75 hp Conveyor Motor Tripping on Overload: Conductor Size and the Temptation to Crank the Dial
A 75 hp, 480 V, 3-phase TEFC conveyor motor has a nameplate FLA of 96 A at a 40 °C ambient, service factor 1.15, design B. It is fed from an MCC about 60 m away through three copper XLPE conductors in an open cable tray where the ambient is 30 °C. The starter is in a non-air-conditioned electrical room that regularly reaches 45 °C in summer. The Class 10 overload relay is set at 100 A. The conveyor now trips on overload after 20–30 minutes of steady running, and maintenance wants to turn the dial up to 120 A. Available 75 °C copper ampacities: 4 AWG 85 A, 3 AWG 100 A, 2 AWG 115 A, 1 AWG 130 A. The driven equipment has not changed, and the motor terminals and starter lugs are rated 75 °C.
1. Determine the minimum copper conductor size for the motor branch circuit and state whether the 30 °C tray ambient changes your answer.
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Step 1 - Minimum conductor ampacity. Motor branch-circuit conductors are sized at not less than 125% of the motor's nameplate full-load current (the nameplate value is used even when the motor is thermally protected):
125% × 96 A = 120 A
Step 2 - Select from the 75 °C column. Terminations are 75 °C at both ends, so the 75 °C ampacity column applies:
| Size | 75 °C ampacity | ≥ 120 A? |
|---|---|---|
| 3 AWG | 100 A | No |
| 2 AWG | 115 A | No |
| 1 AWG | 130 A | Yes |
Minimum size is 1 AWG copper (protected at the branch-circuit overcurrent device, which is separately sized from the short-circuit/ground-fault table, not from conductor ampacity).
Step 3 - Ambient correction. 30 °C is the reference ambient for the 75 °C ampacity column, so the correction factor is 1.00 and no derating is applied. Three current-carrying conductors in the tray also require no adjustment factor.
Checks worth stating: the 45 °C electrical room does not affect the conductor (the conductors are in the 30 °C tray), but it does affect the overload relay and the motor; if the terminations were 60 °C rated, the size would grow to 1/0 AWG; and the equipment grounding conductor and the motor's short-circuit protection must be verified separately.
2. Evaluate the proposal to raise the overload relay setting to 120 A. Is it permitted, and is it the right fix?
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Permitted? Codes commonly allow overload protection to be sized up to about 125% of nameplate FLA where the service factor is 1.15 or greater, so a 120 A setting is not automatically a violation. That is the wrong question, though.
Right fix? No. Overload protection exists to keep the motor's insulation and windings below their thermal limit; it is not a production adjustment. Two facts matter here:
- The motor is applied in the wrong ambient. A 40 °C-rated motor running in 45 °C air has already lost roughly 5% of its thermal capacity (NEMA derating in that range is on the order of 0.95). Its true continuous capability in that room is closer to 0.95 × 96 A ≈ 91 A, not 96 A.
- The existing 100 A setting is already about 104% of FLA. Moving to 120 A (125%) would allow the motor to run indefinitely at a current it cannot thermally survive in that ambient, converting a nuisance trip into a burned winding or a fire.
Conclusion: the trip is a symptom. The relay is doing its job; the motor is being asked to deliver its rated torque in air it was not rated for, and possibly against a load that has crept up (bearing drag, product buildup, belt tension, chute restriction). Correct the ambient (ventilation, filters, ducting) or replace with a higher-temperature-rise or larger motor, and investigate the mechanical load before touching the setpoint.
3. List the diagnostic actions you would take before changing anything, and what each one tells you.
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Electrical:
- Clamp all three phase currents under normal load. A current unbalance of 2% can produce roughly 8% additional rotor heating - a classic cause of a motor that 'should' carry its load but trips.
- Record line-to-line voltages at the motor terminals to find the source of any unbalance (single-phasing risk, blown fuse in a corner-grounded arrangement, loose lug).
- Verify the overload relay is ambient-compensated and correctly matched to the motor's service factor and Class (Class 10 is fast; a high-inertia conveyor may legitimately need Class 20 - but only after the thermal margin is verified).
- Confirm the relay is set in amps, not as a percentage of a different CT ratio, and that the CT/current transformer wiring is correct.
Thermal/mechanical:
- Infrared the motor frame and the starter lugs; compare the three phases and the frame hot spots.
- Confirm airflow: fan cowl clear, filters clean, room exhaust running, motor not recirculating hot air.
- Check mechanical load: bearing temperature and grease condition, gearbox oil, belt/chain tension, roller freedom, product buildup on the belt.
Documentation:
- Log ambient, currents, voltages, and the time-to-trip. If the corrected data still shows a genuine overload, the correct answer is a larger motor or a corrected driven load - not a higher dial setting.