Breaker Size Calculator

A breaker size calculator converts entered branch-circuit or feeder load values into an adjusted load and compares that load with a Selected breaker rating. The result identifies the calculated current demand, the amperes remaining between that demand and the entered breaker rating, and the percentage of the selected rating being used.

For the values shown below, the calculator produces an adjusted load of 40 A. Compared with a selected 45 A breaker rating, the worksheet shows 5 A of rating margin and 88.8889% selected-rating utilization.

This calculation is useful during load review before completing related design and installation checks, including conductor ampacity selection, terminal rating review, voltage-drop evaluation, raceway fill, and equipment-specific overcurrent protection requirements.

Adjusted Load

Electrical loads are commonly separated into noncontinuous and continuous portions for calculation. The worksheet accepts each portion as amperes:

Input	Entered Value	Electrical Use
Noncontinuous load (A)	20 A	Load not treated as continuous by the reviewed method
Continuous load (A)	16 A	Load treated as continuous by the reviewed method
Continuous-load factor (%)	125%	Multiplier applied to the entered continuous load
Selected breaker rating (A)	45 A	Rating represented by the selected device record
Source reference	breaker-record-a	Record identifying the entered load and selected rating

The calculator applies the Continuous-load factor (%) only to the entered Continuous load (A). It then adds the factored continuous portion to the Noncontinuous load (A).

\(\displaystyle \text{Adjusted load} = \text{Noncontinuous load} + \left(\text{Continuous load} \times \frac{\text{Continuous-load factor}}{100}\right)\)

The worksheet does not choose a breaker size. The Selected breaker rating (A) is an entered value representing a device record already selected or being reviewed elsewhere.

Calculation Example

Using the entered values:

\(\displaystyle \text{Adjusted load} = 20\text{ A} + \left(16\text{ A} \times \frac{125}{100}\right)\)

\(\displaystyle \text{Adjusted load} = 20\text{ A} + 20\text{ A} = \mathbf{40\text{ A}}\)

The selected breaker comparison is then:

\(\displaystyle \text{Rating margin} = \text{Selected breaker rating} - \text{Adjusted load}\)

\(\displaystyle \text{Rating margin} = 45\text{ A} - 40\text{ A} = \mathbf{5\text{ A}}\)

Selected-rating utilization is:

\(\displaystyle \text{Selected rating utilization} = \frac{\text{Adjusted load}}{\text{Selected breaker rating}} \times 100\)

\(\displaystyle \text{Selected rating utilization} = \frac{40}{45} \times 100 = \mathbf{88.8889\%}\)

Result	Value	Meaning
Adjusted load	40 A	Noncontinuous load plus factored continuous load
Selected breaker rating	45 A	Entered rating represented by the selected device record
Rating margin	5 A	Difference between the selected rating and adjusted load
Selected rating utilization	88.8889%	Adjusted load as a percentage of the entered selected rating
Source note	Source recorded	A Source reference was entered

Breaker Rating Review

The adjusted-load result can be carried into a branch-circuit or feeder review alongside conductor ampacity. A conductor selection often requires evaluation of conductor AWG or kcmil, insulation temperature rating, equipment terminal rating, ambient-temperature correction factor, and adjustment factor for current-carrying conductors.

A 40 A adjusted load does not by itself establish that any particular conductor, breaker, panelboard, disconnect, or equipment assembly is acceptable. The entered 45 A value is used solely for the arithmetic comparison shown on the worksheet.

The rating margin is positive when the entered selected rating exceeds the adjusted load, zero when the two values are equal, and negative when the adjusted load exceeds the entered selected rating. Utilization expresses the same comparison as a percentage and can help identify records requiring closer review.

Related Installation Checks

The calculated adjusted load may be used as an input to other electrical design work:

• Conductor ampacity: Verify the selected conductor ampacity under the applicable installation conditions, including insulation temperature rating and termination limitations.
• Voltage drop: Calculate voltage drop separately using conductor length, conductor material, AWG or kcmil, circuit configuration, and actual load current.
• Raceway fill: Size conduit or other raceway from conductor dimensions, conductor count, and the installed raceway method; breaker rating does not determine fill by itself.
• Current-carrying conductors: Apply any required adjustment factor separately based on the installed conductor arrangement and conditions of use.
• Equipment protection: Confirm the selected overcurrent protective device is suitable for the equipment and circuit arrangement, including any applicable motor, transformer, or equipment-specific protection rules.
• Available fault current and interrupting rating: Confirm the protective device’s interrupting rating and the available fault current at its installation point through a separate short-circuit and equipment evaluation.

Field Verification

This is an entered-value worksheet. It records Noncontinuous load (A), Continuous load (A), Continuous-load factor (%), Selected breaker rating (A), and Source reference, then calculates the resulting load comparison.

It does not select a device or interpret a device table. It does not determine final overcurrent protection, conductor ampacity, interrupting rating, coordination, equipment listing, terminal limitations, or installation compliance. Final circuit and equipment decisions require the applicable project documents, manufacturer instructions, field conditions, and AHJ requirements.

For load treatment and protection context, compare this worksheet with the Continuous Load Calculator and the Breaker Interrupting Rating Calculator.