Safety & Protection calculator
Circuit Breaker Sizing Calculator
Use this breaker size calculator as a NEC OCPD finder. Enter load current, load type, selected conductor, ambient factor, and current-carrying conductor adjustment; the result screens the next standard breaker review point and flags conductor conflicts before you use the reference tables.
Updated August 4, 2026
Enter load current, load type, selected conductor, ambient factor, and current-carrying conductor adjustment to screen the next NEC standard breaker review point.
Use the preset values as calculator inputs, then compare the returned OCPD review point with the conductor check
Enter load current, load type, and selected conductor below to screen the next standard breaker review point and catch wire-limit conflicts early
Source: NFPA 70, National Electrical CodeOpens in a new tab | Last reviewed: 2026-08-04
Final installation decisions must use the adopted NEC edition, equipment listings, and AHJ requirements.
Calculator Inputs
Quick Presets
Calculation Results
Enter values above to see calculation results
Field kit
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Use the breaker result as a review point, then compare non-invasive tools for tracing, measuring, and documenting circuits.
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Calculation history
Example Calculations
How to Use
How This Breaker Size Calculator Works
Start with the calculator inputs above. Enter the load current, choose the load type, select the intended conductor, and apply any ambient-temperature or current-carrying-conductor adjustment that belongs to the installation. The page keeps NEC 240.6 standard ratings and small-conductor references below the workflow so the project-specific answer comes from your entered load and conductor basis, not from a generic chart.
Inputs used by the calculator
- Load current is the circuit load or motor full-load current in amperes.
- Load type selects the sizing path: non-continuous, continuous, mixed, or motor.
- Selected copper conductor is used for the conductor compatibility review after derating.
- Ambient temperature factor and current-carrying conductor factor reduce the simplified conductor ampacity check when the installation runs hotter than the base condition.
General-purpose load path
- Non-continuous loads are sized at 100% of load current.
- Continuous loads are sized at 125% of load current.
- The result is rounded up to the next standard breaker size for review.
- The selected conductor is checked against a simplified ampacity review plus the small-conductor breaker limits used on common branch circuits.
Motor load path
Motor branch circuits are not checked the same way as ordinary branch circuits. This calculator uses a simplified inverse-time breaker path for motors and then performs a separate conductor ampacity review. That keeps the breaker screening from incorrectly treating the motor breaker rating as if it had to match the general-purpose conductor limit one-for-one.
NEC 240.6 reference after using the calculator
Use this table as a reference after the calculator has screened your load type and conductor basis. A standard rating by itself does not finish the branch-circuit review.
| Common standard sizes | Typical use |
|---|---|
| 10A, 15A, 20A, 25A, 30A | Small branch circuits, controls, receptacle and appliance circuits |
| 35A, 40A, 45A, 50A, 60A | Larger branch circuits and small feeders |
| 70A through 225A | Feeders, panelboards, and larger equipment circuits |
| 250A and above | Larger feeders, services, and industrial distribution |
For a field-reference version of the same standard-size sequence, open the Breaker Size Chart. After the breaker screen, continue to the Wire Size Calculator and Ampacity Calculator before treating the result as ready for installation.
Small-conductor breaker limits used in this calculator
| Copper conductor | General branch-circuit breaker limit | Common use |
|---|---|---|
| 14 AWG | 15A | Lighting and general 15A branch circuits |
| 12 AWG | 20A | 20A kitchen, bathroom, garage, and general-use circuits |
| 10 AWG | 30A | Dryers, water heaters, and similar 30A circuits |
A 12 AWG copper conductor may show more than 20A in a table column, but that does not automatically make it acceptable on a 25A general branch-circuit breaker. That is why this page treats the small-conductor rule as a separate check instead of relying only on ampacity numbers.
What this calculator does not model automatically
- Detailed motor overload sizing and controller coordination.
- Field verification of terminal temperature ratings and manufacturer markings.
- Every exception that can permit or restrict the next-higher standard breaker size in special circuit conditions.
- Interrupting-rating checks against available fault current, SCCR review, selective coordination, or permit-level engineering approval.
Technical notes. Open for formula basis, assumptions, and validation notes.
Why standard breaker size questions matter
Many searchers arrive here looking for a chart of standard breaker sizes, but the real field decision is always a combination of load type, the next standard size for review, and the conductor you intend to install. A standard size list by itself does not tell you whether the breaker screen is appropriate for a continuous load or whether the selected wire is still inside its general branch-circuit limit.
Why this page separates motor circuits from ordinary branch circuits
Motor branch circuits start differently, protect differently, and are reviewed under a different workflow than a normal lighting or receptacle circuit. That is why the calculator keeps motor inverse-time breaker sizing separate from the conductor ampacity review instead of forcing both through one general branch-circuit rule.
What to check after the breaker result
After the calculator identifies a standard breaker review point, continue through the conductor and system checks before treating the result as ready for the field. Use the Wire Size Calculator for the conductor gauge, the Ampacity Calculator when temperature or conductor count changes the ampacity, the Short Circuit Calculator for available-fault-current screening, and the Panel Load Calculator when the open question is spare capacity.
Common Applications
More applications. Open to review 1 additional use case.
Frequently Asked Questions
How should I use a standard breaker-size list?
Use the list only after you enter the load current, load type, selected conductor, ambient factor, and conductor-count adjustment. The calculator identifies the standard breaker review point that belongs to that input set, then the table serves as a reference.
How does the calculator screen a continuous load?
Choose the continuous-load path, enter the actual load current, and select the conductor you plan to use. The calculator applies the continuous-load multiplier before the standard breaker review and then keeps the conductor check visible.
How should I check a small copper conductor with a larger breaker?
Run the intended conductor through the calculator instead of reading a breaker rating from the chart alone. Small-conductor rules, terminal temperature, ambient correction, and conductor-count adjustment can stop an otherwise tempting breaker choice.
Why can a motor breaker look much larger than the motor conductor ampacity?
Motor branch-circuit short-circuit and ground-fault protection does not work like ordinary branch-circuit overcurrent protection. The breaker can be significantly larger than the conductor ampacity because overload protection is handled separately.
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