Motor calculations
Every motor problem starts with the full-load current from the tables in Article 430, not from the nameplate. From that one number you size the conductors at 125% and the branch-circuit protective device at the percentage Table 430.52 gives for the type of device.
The method
- Find the full-load current in Table 430.248 for a single-phase motor or Table 430.250 for a three-phase motor.
- Branch-circuit conductors: at least 125% of that table current, under 430.22.
- Short-circuit and ground-fault protection: the table current times the percentage in Table 430.52 for the device, a fuse or a breaker.
- If the result is not a standard rating, 430.52 allows the next standard size up. The standard ratings are listed in 240.6(A).
- Overload protection is the one calculation that uses the nameplate current.
Two mistakes cost most of the points: reading the nameplate where the table is required, and using the conductor percentage for the protective device.
Worked problems
Problem 1. A 100 HP, 460-volt three-phase squirrel-cage motor is protected by a dual-element (time-delay) fuse. What is the maximum standard rating the Code permits for the branch-circuit short-circuit and ground-fault protective device?
Answer: 225 A
Table 430.250 gives 124 A full-load current. Table 430.52 permits up to 175% for a dual-element (time-delay) fuse: 124 x 1.75 = 217.0 A. Since that is not a standard rating, 430.52(C)(1) Exception No. 1 permits going up to the next standard size: 225 A. Do not confuse this with the conductor, sized at 125% = 155.0 A: they are two different calculations on the same motor.
Reference: NEC 430.52 and Table 430.250
Problem 2. A 60 HP, 230-volt three-phase squirrel-cage motor is protected by a dual-element (time-delay) fuse. What is the maximum standard rating the Code permits for the branch-circuit short-circuit and ground-fault protective device?
Answer: 300 A
Table 430.250 gives 154 A full-load current. Table 430.52 permits up to 175% for a dual-element (time-delay) fuse: 154 x 1.75 = 269.5 A. Since that is not a standard rating, 430.52(C)(1) Exception No. 1 permits going up to the next standard size: 300 A. Do not confuse this with the conductor, sized at 125% = 192.5 A: they are two different calculations on the same motor.
Reference: NEC 430.52 and Table 430.250
Problem 3. A 2 HP, 460-volt three-phase squirrel-cage motor is protected by a dual-element (time-delay) fuse. What is the maximum standard rating the Code permits for the branch-circuit short-circuit and ground-fault protective device?
Answer: 15 A
Table 430.250 gives 3.4 A full-load current. Table 430.52 permits up to 175% for a dual-element (time-delay) fuse: 3.4 x 1.75 = 6.0 A. Since that is not a standard rating, 430.52(C)(1) Exception No. 1 permits going up to the next standard size: 15 A. Do not confuse this with the conductor, sized at 125% = 4.2 A: they are two different calculations on the same motor.
Reference: NEC 430.52 and Table 430.250
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Common questions
Do I use the nameplate current to size motor conductors?
No. Use the full-load current from Table 430.248 or Table 430.250 and size the conductors at 125% of it under 430.22. The nameplate current is for overload protection.
Can I round up the motor breaker size?
When the calculated value is not a standard rating, 430.52 permits the next higher standard size listed in 240.6(A).