Motor circuits follow Article 430, which uses separate rules for conductors, short-circuit and ground-fault protection, and overload protection. The key idea is that conductors and short-circuit devices use table values, while overloads use the nameplate.
Step 1: Full-load current from the table
Use NEC Table 430.248 for single-phase motors and Table 430.250 for three-phase motors. A 10 HP, 460 V, three-phase motor has a table full-load current of 14 A.
Step 2: Branch-circuit conductors
Per 430.22, size the conductors at 125% of the table value: 14 × 1.25 = 17.5 A. Using the 75°C column, 12 AWG copper (25 A) is enough. Still check voltage drop and terminals.
Step 3: Short-circuit and ground-fault protection
Table 430.52 allows an inverse-time breaker up to 250% of full-load current and a dual-element time-delay fuse up to 175%. For this motor:
- Breaker: 14 × 2.5 = 35 A
- Dual-element fuse: 14 × 1.75 = 24.5 A, rounded to the next standard size, 25 A
If the motor will not start, 430.52(C)(1) exceptions allow higher ratings in some cases.
Step 4: Overload protection
Overloads use the nameplate current. Under 430.32 the trip setting is generally 125% for motors with a 1.15 service factor or 40°C temperature rise or less, and 115% for others. With a 13.2 A nameplate, the 125% setting is 16.5 A.
Step 5: Disconnect and controller
The motor disconnecting means and controller must be rated for the horsepower, and the disconnect ampere rating must be at least 115% of the table full-load current (430.110).
Common mistakes
- Using nameplate current for conductors and breakers.
- Treating the maximum breaker rating as a required size.
- Ignoring variable frequency drives, which follow Part X of Article 430.
Try the calculators
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