NEC 430.52: Sizing the Motor Breaker, and Why You Round Up
In our diagnostic data this is the most-answered hard question we have: fewer than half of candidates size a motor breaker correctly. The reasons are always the same two. They apply 125% like a conductor rule, or they refuse to round up because every other rule they know rounds down. Here is the rule, the why, and the worked math.
Last reviewed September 2026
Why 250% is legal
A motor is the one load that briefly outdraws its own rating every time it starts. Inrush runs on the order of six times full-load current for a second or two. A breaker sized at 125% would see that inrush as a fault and trip on every start, so the code splits the protection job in two. The branch-circuit device under 430.52 is sized high on purpose and catches only short circuits and ground faults. The overload device under 430.32 is sized tight, at roughly 115% to 125% of nameplate, and it is what actually protects the motor from cooking.
The percentages that get tested
- Inverse-time circuit breaker: 250% of table FLC. The everyday answer.
- Dual-element (time-delay) fuse: 175%.
- Nontime-delay fuse: 300%.
- All from Table 430.52, applied to the FLC from the 430.248/430.250 tables, never the nameplate.
The percentage depends entirely on the device named in the question. Exam writers know candidates memorize one number, so they rotate the device type. Read which device is being sized before touching the calculator.
Worked example: the round-up
A 3-phase motor has a table full-load current of 28 amps. Size the inverse-time breaker.
- Table 430.52 for an inverse-time breaker: 250%.
- 28 A x 2.50 = 70 A.
- 70 is a standard rating in 240.6(A), so a 70-amp breaker.
- Now try FLC = 30 A: 30 x 2.50 = 75 A. Not a standard rating.
- Exception 1 permits the next higher standard size: 80 amps.
That second case is where the points die. Candidates trained on conductor rules pick 70 because rounding down feels safe. For this one device, the code says go up, because a breaker that nuisance trips on starting protects nobody.
Where candidates lose the point
- Applying 125% because every other sizing rule uses it. That is the conductor rule (430.22), not the breaker rule.
- Rounding down to a standard size. Exception 1 to 430.52(C)(1) permits the next size UP.
- Using the nameplate current. Branch-circuit calculations run on the 430.248/430.250 table values.
- Using 250% for a fuse. Fuses have their own columns: 175% time-delay, 300% nontime-delay.
- Panicking when the answer looks huge next to the wire. A 10 AWG conductor behind a 70-amp breaker is a correct motor circuit.
The four numbers of every motor circuit
Every motor question is one of four calculations, and each has its own rule: the conductor at 125% of table FLC (430.22), the overload at 115% to 125% of nameplate (430.32), the branch-circuit device from Table 430.52, and the disconnect at 115% (430.110). Four numbers, four jobs, four different sections. The exam's favorite trick is asking for one and offering the other three as wrong answers.
Find out if motor calculations are costing you points
The free 15-minute diagnostic scores Motors and Generators separately. In our data it is the worst section on the exam, and most candidates have no idea it is theirs.