker rating ≥ 125% of the continuous load + 100% of the non-continuous load (NEC 210.20(A)). For a purely continuous load (operating for 3 hours or more), multiply the load current by 1.25 and round up to the next standard breaker size (15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100 A, etc.).

What is a continuous load and why is 125% commonly used?

A continuous load is generally one expected to run at maximum current for three hours or more. The supported general calculation applies 125% to that entered current. Verify the exact rule, listed assembly, conductor, terminal, and equipment requirements; not every load or listed 100%-rated arrangement is handled by this simple mode.

How do breaker rating and conductor ampacity relate?

The overcurrent device and conductor must be selected together under the applicable protection rule, including small-conductor rules and permitted load-specific exceptions. This calculator has no conductor input and does not verify that relationship.

How do I size a breaker for a motor circuit?

Do not use the simple continuous-load mode as a motor-protection calculation. Motor conductors, overload protection, and branch short-circuit/ground-fault protection use separate rules, tables, motor data, device types, and permitted maximums that this calculator does not apply.

What standard breaker sizes are available?

Standard North American breaker trip ratings (NEMA/NEC) are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A and above. The calculator rounds up to the nearest standard size after applying the 125% multiplier.

Is this breaker size calculator free?

Yes. Enter a verified load current and choose continuous or non-continuous to see the supported planning current and first standard rating at or above it. No conductor size, special-load rule, or equipment approval is inferred.

How do I use watts for a 240 V appliance?

First establish design current using the equipment method—for a simple resistive example, I = W ÷ V. Then enter the verified current and choose continuous or non-continuous. Appliance instructions and load-specific code rules may override that simple arithmetic.

Breaker Size Calculator — Free NEC 210.20 Tool

Electrical lab bench with calculator, multimeter, oscilloscope, and low-voltage test circuit used to verify Breaker Size Calculator results. — Calculate the nominal result, then verify assumptions, limits, and real behavior with appropriate measurements.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 1 of 8 — Breaker Size Calculator input-to-result overview.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 2 of 8 — Formula and unit relationship.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 3 of 8 — Worked-example calculation path.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 4 of 8 — Input and assumption checklist.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 5 of 8 — Nominal result and interpretation.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 6 of 8 — Tolerance and worst-case verification.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 7 of 8 — Editable-circuit and related-tool next step.

Breaker Size Calculator formula, inputs, result, verification, or design-boundary visual 8 of 8 — Safety, model, and professional-review boundary.

Breaker sizing method and quick-reference workflow

A breaker is selected from the load, conductor, equipment, and applicable protection rules together. A simple current calculation does not prove the correct protective device. Continuous and noncontinuous portions, motor or transformer rules, permitted next-size provisions, conductor ampacity, terminal ratings, available fault current, interrupting rating, equipment instructions, and local amendments can all change the answer.

Input class	Planning treatment	Separate release check
Noncontinuous load	Use the calculated design current under the governing method.	Verify conductor and equipment requirements.
Continuous load	Where the governing rule uses it, apply the required continuous-load factor.	Confirm device listing, conductor ampacity, and code exceptions.
Motor, HVAC, transformer, welder, or EVSE	Use the equipment-specific article and nameplate/manual data.	Coordinate overload and short-circuit protection separately.
Electronic or high-inrush load	Record steady current and startup/inrush behavior.	Check trip curve, harmonics, neutral loading, and manufacturer guidance.

Worked planning example

If a documented method requires 125% treatment for a 24 A continuous load, the planning current is 30 A. That arithmetic alone does not authorize a 30 A breaker: the conductor, terminals, equipment listing, ambient/bundling corrections, device duty, and any load-specific rules still have to support the selection.

Five-step selection check

Identify the load type and obtain nameplate or design data.
Separate continuous, noncontinuous, largest-motor, and other rule-specific portions.
Calculate the planning current without prematurely rounding.
Check conductor ampacity, termination temperature, equipment maximum protection, and the permitted standard rating.
Verify interrupting rating and coordination against the available fault current.

Verification and source boundary

Use the current edition of NFPA 70 through NFPA LiNK or the governing local standard, plus current equipment instructions. This tool does not perform a short-circuit, arc-flash, selective-coordination, motor-protection, or conductor-derating study.