NEC Table 430.250: motor full-load current
Motor questions follow the same order on every exam: look up the table current, size the branch-circuit conductors, then the short-circuit and ground-fault protection, then the overloads. Only the overloads use the nameplate current.
A timed 80-question exam in the ICC R17 format on the 2023 NEC, or your state’s exact format. Every answer has a worked explanation.
Or start with the free motors and transformers drillThree-phase full-load current: Table 430.250
| Horsepower | 208 V | 230 V | 460 V |
|---|---|---|---|
| 5 | 16.7 | 15.2 | 7.6 |
| 7-1/2 | 24.2 | 22 | 11 |
| 10 | 30.8 | 28 | 14 |
| 15 | 46.2 | 42 | 21 |
| 20 | 59.4 | 54 | 27 |
| 25 | 74.8 | 68 | 34 |
| 30 | 88 | 80 | 40 |
| 40 | — | — | 52 |
| 50 | — | — | 65 |
The four sizing steps
- Use the table current, not the nameplate, for conductors and short-circuit protection (430.6(A)(1)).
- Branch-circuit conductors: at least 125% of the table current (430.22).
- Short-circuit and ground-fault protection: Table 430.52 percentages, for example 250% for an inverse-time breaker and 175% for a dual-element time-delay fuse. When the result is not a standard size, the next larger standard size is permitted (430.52(C)(1) Exception No. 1, 240.6(A)).
- Overload protection: 125% of the nameplate current for motors with a service factor of 1.15 or more or a temperature rise of 40°C or less; 115% for all other motors (430.32(A)(1)).
Worked example
20 hp, 460 V, three-phase motor on an inverse-time breaker
- Table 430.250: 27 A.
- Conductors: 27 × 1.25 = 33.75 A. 10 AWG copper at 75°C is rated 35 A (Table 310.16), so 10 AWG.
- Breaker: 27 × 2.50 = 67.5 A. The next standard size is 70 A.
- Overload with a 25 A nameplate and a 1.15 service factor: 25 × 1.25 = 31.25 A.
Practice motor calculations on a timed exam
A timed 80-question exam in the ICC R17 format on the 2023 NEC, or your state’s exact format. Every answer has a worked explanation.
Or start with the free motors and transformers drillSeveral motors on one feeder
Feeder conductors for several motors must carry 125% of the largest motor full-load current plus the sum of the other motors (430.24). For three 460 V motors of 10, 15 and 20 hp: 27 × 1.25 + 21 + 14 = 68.75 A.
Common questions
Do I use the nameplate or Table 430.250 current?
Use Table 430.250 for conductors and short-circuit protection, and the nameplate current for overload protection (430.6(A)).
What size breaker for a motor?
Multiply the table current by 250% for an inverse-time breaker (Table 430.52). If the result is not a standard size, you may use the next larger standard size.
What is the full-load current of a 10 hp 460 V three-phase motor?
14 amperes (Table 430.250).
Practice motor calculations on a timed exam
A timed 80-question exam in the ICC R17 format on the 2023 NEC, or your state’s exact format. Every answer has a worked explanation.
Or start with the free motors and transformers drillMore journeyman exam guides
Section numbers follow the 2023 National Electrical Code (NFPA 70). Independent preparation, not affiliated with NFPA, ICC, PSI, Prometric, Pearson VUE or any state board.