How to Size a Motor Circuit: Fuse, Breaker, Overload and Wire (NEC)

A motor circuit is more than a wire and a breaker. This guide walks through each part, what sets its size and how the NEC rules fit together, with a worked example.

This is a learning guide. It describes common NEC rules for a standard motor. Real designs involve many details that vary by case, edition and local amendment. Always design from the code edition that applies where the work is done, and have a qualified person review it.

The parts of a motor branch circuit

Part	What it does	What sizes it
Branch-circuit short-circuit and ground-fault protection (fuse or breaker)	Opens quickly in a short circuit or ground fault	A percentage of the table full-load current (Table 430.52)
Overload protection	Opens when the motor draws too much current for too long	The motor nameplate current (430.32)
Conductors	Carry the motor current	125% of the table full-load current (430.22)
Disconnecting means	Isolates the motor and controller for work	At least 115% of the table full-load current or the horsepower rating (430.110)
Motor controller (starter or drive)	Starts and stops the motor	Horsepower and current rating for the motor, and a listing that suits the protective device

Two currents: table and nameplate

The NEC uses two different values on purpose. The table full-load current (Table 430.250 for three-phase motors) is used for the conductors, the short-circuit protection and the disconnect. The nameplate current is used for the overload. Using the table for the first three means every motor of a given horsepower and voltage is treated the same, however efficient it is.

Worked example: 10 hp, 460 V, three-phase

Table full-load current. Table 430.250 gives 14 A.
Short-circuit protection. With a dual-element (time-delay) fuse, the limit is 175% of 14 A = 24.5 A. That is not a standard size, so the NEC allows the next one up, a 25 A fuse. With an inverse-time breaker, the limit is 250%, which is 35 A, a standard size.
If the motor will not start. The NEC permits larger devices up to a stated maximum, for example 225% for a dual-element fuse. Do this only when needed, and note that a larger device gives less protection.
Overload. Say the nameplate shows 12.8 A with a service factor of 1.15. The overload trip setting is at most 125% of 12.8 A = 16 A.
Conductors. 125% of 14 A = 17.5 A minimum ampacity. A 14 AWG copper conductor in the 75 °C column of Table 310.16 is rated 20 A, so it meets the ampacity, but only if the equipment terminals are rated for 75 °C (see NEC 110.14(C); in the 60 °C column 14 AWG is only 15 A). Many designers choose a larger size anyway for voltage drop and mechanical strength.
Disconnect. At least 115% of 14 A = 16.1 A, so a standard 30 A disconnect would be an ordinary choice.

The breaker and fuse sizing calculator does steps 1 to 5 and shows the rule for each. The motor current calculator gives the table value.

Things that change the answer

The type of motor. Table 430.52 differs for wound-rotor and DC motors, and some high-efficiency designs have their own percentages.
Drives and soft starters. Equipment like variable frequency drives often has its own required fuse or breaker type and size in the manufacturer's instructions. Those listing instructions take priority.
Combination starters. A listed combination motor controller may use an instantaneous-trip breaker, and its listing sets the limits.
Several motors on one feeder. Feeder sizing follows its own rules (430.24 and 430.62).
Temperature and bundling. Conductors in hot places or in large bundles need derating.
Fault current. The device must be rated for the fault current available at its location. See fault current and interrupting ratings.

This is general guidance for training and reference and not a substitute for the electrical code or a qualified engineer.