430.32
Motor Overload Protection — Continuous-Duty Motors
The motor overload relay (or heater) is the last line of defense for the motor itself. It sits in the motor starter and monitors running current. If the motor is mechanically bogged down — a pump seizes, a belt jams, a bearing fails — the current climbs above normal running levels. The overload relay detects this slow, sustained overcurrent and disconnects the motor before the windings overheat and burn out. Because the overload must be tuned to the specific motor's actual operating current, you use the nameplate FLA as the basis, not the NEC table FLC. For motors with a service factor of 1.15 or greater (which is most standard motors), the overload must trip at no more than 125% of nameplate FLA. For motors with a service factor below 1.15, the overload must trip at no more than 115% of nameplate FLA. If a standard overload heater is not available at the exact calculated value, 430.32(C) permits going to the next higher commercially available size, but not exceeding 140% of nameplate FLA for motors with SF >= 1.15, or 130% for SF < 1.15.
When You Need This
- Selecting overload relay heater elements during motor starter installation
- Replacing a motor and re-sizing the overload heaters for the new nameplate rating
- Troubleshooting nuisance overload trips — the heater may be undersized for the actual motor nameplate FLA
- Answering exam questions that test whether you know the difference between short-circuit and overload protection
Key Points
Common Mistakes
Using the NEC table FLC value for overload sizing — this is the one place where you must use the nameplate FLA
Confusing the overload relay with the branch-circuit breaker — they serve different purposes and are sized differently
Not checking the motor service factor before choosing the 125% or 115% multiplier
Setting the overload too high to stop nuisance trips — this defeats the purpose and risks burning out the motor windings
Exam Tip
Remember: TABLE for the circuit, NAMEPLATE for the overload. If the exam gives you a motor nameplate FLA of 27A with SF 1.15, the max overload is 27 x 1.25 = 33.75A. If it gives you a table FLC of 28A, that 28A is used for conductors and breakers, NOT overloads.
Frequently Asked Questions
Overload protection (the relay in the motor starter) protects the motor from sustained moderate overcurrent — like a jammed conveyor belt causing the motor to draw 150% of its normal current for minutes. Short-circuit protection (the breaker at the panel) protects against violent, immediate faults where thousands of amps flow for fractions of a second. They are two separate devices sized by two separate rules.
No. The overload must be set to trip at 125% (SF >= 1.15) or 115% (SF < 1.15) of the nameplate FLA. Setting it at exactly the nameplate value would cause nuisance trips during normal motor operation because motors routinely draw brief surges slightly above their rated FLA.
Inline Tools
Motor FLC Calculator
Look up full-load current for any motor HP and voltage
Related Code Sections
This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.