Rooftop Motor Breaker Trips on Hot Afternoons
A 25 hp, 460 V, three-phase motor drives a rooftop exhaust fan. The motor nameplate shows 35.5 A FLA, service factor 1.15, and a 40°C ambient rating. The branch circuit is 8 AWG THWN-2 copper in EMT, protected by a 60 A inverse-time circuit breaker. The overload relay is set at 34 A. The run is 60 ft, with three current-carrying conductors in the conduit. On hot afternoons, the 60 A breaker trips 2–3 seconds after the fan starts; the overload relay has not tripped. The rooftop ambient is measured at 45°C. The motor terminals are rated 75°C. The breaker is a standard thermal-magnetic type.
1. Is the 60 A breaker permitted for this motor branch circuit? What is the maximum standard rating allowed, and what would you check before increasing it?
Read the worked answer
Table 430.250 gives 34 A full-load current for a 25 hp, 460 V motor. An inverse-time breaker may be sized up to 250% of full-load current under 430.52(C)(1): 34 A × 2.50 = 85 A. The next standard rating is 90 A, so the 60 A breaker is permitted but not required. At only 176% of full-load current, it may nuisance trip on starting current, especially at 45°C. Before increasing to 80 or 90 A, check conductor ampacity: 8 AWG THWN-2 has 55 A at 90°C; the 45°C correction factor is 0.87, giving 47.9 A. That exceeds 125% of full-load current (42.5 A) and is below the 75°C terminal limit of 50 A, so the conductor is acceptable. Also check voltage unbalance, lug torque, and breaker enclosure temperature.
2. The overload relay is set at 34 A. Is that acceptable given the nameplate data? What setting would you recommend, and why might the breaker still trip first?
Read the worked answer
Overload protection is based on the motor nameplate FLA, not the table full-load current. With 35.5 A FLA and a 1.15 service factor, 430.32(A)(1) allows a maximum overload setting of 125%: 35.5 A × 1.25 = 44.4 A. The 34 A setting is only 96% of FLA and is too low for a motor expected to run into its service factor on a hot roof. A setting around 44 A is reasonable if the motor actually draws that current. However, the reported trip is the circuit breaker, not the overload, so raising the overload alone will not solve it. The breaker thermal element is responding to starting current and elevated ambient; investigate breaker size and starting conditions first.
3. The owner wants to replace the 60 A breaker with a 100 A breaker because a spare exists. Is that permitted? What is the maximum and what else must be checked?
Read the worked answer
A 100 A breaker is not permitted. The maximum inverse-time breaker rating is 250% of 34 A, or 85 A; the next standard size is 90 A. A 100 A breaker exceeds that allowance. Increasing to 90 A is permitted and may hold the starting current. Also verify the disconnect is rated at least 115% of full-load current (39.1 A, so a 60 A disconnect is fine), the starter is suitable, and the conductor remains protected under the motor branch-circuit rules. If 90 A still trips, look for a mechanical overload, single-phasing, high-resistance connections, or a breaker derated by the hot enclosure. Do not keep oversizing the breaker to compensate for a fault.