Motor calculations (Article 430)
Motor questions are the largest block in the Texas Calculations portion. Every motor calculation starts with the right current: the table full-load current for conductors and short-circuit protection, and the nameplate current for overloads.
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free motors, generators and controls drillWhich current to use
- Conductors, short-circuit and ground-fault protection, and disconnects: the full-load current from Tables 430.247–430.250, not the nameplate (430.6(A)(1)).
- Overload protection: the motor nameplate full-load current (430.6(A)(2)).
The rules in order
| Item | Rule | Section |
|---|---|---|
| Branch-circuit conductors | 125% of table FLC | 430.22 |
| Overloads | 125% of nameplate (service factor 1.15 or more, or 40°C rise); 115% otherwise | 430.32(A)(1) |
| Inverse-time breaker | Up to 250% of table FLC, next standard size up permitted | 430.52(C)(1) |
| Dual-element fuse | Up to 175% of table FLC, next standard size up permitted | 430.52(C)(1) |
| Disconnect (rated in amperes) | At least 115% of table FLC | 430.110(A) |
Worked example: one 40 hp motor
40 hp, 460 V, three-phase squirrel-cage motor; nameplate 50 A, service factor 1.15
- Table 430.250 full-load current: 52 A.
- Conductors: 52 × 125% = 65 A → 6 AWG copper in the 75°C column.
- Inverse-time breaker: 52 × 250% = 130 A → next standard size 150 A.
- Overloads: nameplate 50 × 125% = 62.5 A.
Practice motor calculations on a timed exam
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free motors, generators and controls drillSeveral motors on one feeder
Feeder conductors carry 125% of the largest motor’s full-load current plus the sum of the others (430.24). The feeder protective device may not exceed the largest branch-circuit device rating plus the sum of the other motors’ full-load currents, and there is no next-size-up allowance: round down to a standard size (430.62(A)).
10, 15 and 25 hp motors at 460 V
- Full-load currents: 14 A, 21 A and 34 A.
- Feeder conductors: 34 × 125% + 14 + 21 = 77.5 A.
- Largest branch breaker: 34 × 250% = 85 A → 90 A. Feeder breaker: 90 + 14 + 21 = 125 A, a standard size.
Common questions
Do I use the nameplate or the table current for motor conductors?
The table: Tables 430.247 through 430.250 for conductors and short-circuit protection. The nameplate current is for overload protection (430.6(A)).
Can I round a motor feeder breaker up?
No. Unlike the branch-circuit device, the feeder device under 430.62(A) must not exceed the calculated value, so you go down to the next standard size.
What is the overload setting for a motor with a 1.15 service factor?
125% of the nameplate full-load current (430.32(A)(1)).
Practice motor calculations on a timed exam
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free motors, generators and controls drillMore master exam guides
- Master electrician load calculations
- Services, feeders and transformers
- Grounding and bonding for the master exam
- Special occupancies and emergency systems
- Hazardous (classified) locations
- Master electrician practice test
- ICC Master Electrician Practice Test (R16)
- Texas Master Electrician Practice Test (TDLR)
Section numbers follow the 2023 National Electrical Code (NFPA 70); rules are paraphrased, not quoted. Independent preparation, not affiliated with NFPA, ICC, PSI, Pearson VUE or any state board.