Motor Overload Sizing: How NEC 430.32 Actually Works

The trap: two currents, one wrong grab

Almost every rule in Article 430 wants the table FLC. NEC 430.6(A)(1) sends you to Tables 430.248 and 430.250 for conductor sizing, switch ratings, and branch-circuit protection, no matter what the nameplate says. Then 430.6(A)(2) flips it for one job: separate overload protection is based on the nameplate current rating.

That flip is the whole exam question. The writers give you a motor with a nameplate FLA of 25A and a table FLC of 28A, then put both answers in the choices. Multiply the wrong current and your arithmetic is perfect and your answer is wrong. Before you touch the calculator, say it out loud: overloads read the nameplate.

The 430.32(A)(1) percentages

For a continuous-duty motor rated more than 1 horsepower, 430.32(A)(1) caps the overload device at a percentage of the nameplate FLA. Three buckets:

• Service factor 1.15 or more (marked on the nameplate): 125% of nameplate FLA.
• Temperature rise 40 degrees C or less (marked on the nameplate): 125% of nameplate FLA.
• All other motors: 115% of nameplate FLA.

The motor only needs to hit one of the two conditions to earn 125%. A motor with SF 1.15 and no marked temperature rise gets 125%. A motor with a 40 degree C rise and SF 1.0 gets 125%. A motor with neither marking gets 115%. Read the nameplate data in the question stem carefully. The exam loves to hand you a motor with no service factor listed and watch you default to 125% out of habit.

The 430.32(C) bump: when the motor will not start

An overload sized at 125% can trip on a hard-starting load before the motor ever gets to speed. 430.32(C) covers that case. If the selection under 430.32(A)(1) is not sufficient to start the motor or carry the load, you are permitted to go up a size, with new ceilings:

• Service factor 1.15 or more, or temperature rise 40 degrees C or less: up to 140% of nameplate FLA.
• All other motors: up to 130% of nameplate FLA.

Which number does the exam want?

Default to the 430.32(A)(1) number. That is the answer unless the question hands you a starting problem. Watch for language like “the overload trips when the motor starts,” “the selection is not sufficient to start the motor,” or “what is the maximum setting permitted if the motor fails to start.” Only then do you reach for the 140% or 130% ceiling in 430.32(C). A question that just asks for the maximum overload size, with no starting trouble mentioned, is still a 430.32(A)(1) question, because those percentages are already stated as maximums.

Worked example: 25A nameplate, SF 1.15

A continuous-duty motor has a nameplate FLA of 25A and a marked service factor of 1.15. Size the overload.

• Step 1. Pick the current. Overloads use nameplate FLA per 430.6(A)(2). That is 25A. Ignore the table FLC even if the question prints it.
• Step 2. Pick the percentage. Service factor is 1.15, which meets the 1.15-or-more condition in 430.32(A)(1). That is 125%.
• Step 3. Multiply. 25 x 1.25 = 31.25A. The overload is selected to trip at no more than 31.25A.
• Step 4. The bump, only if asked. The question adds that the motor trips on starting. 430.32(C) allows up to 140% for this motor: 25 x 1.40 = 35A. That is the absolute ceiling.

Notice the answer choices this setup produces: 31.25A from the correct math, 35A from the 430.32(C) ceiling, 28.75A from applying 115% to the right current, and something like 35.0A or 39.2A from running the 125 or 140 percent multipliers against the 28A table FLC. Every one of those wrong answers is a specific mistake the writers expect you to make. Know which number answers which question and the choices stop being scary.

Overloads vs branch-circuit protection: two different jobs

The overload device and the branch-circuit breaker or fuses protect different things, and that is why they read different currents.

• Overload (430.32): protects the motor windings from cooking under a sustained running overcurrent. Sized from nameplate FLA at 115% to 125%, up to 130% or 140% under 430.32(C).
• Branch-circuit short-circuit and ground-fault protection (430.52): protects the circuit from faults, not from overload. Sized from the table FLC in 430.248 or 430.250, and the Table 430.52(C)(1) percentages run far higher, up to 250% of FLC for an inverse time breaker, because the device has to ride through motor inrush.

That split is why a motor circuit can legally have a 50A breaker and a 31.25A overload on the same 25A motor. The breaker is not undersized protection and the overload is not a redundant one. Each device covers the failure mode the other cannot.

The job-site memory hook

The overload protects the motor, so it reads the motor. The breaker protects the wire, so it reads the book.

Nameplate for the overload. Table for everything else. Six seconds to recall, and it settles the first and biggest decision in every 430.32 problem before the calculator comes out.

The mistakes that cost points on 430.32

• Using table FLC to size the overload. Overloads read the nameplate per 430.6(A)(2).
• Using nameplate FLA to size conductors or the breaker. Those read the table per 430.6(A)(1).
• Grabbing 115% for a motor marked SF 1.15. Service factor 1.15 or more earns 125%.
• Jumping to 140% when the question never said the motor fails to start. 430.32(C) needs a starting problem.
• Forgetting that temperature rise of 40 degrees C or less also earns 125%, even at SF 1.0.
• Rounding up past the calculated value. The 430.32 percentages are ceilings. 31.25A means no more than 31.25A.