Nuisance Overload Trips on a 75 hp Fan in a Hot Mezzanine
A 75 hp, 460 V, 3-phase, Design B fan motor drives a rooftop exhaust fan. Nameplate data: 96 A FLC, 1.15 service factor, 40°C ambient rating, Class F insulation. The starter is in a mezzanine at 48°C and contains a non-ambient-compensated Class 20 thermal overload set at 95 A. The branch circuit is three 1/0 Cu THWN conductors in 1-1/4 in. PVC, 85 ft long, with 75°C terminations, protected by a 250 A inverse-time breaker. The motor draws 88 A per phase at 472 V and trips after 25 to 35 minutes at about 80% of rated airflow. Voltage balance is within 1%. The maintenance technician wants to raise the overload to 110 A and, if that fails, replace the 250 A breaker with a 300 A unit.
1. Evaluate the overload setting considering the nameplate FLC, service factor, and 48°C ambient.
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The overload is the likely cause. For a 1.15 service-factor motor, the overload may be selected up to 125% of nameplate FLC: 96 A × 1.25 = 120 A. A 95 A setting is below the motor nameplate FLC, so it has little margin and may trip on normal variation. More importantly, the overload is non-ambient-compensated and located at 48°C, 8°C above the 40°C calibration ambient. A thermal overload trips at a lower current in a hotter enclosure; a rough correction is 0.90 to 0.93, so a 95 A setting behaves like roughly 86 to 88 A. The motor is drawing 88 A, right at that effective trip point, so a 25 to 35 minute trip is consistent. The 80% airflow does not necessarily mean the motor is overloaded; the measured current is only about 92% of nameplate. The fix is not to raise the breaker. Use an ambient-compensated overload or apply the manufacturer's ambient correction, and set it high enough to carry the motor but not above 120 A.
2. Verify whether the branch-circuit conductors and the 250 A breaker are adequate.
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The branch-circuit conductors are acceptable. 125% of 96 A is 120 A. 1/0 Cu THWN at 75°C has a base ampacity of 150 A. At 48°C, the 75°C correction factor is about 0.93, giving roughly 139 A, still above 120 A, and the 75°C terminations are not exceeded. The 250 A inverse-time breaker is within the motor branch-circuit short-circuit and ground-fault protection limits: 250% of 96 A is 240 A, and 250 A is the next standard rating, so it is permitted as a maximum, assuming the motor will start. The breaker protects against short circuits and ground faults, not overload. A 300 A breaker would exceed 250% and would not be justified by a thermal overload trip. If the 250 A breaker trips on starting, that is a separate issue requiring checks of acceleration time and breaker type, not a blind upgrade.
3. Recommend a corrective sequence and explain why raising the breaker is the wrong response.
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Measure all three phase currents and voltage unbalance under load; confirm the motor is not actually overloaded by airflow restriction, belt tension, or a failing bearing. Verify the starter enclosure temperature and ventilation. Replace the non-compensated overload with an ambient-compensated Class 20 device, or a Class 30 device if starting is long, and set it near 110 to 115 A but not above 120 A. Check the overload current transformer ratio and heater selection if it is a separate-heater type. Do not raise the 250 A breaker to 300 A to solve a thermal trip; the breaker is not sensing overload. If the motor still trips at 88 A with a correct overload, investigate the motor, load, or voltage unbalance. A small voltage unbalance can cause significant current unbalance and heating, so check all phase-to-phase voltages, not just the average.