Motor overload calculator
Enter the nameplate full-load amps, service factor, and temperature rise. You get the maximum overload rating or trip setting.
Built on the 2026 NEC · Free, no sign-up · Updated October 2026
Result
35.00 A max (125% of FLA)
How it was calculated
- Select base overload multiplier: 125% (qualified for 125%).
- Use base multiplier (standard tier).
- Max overload = 28 A × 1.25 = 35.00 A.
Assumptions
- Service factor 1.15 ≥ 1.15 → 125% per 430.32(A)(1).
- Standard tier — base multiplier 125%.
- Calculation is per 430.32(A)(1) (1 hp+ continuous-duty motors). Different rules apply to motors under 1 hp (430.32(B)) and intermittent-duty motors (430.33).
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
How overloads are sized
For a continuous-duty motor over 1 hp with a separate overload device, 430.32(A)(1) caps the overload at a percentage of the nameplate full-load current:
- 125% for a motor with a marked service factor of 1.15 or more.
- 125% for a motor with a marked temperature rise of 40°C or less.
- 115% for all other motors.
Unlike conductor sizing, overloads use the nameplate current, not the table current.
If the motor trips on start
If the overload sized under 430.32(A)(1) isn't enough to start the motor or carry the load, 430.32(C) lets you use the next higher size or setting, up to 140% for motors that qualified for 125%, and 130% for the rest. Select "Permitted increase" in the calculator to see that ceiling.
2023 to 2026: our list has 8 changes in Article 430. See the changes
Worked example
A motor with a 28 A nameplate current, a 1.15 service factor, and a 40°C rise:
- Nameplate full-load amps (amps): 28
- Service factor: 1.15
- Temperature rise (°C): 40
- Tier: Standard
Result
35.00 A max (125% of FLA)
How it was calculated
- Select base overload multiplier: 125% (qualified for 125%).
- Use base multiplier (standard tier).
- Max overload = 28 A × 1.25 = 35.00 A.
Assumptions
- Service factor 1.15 ≥ 1.15 → 125% per 430.32(A)(1).
- Standard tier — base multiplier 125%.
- Calculation is per 430.32(A)(1) (1 hp+ continuous-duty motors). Different rules apply to motors under 1 hp (430.32(B)) and intermittent-duty motors (430.33).
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
Questions electricians ask
What does a motor overload calculator set, and does this one follow the NEC?
It finds the maximum overload device rating or trip setting from the nameplate full-load current, the service factor, and the temperature rise. This calculator follows the NEC 2026 method in 430.32(A)(1), with the 430.32(C) increase for a motor that trips while starting or carrying its load.
Is there a motor overload sizing chart?
The NEC has no chart for this. The answer comes from a percentage of the nameplate current: 125% for a service factor of 1.15 or more or a temperature rise of 40°C or less, and 115% for all other motors (430.32(A)(1)). The calculator applies those percentages for you. For the rating of a relay or heater, use the manufacturer's selection chart for that device.
How do I set the overload when the nameplate shows a service factor?
Multiply the nameplate full-load current by 125% if the service factor is 1.15 or more, or by 115% for all other motors, such as one with a service factor below 1.15 and a rise over 40°C. That is the maximum setting. If the motor still trips while starting, 430.32(C) allows the next higher size or setting, up to 140% or 130%.
Should I use nameplate or table current for overloads?
Nameplate. Section 430.32 sizes overloads from the motor's marked full-load current rating.
What is the maximum overload setting allowed?
Normally 125% or 115% of nameplate current. If that trips the motor while starting or carrying its load, 430.32(C) allows up to 140% or 130%.
Is the overload the same as the breaker?
No. The overload protects the motor and conductors from running overloads. The branch-circuit breaker or fuse protects against short circuits and ground faults, and is sized with the motor circuit calculator.
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How this calculator was built
The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.
How we test the calculators. See something wrong? Tell us and we will check it against the code.
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