Practice: motor short-circuit protection
Each problem uses new numbers, so you practice the method rather than memorizing an answer.
Pick the answer: sizes and ratings come from your code book.
Worked solution
Formula
| Symbol | Meaning | Unit |
|---|---|---|
| FLC | Table full-load current, Table 430.248 or 430.250 | A |
| % | Table 430.52(C)(1) maximum percent for the device type | % |
| Rating | Standard fuse or breaker rating, 240.6(A) | A |
Worked example
A 20 hp, 230 V three-phase squirrel-cage motor has a nameplate current of 50.8 A. Its branch-circuit short-circuit and ground-fault protection will be an inverse time circuit breaker. Using Table 430.52(C)(1), and the next higher standard rating that 430.52(C)(1) Exception No. 1 allows when the result is not a standard size, what is the largest rating permitted?
- Look up Table 430.250, 230 V column, 20 hp: 54.0 A. Use the table, not the nameplate (430.6(A)(1)).
- Look up Table 430.52(C)(1), inverse time breaker column: 250% of full-load current.
- 54.0 × 250% = 135.00 A.
- 135.00 A is not a standard rating, so Exception No. 1 lets you go up to the next standard size in 240.6(A).
- Largest rating: 150 A.
Answer: 150 A
Motor short-circuit protection, explained
A motor draws several times its running current while it starts. If the branch-circuit breaker or fuse were sized like a normal circuit, it would trip every start. So Article 430 splits the job: overloads (430.32) protect against long overloads, and the branch-circuit short-circuit and ground-fault device (430.52) only has to clear faults. That device can be much larger than the full-load current.
Table 430.52(C)(1) gives the maximum as a percent of the table full-load current for each device type. For ordinary single-phase and squirrel-cage motors it is 300% for a non-time-delay fuse, 175% for a dual-element (time-delay) fuse and 250% for an inverse time breaker. Multiply the table full-load current by that percent.
If the answer is not a standard rating from 240.6(A), Exception No. 1 to 430.52(C)(1) allows the next standard size up. Exception No. 2 allows a larger device again if the motor still will not start, up to set limits. On the exam, use an exception only when the question says to. The 2026 NEC also adds Design BE and CE motors to the table; their fuse and inverse time breaker percentages match the ones here.
How to solve it on the exam
- Look up the full-load current in Table 430.248 or 430.250, not the nameplate.
- Find the device type in Table 430.52(C)(1) and read its percent for the motor type.
- Multiply the full-load current by that percent.
- If the question allows Exception No. 1 and the result is not a standard size, go up to the next standard rating in 240.6(A).
Common mistakes
- Rounding down to the next standard size, as for a feeder, when Exception No. 1 allows rounding up.
- Using the nameplate current instead of the table full-load current.
- Using the percent for the wrong device type, such as 250% for a dual-element fuse.
- Sizing the breaker at 125% like the conductors.
- Applying Exception No. 2 when the question does not say the motor fails to start.
Exam tips
- Tab Table 430.52(C)(1) and 240.6(A) together. Every motor breaker question uses both.
- Read the device type twice. "Fuse", "time-delay fuse" and "inverse time breaker" each have their own column.
Questions people ask
How do you size a breaker for a motor?
Multiply the motor's table full-load current by 250% for an inverse time breaker (Table 430.52(C)(1)). If the result is not a standard size, Exception No. 1 to 430.52(C)(1) allows the next standard size up.
What size dual-element fuse for a motor?
Use up to 175% of the table full-load current for a dual-element (time-delay) fuse, then the next standard size up if the result is not standard.
Why can a motor breaker be larger than the wire ampacity?
Because the branch-circuit breaker only protects against short circuits and ground faults. The overload relay protects the motor and conductors from overload, so the breaker can be large enough to let the motor start.
Can I go higher if the motor will not start?
Yes, Exception No. 2 to 430.52(C)(1) allows a larger device when the motor will not start, up to set maximum percentages. On the exam, apply it only when the question says so.
Sources
- TDLR: Electrician examination information
- PSI: Texas electrician candidate information bulletin
- IEC: 2026 NEC changes to Chapter 4
About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.