Motor Branch Circuit Protection Calculator

A motor branch circuit protection calculation starts with the reviewed motor full-load current and the protection factor used in the selected protection method. The worksheet produces an entered reference protection value in amperes, then compares that value with the Selected protection rating recorded for the circuit.

The result supports documentation and design review for motor branch circuits. It can be used alongside conductor ampacity calculations, branch-circuit layout, feeder load review, available-fault-current review, and motor control equipment selection. It does not select a fuse or circuit breaker and does not establish the final overcurrent protective device for a motor circuit.

The calculator records four input values:

Input	Electrical purpose
Motor FLA (A)	The reviewed motor full-load current used as the calculation current.
Entered protection factor (%)	The percentage factor from the reviewed motor protection method.
Selected protection rating (A)	The ampere rating represented by the selected protective-device record.
Source reference	The record identifying the source for the FLA, factor, and selected rating.

Reference Protection Calculation

The calculator applies the entered protection factor directly to Motor FLA (A):

\(\displaystyle \text{Entered reference protection} = \frac{\text{Motor FLA} \times \text{Entered protection factor}}{100}\)

The entered reference protection is a comparison value. It represents the ampere result produced by the motor FLA and the reviewed percentage factor, without determining whether a particular protective device is permitted, properly listed, or suitable for the installation.

The calculator then calculates the difference between the selected device record and the entered reference protection:

\(\displaystyle \text{Protection margin} = \text{Selected protection rating} - \text{Entered reference protection}\)

\(\displaystyle \text{Protection margin percent} = \frac{\text{Protection margin}} {\text{Entered reference protection}} \times 100\)

A positive margin means the entered selected rating is above the calculated reference value. A zero margin means the selected rating equals the entered reference protection. A negative margin means the selected rating is below that reference value.

Calculation Example

Assume the reviewed motor protection record provides the following values:

Field	Entered value
Motor FLA (A)	28 A
Entered protection factor (%)	250%
Selected protection rating (A)	70 A
Source reference	motor-protection-record-a

First, calculate the entered reference protection:

\(\displaystyle 28\text{ A} \times \frac{250}{100} = \text{70 A}\)

Then compare the selected protection rating:

\(\displaystyle 70\text{ A} - 70\text{ A} = \text{0 A protection margin}\)

\(\displaystyle \frac{0\text{ A}}{70\text{ A}} \times 100 = \text{0\% protection margin}\)

The worksheet result is:

Result	Value
Entered reference protection	70 A
Selected protection rating	70 A
Protection margin	0 A
Protection margin	0%
Record comparison	Source recorded

This result documents that the selected protection rating in the referenced record matches the arithmetic value generated from 28 A Motor FLA and a 250% entered protection factor.

Motor Circuit Coordination With Other Work

Motor branch-circuit protection is not the same calculation as branch-circuit conductor ampacity. A protective-device rating may be associated with motor starting and short-circuit or ground-fault protection, while conductor sizing requires its own review of conductor ampacity, AWG or kcmil size, insulation temperature rating, terminal rating, correction factor, adjustment factor, and current-carrying conductors.

The recorded protection value can be carried into related electrical checks, including:

• Branch-circuit conductor sizing and termination review.
• Raceway fill and conduit layout when conductors and equipment grounding conductors are identified.
• Motor control equipment review, including disconnecting means, starters, drives, and listed overcurrent protection components.
• Feeder load calculations where multiple motors and nonmotor loads are combined.
• Voltage-drop review for long motor branch circuits and feeders.
• Short-circuit and ground-fault protection review using available fault current and the protective device’s interrupting rating.
• Selective coordination or coordination review where the motor circuit interacts with upstream protective devices.

A 70 A selected protection rating does not establish that a conductor has 70 A ampacity, that a 70 A device has adequate interrupting capacity, or that the motor circuit will coordinate with upstream equipment. Those are separate design and field-verification decisions.

Field Verification

Use the Source reference field to identify the reviewed basis for all three electrical inputs: motor FLA, protection factor, and selected protection rating. The source may be a motor schedule, approved equipment submittal, one-line diagram, panel schedule, manufacturer documentation, engineered calculation, or another controlled project record.

Verify separately that the entered Motor FLA is the correct reviewed value for the specific motor and supply conditions. Nameplate data, motor controller information, voltage, phase, motor duty, and equipment listing can affect the applicable design basis. Do not substitute a running-current measurement for the documented FLA unless the project’s reviewed method specifically calls for it.

The worksheet performs arithmetic only. It does not use a protective-device table; determine device type; evaluate a fuse or circuit breaker’s interrupting rating; evaluate available fault current; verify coordination; verify equipment listing; apply conductor ampacity rules; or issue a final motor protection decision. The final installation must be verified against the applicable electrical code, manufacturer instructions, construction documents, and AHJ requirements.