Thermal Overload Relay Calculator
Enter FLA, service factor and trip class to get the NEC maximum trip current and recommended relay dial.
Step-by-step
- Enter valid values to begin.
NEC 430.32(A)(1): 125% for marked SF ≥ 1.15 or temp rise ≤ 40 °C, otherwise 115%. 430.32(C) allows 140% / 130% only if the first value will not carry the load. Trip times are the IEC 60947-4-1 class bands at 7.2 × setting, scaled by I²t for other multiples — indicative only; use the manufacturer trip curve. Percentages are typed from memory: verify against the adopted NEC edition.
Thermal Overload Relay: Setting the Dial Without Nuisance Trips
Core Engineering Principles
A thermal overload relay is a model of the motor’s winding temperature, built from a bimetal strip heated by the motor current. It is meant to trip before the motor insulation is damaged and not before the motor has done its legitimate work. The code handles this by tying the maximum trip setting to the nameplate full-load current and to the motor’s thermal rating: 125% for motors with a service factor of 1.15 or more or a temperature rise of 40 °C or less, and 115% for everything else. These percentages are limits, not targets. Most good electricians start at the nameplate current and only move up if the motor trips on a normal start.
The trip class describes how long the relay takes to trip on a heavy overload. Class 10 trips within 10 s at 7.2 times the setting, class 20 within 20 s and class 30 within 30 s. Match the class to the motor’s starting time: a fan with a 12 s run-up trips a class 10 relay every time but sits happily on class 20 or 30. Pick a class too slow, though, and the relay can’t protect a low-thermal-mass motor such as a submersible.
Imax,430.32(C) = 140% (or 130%) × FLA • t ≈ t7.2× × (7.2 / m)²
NEC & Standard References
NEC 430.32(A)(1) sets the separate overload device at no more than 125% of nameplate current for motors with a marked service factor of 1.15 or greater or a marked temperature rise of 40 °C or less, and 115% for all other motors. NEC 430.32(C) allows a higher setting, up to 140% or 130% respectively, if the first setting is not enough to carry the load. IEC 60947-4-1 defines trip classes 10A, 10, 20 and 30. NEMA ICS 2 does the same for North American relays. Percentages typed from memory should be checked against the adopted code edition.1. Limit under 430.32(A)(1) = 1.25 × 52 = 65.0 A.
2. Recommended dial = nameplate = 52 A (100%).
3. Raised limit under 430.32(C) = 1.40 × 52 = 72.8 A, to be used only after nuisance trips.
4. At 1.2× the setting the relay should trip within two hours, at 62.4 A.
5. The class 20 band scaled by I²t gives about 8.6 s minimum at 6×, so a 5 s start uses about 58% of the estimated minimum trip time and is acceptable.
- Don’t default to the maximum. Setting 125% on every motor gives you a relay that protects nothing until the insulation is already stressed.
- Ambient temperature matters. A bimetal relay in a hot panel trips early. Use a compensated relay or derate the dial.
- Single-phasing needs differential tripping. Choose a relay with phase-loss sensitivity, since an unbalanced supply makes one winding run far hotter than the others.
- A trip is information. Repeated trips mean a jam, a dragging bearing or low voltage; raising the dial just burns the winding instead.
- Reset only after cooling. A thermal relay needs a few minutes to cool, and forced resets against a hot bimetal defeat the whole protection.