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Breaker Size Calculator

Audit one documented protection-selection record from source-labelled current components, an explicit multiplier, an already selected device rating, and a documented conductor ampacity—without choosing a breaker or declaring compliance.

Use this result well

Inputs that matter
One de-identified reviewed protection calculation; named load and multiplier sources; an already selected device record; a final conductor-ampacity record; and the copied current components, multiplier, adder, device rating and ampacity
Output to expect
Source-preserving documented load arithmetic plus signed differences to the entered selected-device rating and entered conductor ampacity
How it works
Add noncontinuous current to continuous current × only the entered multiplier and any separate entered adder, then subtract against the two entered records; do not choose a breaker, fuse, standard rating, next-size exception, conductor or compliance verdict
  • Confirm simultaneous-load classification, exact continuous-load treatment, any listed 100%-rated assembly, equipment-specific rules and every current component against the responsible calculation and adopted requirements.
  • Verify the complete conductor ampacity calculation, terminations, adjustments/corrections, device type/characteristics, interrupting and fault-current duties, poles/voltage, equipment protection, listing, manufacturer instructions and AHJ acceptance separately.

Choose your path

Built around the job you need to finish

Reproduce one source-labelled protection-selection record and compare its entered load arithmetic with an already selected device and documented conductor ampacity without choosing equipment or declaring compliance.

Electrical designer checking a protection schedule

Find transcription or arithmetic differences without a generic calculator replacing the responsible selection method.

Name the load, factor, selected-device and conductor records; copy each entered value; inspect the arithmetic and two signed differences.

Can trace every number while device selection and final coordination remain in the complete design.

Plan reviewer checking provenance

Distinguish a documented record from a universal 125% rule, standard-size lookup or AWG shortcut.

Inspect the basis/source cards and verify that the selected rating and final allowable ampacity came from the submitted calculation.

No standard size, next-size exception, conductor or compliance claim is generated.

Apprentice auditing an education exercise

See how an explicitly supplied continuous multiplier changes only the entered continuous component.

Run separate records with different copied multipliers and interpret positive or negative differences as arithmetic only.

Can reproduce the math and identify every classification, listing, equipment and authority decision the tool omits.

Authoritative checks for this tool

Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.

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Reference & details

How it works

Keep Every Decision With Its Source

The form requires four separate records because load classification, multiplier treatment, equipment selection and conductor allowable ampacity are not interchangeable. Use de-identified labels and copy only values already established by the responsible documents.

Record = load source + factor source + selected-device source + conductor source

Reproduce Entered Load Arithmetic

Only the continuous component receives the percentage copied from the named source. The calculator does not assume 125%, infer a 100%-rated assembly, classify a load or create an equipment-specific adder.

Load arithmetic = noncontinuous A + continuous A × entered % + other entered A

Read Differences as Arithmetic Only

A positive, zero or negative difference is not a pass/fail verdict. Standard ratings, conditional next-higher provisions, conductor limitations, equipment protection and device characteristics require the complete adopted method and responsible review.

Device difference = entered device A − load arithmetic A; conductor difference = entered ampacity A − entered device A

Updated: August 2026

Example Scenarios

A designer copies the noncontinuous and continuous currents, exact multiplier, separate adder, already selected device rating and final allowable ampacity from a reviewed calculation. The result exposes any arithmetic difference without replacing the design.

A reviewer uses the multiplier and selected rating stated by the controlling worksheet rather than forcing a generic factor. Product listing, equipment limits and final coordination remain in that source and the adopted requirements.

An apprentice enters a de-identified exercise whose selected-device rating is below the reproduced load arithmetic or above the entered conductor ampacity. The negative value is a question to reconcile with the instructor, not an automatic unsafe or noncompliant judgment.

FAQ

No. Enter the rating of a device already selected by the responsible calculation. The tool neither rounds to a standard rating nor selects a breaker, fuse, conductor or exception.

The applicable treatment depends on the controlling calculation, adopted requirements, equipment and listings. Copy the exact percentage from the named source instead of accepting a hidden universal factor.

Only a separate current component explicitly required by the named calculation. Enter zero when the source has no separate adder; do not use it as an unlabeled contingency or guessed demand.

No. Both outputs are signed arithmetic only. They do not verify conductor conditions, device characteristics, equipment protection, listing, interrupting duty, fault current, voltage/poles, special loads or code compliance.

Copy the final allowable ampacity from the complete named conductor calculation after every applicable material, insulation, termination, temperature, correction, adjustment and protection limitation. Do not substitute a generic wire-size table.

About Breaker Size Calculator

Reproduce the arithmetic in one reviewed protection-selection record. Name the source for the load classification, continuous multiplier, already selected device and final conductor ampacity; then compare the entered records with transparent signed differences. This tool does not supply a standard breaker size, conductor lookup or approval decision.