Motor Overload Protection Calculator
Size the running-overload device for a continuous-duty motor per NEC 430.32. Enter the motor nameplate full-load current and its overload class to get the maximum overload trip at 125 percent or 115 percent, plus the 430.32(C) increase permitted when the standard size will not start the motor.
How to use this calculator
- Enter the motor nameplate full-load current in amperes. Read it from the nameplate, not the NEC table (NEC 430.6(A)(2)).
- Pick the overload class from the nameplate: a marked service factor of 1.15 or greater, a marked temperature rise of 40C or less, or all other motors.
- Read the standard maximum overload trip (430.32(A)(1)) and the maximum permitted increase (430.32(C)).
- Select an overload relay or heater element that trips at or below the standard maximum; step up toward the 430.32(C) ceiling only if the motor nuisance-trips on starting.
NEC reference
This tool applies NEC 2020 430.32(A)(1) (separate overload device for a continuous-duty motor rated more than 1 hp) and 430.32(B)(1) (the same percentages for a motor of 1 hp or less, nonautomatically started), with the 430.32(C) permitted increase, all on the motor nameplate full-load current per 430.6(A)(2). The percentages are unchanged in the 2017 and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
How NEC 430.32 sizes the overload
A motor branch circuit carries two separate protective functions. The overload device protects the motor windings against a sustained running overcurrent and sits close to the nameplate current. The branch-circuit short-circuit and ground-fault device (a breaker or fuse per 430.52) is sized several times higher so the motor can start without tripping while still clearing a fault. This page sizes the first device; the Motor Breaker Sizing Calculator sizes the second.
NEC 430.32(A)(1) caps a separate overload device at a percent of the motor nameplate full-load current:
max overload trip = nameplate FLA x 125% (or 115% for all other motors)
The 125 percent class covers motors marked with a service factor of 1.15 or greater or a temperature rise of 40C or less. Everything else is the 115 percent class. The two markings are alternatives: either one puts the motor in the higher class.
NEC 430.32 overload percentages
| Motor nameplate marking | 430.32(A)(1) max | 430.32(C) max increase |
|---|---|---|
| Service factor 1.15 or greater | 125% | 140% |
| Temperature rise 40C or less | 125% | 140% |
| All other motors | 115% | 130% |
Percent of motor nameplate full-load current per NEC 2020 430.32(A)(1)/(B)(1) and 430.32(C).
Worked example
A continuous-duty motor has a 28 A nameplate full-load current and a marked service factor of 1.15. It falls in the 125 percent class, so the standard maximum overload trip is:
28 A x 125% = 35 A maximum overload
If a 35 A overload nuisance-trips while the motor comes up to speed, NEC 430.32(C) permits stepping up to the next size, but no higher than:
28 A x 140% = 39.2 A maximum permitted
Start at 35 A and only move toward 39.2 A if the motor will not start. Because overload heaters are chosen from manufacturer tables rather than standard breaker ratings, pick the largest listed element that trips at or below the applicable maximum.
How this differs from the Motor Breaker Sizing Calculator
The two tools size two different devices on the same motor circuit, from two different currents. This overload tool uses the motor nameplate full-load current (NEC 430.6(A)(2)) and the small 125/115 percent overload window. The Motor Breaker Sizing Calculator uses the table full-load current from Table 430.248 or 430.250 (NEC 430.6(A)(1)) and the much larger Table 430.52 percentages (up to 250 or 300 percent) for the short-circuit and ground-fault device, plus the 430.22 conductor. A complete motor branch circuit needs both: the overload from this page and the breaker or fuse plus conductor from that one.
Frequently asked questions
How do you size motor overload protection under NEC 430.32?
NEC 430.32(A)(1) limits a separate overload device to a percent of the motor nameplate full-load current: 125 percent for a motor marked with a service factor of 1.15 or greater or a temperature rise of 40C or less, and 115 percent for all other motors. A 28 A nameplate motor with a 1.15 service factor gets a maximum overload of 28 times 1.25, which is 35 A.
Nameplate current or table FLC?
Nameplate. NEC 430.6(A)(2) bases separate motor overload protection on the nameplate full-load current rating. The Table 430.248/430.250 values are for the branch-circuit device and conductors, not the overload.
When can I use the 430.32(C) increase?
Only when the 430.32(A)(1) size will not start the motor or carry the load. Then you may step up to the next overload size, capped at 140 percent of nameplate for a 125 percent motor or 130 percent for a 115 percent motor. It is a ceiling for nuisance tripping, not the default.
Does this cover integral thermal protectors?
No. This tool sizes a separate overload device (an overload relay). Thermal protectors integral to the motor (430.32(A)(2)) and automatically started motors of 1 hp or less protected by the branch-circuit device (430.32(D)) follow different rules and are out of scope for v1.
Related calculators
- Motor Breaker Sizing Calculator: the branch-circuit breaker or fuse and conductor per NEC Article 430 Table 430.52 and 430.22.
- MCA & MOCP Calculator: HVAC minimum circuit ampacity and maximum overcurrent protection per NEC Article 440.
- Breaker Size Calculator: standard NEC 240.6(A) breaker for a general load with the 125 percent continuous rule.
- Wire Ampacity Calculator: conductor ampacity with the 110.14(C) termination and 310.15 derating rules.
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