Sizing a motor branch circuit under NEC Article 430

Two currents: table FLC and nameplate

The most common error in motor work is using the wrong current. NEC 430.6(A)(1) says that conductors, switches, and branch-circuit short-circuit and ground-fault protection are sized from the full-load current in the NEC tables, not from the motor nameplate. Overload protection, on the other hand, is sized from the nameplate current (430.6(A)(2) and 430.32).

A few values from those tables: a 5 hp single-phase motor at 230 V is 28 A; a 10 hp three-phase motor at 460 V is 14 A; a 25 hp synchronous motor at 575 V and unity power factor is 21 A; a 7-1/2 hp two-phase motor at 460 V is 9 A.

Step 1: branch-circuit conductors (430.22)

Conductors supplying a single continuous-duty motor need an ampacity of at least 125% of the table full-load current.

Minimum ampacity = 1.25 × table FLC
10 hp, 460 V, three-phase: 1.25 × 14 A = 17.5 A

Then choose a conductor from Table 310.16 using the column that matches the terminal temperature rating of the equipment (60 °C unless the equipment is marked 75 °C, for circuits of 100 A or less), and apply any ambient or bundling correction before comparing.

Step 2: short-circuit and ground-fault protection (430.52)

The branch-circuit breaker or fuse protects against short circuits and ground faults only, so it is allowed to be far larger than the conductor ampacity; the overloads protect the motor and wire from overheating. Table 430.52 gives the maximum setting as a percentage of table FLC:

Design B energy-efficient motors allow 1,100% for instantaneous-trip breakers; wound-rotor and DC motors have much lower limits (150% for fuses and inverse-time breakers). Instantaneous-trip breakers are only permitted as part of a listed combination motor controller.

If the percentage does not land on a standard rating, Exception No. 1 permits the next higher standard size. 10 hp at 460 V: inverse-time breaker 14 × 2.5 = 35 A, a standard size, so 35 A; dual-element fuse 14 × 1.75 = 24.5 A, next standard size 25 A. Exception No. 2 allows more if the motor will not start, within stated ceilings.

Step 3: overload protection (430.32)

Separate overload devices for a continuous-duty motor over 1 hp are sized from the nameplate full-load current:

• Motors with a marked service factor of 1.15 or more, or a marked temperature rise of 40 °C or less: not more than 125% of nameplate current.
• All other motors: not more than 115%.

Example: nameplate 48 A, service factor 1.15: 48 × 1.25 = 60 A maximum. With a service factor of 1.0 the limit is 48 × 1.15 = 55.2 A. If the selected relay trips during normal starting, the code allows a higher setting up to 140% and 130% respectively.

Step 4: motors that do not run continuously (Table 430.22(E))

For motors on short-time, intermittent, periodic or varying duty, conductors are sized as a percentage of the nameplate current from Table 430.22(E):

Example: a 7.5 hp, 460 V, 15-minute rated motor on varying duty with a nameplate current of 11.1 A needs conductors of at least 11.1 × 1.20 = 13.32 A. The 125% rule of 430.22 does not apply to it.

Step 5: several motors on one conductor (430.24)

Conductors supplying several motors need an ampacity of at least 125% of the full-load current of the largest motor plus the full-load currents of all the others, plus any other loads on the same conductors.

Two motors with FLCs of 28.9 A and 47.8 A: 1.25 × 47.8 + 28.9 = 88.65 A. A 25 hp and a 10 hp motor at 460 V (34 A and 14 A): 1.25 × 34 + 14 = 56.5 A.

If two motors are the same size, only one of them is taken at 125%.

Reference: 460 V three-phase induction motors

The protective-device columns are maximums for the branch circuit. Smaller devices are fine as long as the motor starts and runs without nuisance tripping.

Common mistakes

• Sizing conductors or the breaker from the nameplate. Both use the table FLC; only overloads use the nameplate.
• Sizing the breaker at 125% like the wire. Table 430.52 allows up to 250% for an inverse-time breaker because the overloads do the overload job.
• Using the 230 V column for a 208 V system. Use the column for the actual system voltage; Table 430.250 has separate 200 V and 208 V columns.
• Applying 125% to every motor in a group. Only the largest motor is taken at 125% under 430.24.
• Using 430.22 for a 15-minute rated motor. Duty-cycle motors use Table 430.22(E) and the nameplate current.

Questions people ask