15 hp Conveyor Motor: Breaker Trips, Overloads Get Blamed
A 15 hp, 460 V, 3-phase, 1.15 service factor motor drives a conveyor on a 480Y/277 V system. Nameplate FLA is 21 A. The branch circuit is 10 AWG THHN copper in EMT with 75 °C terminations, protected by a 60 A inverse-time circuit breaker. The starter's overload relay is set at 26 A. On cold mornings the 60 A breaker trips during acceleration. The maintenance tech wants to swap in a 100 A breaker "because the motor only draws 21 A." The plant electrician wants to raise the overload relay settings instead. Management wants the conveyor running today.
1. Verify the branch-circuit conductor and short-circuit/ground-fault protection sizing before anyone changes anything.
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Conductor sizing. Motor branch-circuit conductors are sized at 125% of the motor full-load current from the motor tables (not the nameplate alone), and 21 A happens to equal the table value for 15 hp at 460 V.
- 21 A × 1.25 = 26.25 A
- 10 AWG Cu at 60 °C = 30 A; at 75 °C = 35 A → 10 AWG is adequate.
Short-circuit and ground-fault protection. An inverse-time breaker is permitted up to 250% of FLA:
- 21 A × 2.50 = 52.5 A → next standard size up = 60 A. Correct as installed.
The 60 A breaker on 10 AWG is not a violation. This is the point that confuses people. The small-conductor protection rules do not govern motor branch circuits; the motor article permits the short-circuit device to exceed the conductor ampacity because the overload relay, not the breaker, protects the conductors and the motor from overload. The breaker only clears faults.
Conclusion: the conductors and the breaker are both correctly sized as installed. Do not start by changing either one - the installation was compliant, so the problem is the starting current, not the sizing.
2. The breaker trips on cold-morning starts. May it be increased to 100 A? What is the maximum, and what is the correct sequence of steps?
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No - 100 A is not permitted. The ceiling for an inverse-time breaker on a motor branch circuit is 400% of FLA:
- 21 A × 4.00 = 84 A maximum
Permitted escalation, stopping as soon as the nuisance tripping ends:
- 60 A (250%, as installed) - tripping.
- 70 A (about 333%) - try this first.
- 80 A (about 380%) - permitted if 70 A is not enough.
- 84 A - the absolute maximum; do not exceed it.
Before changing the breaker at all, check the cheap things:
- Actual voltage at the motor terminals during the start (a 480 V system sagging badly on a cold morning start is a supply problem, not a breaker problem).
- Conveyor load - cold grease and frozen product dramatically increase breakaway torque.
- Breaker condition and ambient - a breaker in a cold outdoor enclosure has a different trip curve than one in a warm room.
- Motor connections and any loose terminations causing a high-resistance start.
If 84 A still trips, stop escalating. The options are a reduced-voltage or soft starter, a VFD, a higher-magnetic-element breaker, or fixing the mechanical load. A 100 A breaker would leave the conductors and starter unprotected against fault current and voids the design basis for the whole circuit.
3. Separately, the overload relay trips after about 20 minutes of steady running. What may be done, and what may not?
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Overload protection is a different question from the breaker, and it protects the motor - not the circuit.
Permitted setting. For a continuous-duty motor with a service factor of 1.15 or greater, the overload device is set at 125% of FLA:
- 21 A × 1.25 = 26.25 A → the 26 A setting is correct.
Permitted increase if it trips. The setting may be raised to a maximum of 140% of FLA for this motor (the 140% ceiling applies to motors with service factor 1.15 or a 40 °C rise):
- 21 A × 1.40 = 29.4 A maximum.
What may not be done: the overload cannot be increased beyond 140%, jumpered out, or replaced with a larger heater "to get through the shift." Above that point the motor is being asked to run outside its thermal capability, and the relay is telling the truth.
Investigate rather than escalate:
- Voltage unbalance - a 3% unbalance can raise motor heating dramatically; check all three phases under load.
- Single phasing - a blown fuse in a fused disconnect, or a bad contact in one pole of a contactor.
- Mechanical drag - bearings, alignment, and belt tension all show up as overload tripping long after startup.
- Ambient at the starter - overload relays are ambient-compensated to varying degrees; a relay in a hot enclosure trips early with nothing wrong with the motor.
- Duty cycle - a conveyor that starts and stops frequently may exceed the motor's rated duty and need a larger frame, not a larger heater.
Raise to 29.4 A only after the electrical and mechanical checks come back clean, and document the change.