Rooftop Fan Motor Branch Circuit Trips on Start
A 25 hp, 460 V, three-phase rooftop supply fan motor has a nameplate full-load current of 34 A, service factor 1.15, code letter G, and a 40°C ambient rating. The existing branch circuit consists of 10 AWG THHN copper conductors in EMT, a 50 A inverse-time circuit breaker, and a combination starter with Class 10 overload heaters set at 38 A. The starter is listed for 75°C terminations and for 10 AWG conductors. During hot afternoon starts, the circuit breaker trips roughly once a week; the overload heaters have never tripped. The maintenance technician wants to replace the 50 A breaker with an 80 A breaker to stop the tripping. Available standard ratings are 45, 50, 60, 70, 80, 90, and 100 A. The motor is continuous duty and the conductors are not bundled.
1. Check the branch-circuit conductors and the 50 A breaker against Code minimums. Is the existing installation compliant? Show the numbers.
Read the worked answer
Conductor minimum ampacity for a continuous-duty motor branch circuit is 125% of nameplate FLA: 34 A x 1.25 = 42.5 A. The installed 10 AWG THHN copper has a 75°C ampacity of 35 A, so it is undersized and the installation is not compliant. A minimum 8 AWG copper conductor is required; at 75°C it provides 50 A, which exceeds 42.5 A.
The 50 A breaker is not theCode violation by itself. Inverse-time motor branch short-circuit and ground-fault protection may be sized up to 250% of FLA: 34 A x 2.50 = 85 A. A 50 A breaker is therefore permitted, and an 80 A standard rating is also permitted because it does not exceed 85 A. Motor branch protection is allowed to exceed the conductor ampacity, but the conductor still must have the required 125% FLA ampacity. The real defect is the 10 AWG conductor, and simply installing an 80 A breaker would leave that defect in place while making it harder to detect an overloaded circuit.
2. Evaluate the 38 A overload setting and the Class 10 selection. What maximum overload setting is allowed, and what would you check before changing it?
Read the worked answer
Overload protection is based on motor nameplate current, not on the branch breaker. Because the motor has a service factor of 1.15, the overload device may be selected up to 125% of nameplate FLA: 34 A x 1.25 = 42.5 A. The existing 38 A setting is about 112% of FLA and is acceptable. It is not required to be raised.
Class 10 overloads trip quickly and can be marginal on a high-inertia fan, but the reported symptom is breaker tripping, not overload tripping. Before changing the overload setting, measure actual running current on all three phases, verify the starter ambient temperature and heater selection table, check that ventilation openings are clear, and confirm the motor is not running above nameplate current because of belt tension, fan imbalance, or restricted airflow. If nuisance overload trips do occur and running current is normal, the setting may be increased only to the 42.5 A maximum for this service factor. Do not use a larger overload to compensate for a bad bearing or a failing motor.
3. The technician proposes only swapping the breaker to 80 A. What is your response, and what sequence of checks and fixes do you recommend?
Read the worked answer
I would reject the breaker-only change. An 80 A breaker is within the 250% allowance for motor short-circuit protection, but it does not correct the undersized 10 AWG conductor and could mask a high starting current or a supply problem. The conductor must be replaced with 8 AWG copper or larger before the branch circuit is acceptable.
Recommended sequence: lock out the circuit and verify the starter is listed for 8 AWG terminations; replace the 10 AWG conductors with 8 AWG; torque all terminations to the manufacturer’s values; then record supply voltage and voltage imbalance at the starter during start. Check the breaker trip curve and age, the starter contactor contacts, and the mechanical load: fan rotation, belt condition, bearing drag, and dampers. Measure starting current with a peak-reading meter. If the branch still trips during normal starting after the conductor correction, the inverse-time breaker may be increased up to 80 A maximum for this 34 A motor. If starting current is excessive or voltage dip is severe, the cause is the motor, starter, supply, or mechanical load, not the breaker rating.