17
A
125
%
17
A
20
1
17
A
125
%
17
A
20
1
The motor overload protection calculator determines the correct setting for thermal overload relays and electronic overload devices that protect AC and DC motors from running overload conditions. While branch circuit fuses and breakers protect against short circuits, overload relays protect the motor winding insulation from the slow thermal damage caused by sustained operation above rated current.
NEC Section 430.32 specifies maximum overload relay settings based on the motor's nameplate FLA (Full Load Amperes — the actual nameplate rating, not the NEC table value). The setting depends on the motor's thermal characteristics as indicated by its service factor (SF) and temperature rise rating. For motors with SF ≥ 1.15 or temperature rise ≤ 40°C: maximum setting = 125% of nameplate FLA. For all other motors: 115% of FLA.
The rationale for different percentages is based on motor thermal design. A motor with SF ≥ 1.15 is built with extra thermal margin — it can tolerate 15% overload continuously without damage. Setting the relay at 125% FLA for these motors prevents nuisance tripping while still protecting against genuine overloads. Motors without this margin (SF = 1.0) receive tighter protection at 115% FLA.
Overload relay trip classes (NEMA Class 10, 20, 30) define how quickly the relay trips at overload. Class 10 trips within 10 seconds at 600% of pickup current — used for fast-response motors (pumps, compressors with low inertia). Class 20 trips within 20 seconds — the most common for general-purpose motors. Class 30 trips within 30 seconds — used for high-inertia loads (centrifuges, large fans, conveyors).
Ambient temperature affects overload relay calibration. Most thermal overload relays are calibrated for 40°C ambient. At higher ambient temperatures, the relay trips at lower currents than its setting because the relay's thermal element is already pre-heated by the environment. Electronic overload relays with temperature compensation avoid this issue. This calculator applies a thermal derating factor for ambient temperatures above 40°C: F = √(T_class / (T_class + ΔT)), where T_class is the motor insulation class temperature and ΔT is the excess ambient temperature above 40°C.
Max overload setting: SF ≥ 1.15 or temp rise ≤ 40°C → 125% × FLA. Otherwise → 115% × FLA. Thermal derating for ambient > 40°C: F = √(T_ins / (T_ins + T_ambient - 40)), where T_ins = 155°C for Class F insulation (typical AC motors). Recommended relay setting = max_setting × thermal_derating.
Set the overload relay at or below the calculated setting. If the motor trips on starting (especially high-inertia loads), check that you are using the correct trip class. For high-inertia applications, use Class 30 relays. If the overload relay repeatedly trips at normal load, check motor cooling, verify nameplate FLA, and ensure ambient temperature is within the relay's compensation range.
Inputs
Results
FLA = 13.6A, SF = 1.15 → set overload relay at 125% × 13.6 = 17.0A. Use Class 20 relay for normal motor starting. No ambient derating at 40°C.
Inputs
Results
FLA = 28A, SF = 1.0, ambient = 50°C: NEC max = 32.2A. With ambient derating at 50°C, actual relay setting = 29.9A to account for pre-heating of relay element in hot environment.
Service Factor is a multiplier on the motor nameplate indicating how much the motor can be overloaded without damage. SF = 1.15 means the motor can run continuously at 115% of rated HP. It does not mean you should operate the motor at 115% load routinely — it provides margin for occasional overloads.
Trip class defines maximum time-to-trip at 600% of overload relay setting (locked rotor test). Class 10 = 10 sec, Class 20 = 20 sec, Class 30 = 30 sec. Use Class 10 for motors that start quickly, Class 20 for standard motors, Class 30 for high-inertia loads.
A three-phase motor that loses one phase (single-phasing) immediately draws about 1.73× rated current in the remaining two phases. This exceeds typical overload relay settings, causing a trip — but only if the relay detects all three phases. Phase-loss detection is better handled by phase-sensitive overload relays or electronic monitoring relays.
Thermal overload relays use a bimetallic element or eutectic alloy that heats with motor current and trips mechanically. They require ambient temperature compensation adjustment. Electronic overload relays use current transformers and microprocessors to calculate motor thermal state, provide phase-loss detection, and are immune to ambient temperature variations.
Setting the relay above 125% FLA (or the applicable NEC maximum) is a code violation and leaves the motor unprotected against overloads. The motor may overheat and fail without tripping the relay — damaging windings and potentially causing a fire. Always use the nameplate FLA, not NEC table FLC, for overload relay settings.
Manual reset requires an operator to physically reset the relay after a trip — preferred in most applications because it forces investigation of the cause before restart. Automatic reset is used in remote locations where immediate restart is more important than investigating the trip cause, and where nuisance tripping is likely (conveyors with momentary jams).
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