Practice: motor overload protection
Each problem uses new numbers, so you practice the method rather than memorizing an answer.
Pick the answer: sizes and ratings come from your code book.
Worked solution
Formula
| Symbol | Meaning | Unit |
|---|---|---|
| FLA | Motor nameplate full-load current | A |
| SF | Service factor marked on the nameplate | |
| Rise | Marked temperature rise of the motor | °C |
Worked example
A 15 hp, 460 V three-phase motor has a nameplate full-load current of 19.3 A, a service factor of 1.25 and a marked temperature rise of 50 °C. It is protected by a separate overload relay. Under NEC 430.32(A)(1), what is the maximum trip rating of the overload relay?
- Separate overload protection uses the nameplate current, not Table 430.250 (430.6(A)(2)): 19.3 A.
- Service factor 1.25: 1.15 or more, which qualifies for 125%.
- Temperature rise 50 °C: more than 40 °C, which does not qualify for 125%.
- At least one condition is met, so the relay may be rated up to 125% of the nameplate current.
- Maximum trip rating: 19.3 × 125% = 24.13 A.
Answer: 24.13 A
Motor overload protection, explained
Overload protection keeps a motor from overheating when it is asked to carry more than it was built for, such as a jammed pump or a dragging belt. It is a separate job from short-circuit protection, so it is sized much closer to the running current.
NEC 430.6(A)(2) says separate overloads are sized from the nameplate current, because the relay must match the actual motor. Then 430.32(A)(1) sets the maximum: 125% of nameplate current when the service factor is 1.15 or more, or when the marked temperature rise is 40 °C or less. Every other motor gets 115%. Only one of the two conditions has to be met for 125%.
If an overload relay sized this way trips while the motor starts or carries its normal load, 430.32(C) allows the next higher size relay, up to 140% for motors that met a 125% condition and 130% for the rest. These are maximums, so you do not round up to a standard breaker size.
How to solve it on the exam
- Find the nameplate full-load current in the question. Do not use Table 430.248 or 430.250.
- Check the service factor: 1.15 or more qualifies for 125%.
- Check the marked temperature rise: 40 °C or less qualifies for 125%.
- If either condition is met, multiply the nameplate current by 1.25; if neither, by 1.15.
- Only if the question says the motor will not start with that relay, use 430.32(C): 1.40 or 1.30.
Common mistakes
- Using the table full-load current instead of the nameplate current.
- Thinking both conditions must be met for 125%. Either one is enough.
- Rounding the result up to a standard breaker size.
- Using the 430.32(C) percentages when the question does not say the motor fails to start.
- Multiplying by the service factor itself instead of the code percentage.
Exam tips
- Underline the service factor and the temperature rise in the question before you start.
- Remember the pairs: 125 and 115 for 430.32(A)(1), 140 and 130 for 430.32(C).
Questions people ask
How do you size motor overload protection?
Multiply the nameplate full-load current by 125% if the service factor is 1.15 or more or the temperature rise is 40 °C or less, otherwise by 115%. This is NEC 430.32(A)(1).
Do overloads use nameplate or table current?
Overloads use the nameplate current. NEC 430.6(A)(2) requires it for separate overload protection, while conductors and breakers use the table value.
What if the overload trips when the motor starts?
NEC 430.32(C) allows a higher setting: up to 140% of nameplate current for motors with a service factor of 1.15 or more or a 40 °C rise, and 130% for all others.
What does service factor mean on a motor?
Service factor is how much the motor can be overloaded without damage. A 1.15 service factor means the motor can carry 115% of its rated horsepower for a time.
Sources
- TDLR: Electrician examination information
- PSI: Texas electrician candidate information bulletin
- IEC: 2026 NEC changes to Chapter 4
About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.