Motor Branch Circuit Protection Calculator

What this motor branch protection calculator does

This page screens motor branch-circuit short-circuit and ground-fault protection. It does not size motor overload relays, does not prove selective coordination, and does not replace the equipment installation instructions. The output is meant to help an electrician or designer organize the NEC Article 430 sizing check before confirming the adopted code edition, AHJ requirements, equipment listing, interrupting rating, and SCCR.

Branch protection is not overload protection

Motor branch short-circuit and ground-fault protection clears faults on the circuit. Running overload protection is a separate task and is normally based on the actual installed motor and overload device. This distinction matters because branch protective devices can be much larger than the motor running current, while overload protection must track the motor thermal limit.

Core calculation

The calculator applies this screening relationship:

Calculated maximum = motor FLC x Table 430.52 percentage

It then shows two standard ampere values:

• Standard size at or below calculation: the largest common standard ampere rating that does not exceed the calculated ampere value.
• First standard size for code review: the next common standard rating when the calculated value falls between standard sizes. Whether that rating is acceptable depends on the adopted NEC standard-size rule, the selected device, and the actual installation.

Common percentage screen

Use the row that matches the adopted NEC edition and the actual motor. If the motor design row, controller listing, or drive manual gives a tighter limit, use the tighter requirement.

Worked example

A 50 HP, 460 V, three-phase squirrel-cage motor is being screened with 65 A as the motor FLC basis and a dual-element time-delay fuse.

• Table percentage: 175%
• Calculated maximum: 65 A x 1.75 = 113.75 A
• Largest common standard size at or below calculation: 110 A
• First standard-size review point: 125 A
• Minimum branch conductor ampacity screen: 65 A x 1.25 = 81.25 A before derating and terminal checks

The calculator intentionally presents the standard-size review separately instead of hiding it as a single recommendation. The final installed device must be checked against the adopted NEC, the equipment instructions, the motor starter or drive listing, available fault current, SCCR, and the AHJ.

VFD and soft-starter note

Starting equipment can change the current seen during acceleration, but it does not automatically change the NEC branch-device percentage used by this screen. For VFDs and other power-conversion equipment, the marked or instructed maximum input protective-device rating can be the controlling limit. Use the drive or starter manual before buying fuses, breakers, or MCPs.