25 hp Conveyor Motor Tripping on Acceleration
A 25 hp, 480 V, three-phase, design B, totally enclosed fan-cooled conveyor motor has a nameplate full-load current of 32.0 A, a service factor of 1.15, and a terminal housing rated 75°C. The branch circuit is 8 AWG THWN-2 copper in EMT protected by a 90 A inverse-time circuit breaker; a thermal overload relay in the starter is set at 34 A. The motor room runs near 40°C for much of the summer. The conveyor nuisance-trips the breaker during acceleration about twice a week, always on a loaded start. The maintenance technician proposes replacing the 90 A breaker with a 125 A breaker and 'turning up' the overload relay to 42 A.
1. Verify the branch-circuit conductor size and the 90 A breaker against the code minimums, including the elevated ambient.
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Motor FLA for sizing: use the table value for 25 hp at 460 V, 34 A - not the 32.0 A nameplate. Nameplate current is used for the overload device only.
Conductors: 125% x 34 A = 42.5 A required ampacity. 8 AWG Cu at 75°C = 50 A. Apply the ambient correction for 40°C on a 75°C conductor (0.88): 50 x 0.88 = 44 A. 44 A >= 42.5 A -> 8 AWG is acceptable. (Margin is thin; a warmer room or a rooftop run would change the answer.)
Short-circuit/ground-fault protection: inverse-time breaker at 250% of 34 A = 85 A. 85 A is not a standard rating, so the next higher standard rating, 90 A, is permitted. The 90 A breaker is code-compliant as installed.
The breaker is therefore not oversized - the tripping is a symptom, not a sizing error.
2. Evaluate the proposed 42 A overload relay setting.
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The overload device is sized on nameplate current, not table current:
- Service factor 1.15 (>= 1.15) -> maximum normal setting = 125% x 32.0 A = 40 A
The existing 34 A setting is fine. 42 A is above the 125% allowance and is not permitted as a routine adjustment.
The only path above 40 A is the 'insufficient to start or carry the load' provision: if the correctly sized device will not carry the load, the sensing element may be raised, but never above 140% of nameplate, i.e. 44.8 A here - and only after the 40 A device has actually been shown to be inadequate. 42 A is under the 44.8 A ceiling but only becomes legal after that demonstration, not as a first move.
Also note the overload relay does not protect against short circuits; raising it does nothing for the acceleration trip and only shortens motor life.
3. The technician wants to install a 125 A breaker. Is that permitted, and what should be checked before any change is made?
Read the worked answer
Permitted, conditionally. The 'next size up' and the 400% allowance exist precisely for starting current: 400% x 34 A = 136 A, so 125 A is within the ceiling for an inverse-time breaker where the 250% device will not carry the starting current.
But before using that allowance:
- Confirm the overload relay is correct (it is set at 34 A, below the 40 A maximum) - a mis-set or tripping overload is often misdiagnosed as a breaker problem.
- Check voltage at the motor terminals during acceleration. A long feeder or an undersized transformer will drop voltage, raise current, and lengthen acceleration time. Fixing voltage drop fixes the trip without weakening protection.
- Check the mechanical side - belt tension, loaded-start condition, brake drag, product buildup on the belt. A conveyor that starts loaded when it was designed to start empty will trip any correctly sized breaker.
- Verify the breaker's trip curve and that it is the type assumed (inverse-time vs. instantaneous-only; motor-circuit protectors have different rules).
- Document the change. If the rating is increased above the standard 250%, the setting must be recorded and made available, and the equipment must still be marked with the maximum setting.
Risk to flag: increasing the breaker raises the let-through energy for a ground fault in the branch circuit. With 8 AWG conductors, a 125 A breaker gives up most of the conductor protection, so ground-fault protection and conductor physical protection deserve a second look. If the real problem is mechanical or voltage, the correct answer is to fix that, not to upsize the breaker.