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Practice question · NEC 430.52
How High Can a Motor Breaker Go When It Won't Start?
A motor breaker that won't start the motor is not a problem you solve by feel. Article 430 gives you a starting percentage in Table 430.52, and when that breaker will not hold through starting, one named exception raises the ceiling. The question is whether you know where that ceiling stops.
Exam-style question (Sparky QOTD)
An inverse-time breaker sized at 250% of FLC will not let a motor start. The Code permits increasing it to a maximum of ___ of FLC.
A. 400%
B. 300%
C. 250%
D. 500%
Answer: A) 400%. Per 430.52(C)(1) Exception 2, where the Table 430.52 rating won't allow the motor to start, an inverse-time breaker may be increased up to 400% of FLC.
Where to Find It
Open your Code book to 430.52(C)(1) Exception 2 in Article 430, working from Table 430.52. The stem starts you at the table value: an inverse-time breaker sized at 250% of FLC. Exception 2 handles exactly the failure the stem describes — that rating won't allow the motor to start — and it sets the limit on your fix: the breaker may be increased up to 400% of FLC. Tab it in your book: Article 430, "430.52(C)(1) Ex. 2" — won't start? 400% breaker is the ceiling.
Why 400 Percent
Read the stem as a two-step story. Step one already happened: somebody sized the breaker at 250% of FLC off Table 430.52, and the motor will not start on it. Option C, 250%, is just that starting point restated. Picking it answers a question nobody asked — you already know that breaker fails.
Step two is the exception, and the exception has a hard stop. Exception 2 permits the increase only because the Table 430.52 rating won't allow the motor to start, and it caps the increase at 400% of FLC. That cap is the answer, because the stem asks for the maximum. Option B, 300%, stays under the ceiling but it is not the maximum the Code permits. Option D, 500%, blows straight through it. When a stem asks for a maximum, find the number the Code itself stops at.
Keep the two 430.52 exceptions straight in your tabs. Exception 1 handles a calculated value that is not a standard size — we drill that one in the 22A motor breaker question. Exception 2 is this one: the breaker is a standard size, it just won't start the motor, and the ceiling is 400%.
The Trap
The trap is treating the increase as open-ended. "The Code permits increasing it" sounds like permission to keep going until the motor starts. It is not. The permission and the limit live in the same sentence: increase it, but not past 400% of FLC. Every word after "maximum" in the stem is pointing at that limit.
Exam Tip
When a motor stem says the breaker "will not let a motor start," go straight to 430.52(C)(1) Exception 2 and look for 400% in the options. That phrase is the pointer, and 400% of FLC is the ceiling for an inverse-time breaker. Spot the pair and this is a fast, free point.
Keep Drilling
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