25 hp Conveyor: Nuisance Trips, Overload Settings, and the Wrong Breaker
A conveyor is driven by a 25 hp, 460 V, 3-phase Design B motor with a nameplate full-load current of 32 A and a marked service factor of 1.15. The branch circuit runs 45 ft in EMT from a combination starter whose lugs are marked 75 °C; the conductors are 8 AWG THHN. The starter's adjustable electronic overload is set at 34 A, and the branch-circuit protection is a 90 A inverse-time breaker. During the last hour of each shift the overload trips while the conveyor is fully loaded; a clamp meter reads a steady 33 A per phase. The maintenance supervisor wants to raise the overload to 45 A and change the breaker to 100 A 'so it stops nuisance tripping.' The starter is not within sight of the motor.
1. Verify the branch-circuit conductors, the overload setting, and the 90 A breaker against Article 430.
Read the worked answer
Motor full-load current: Use Table 430.250, not the nameplate, for conductor and short-circuit protection sizing. 25 hp at 460 V = 34 A FLC (the 32 A nameplate value is used only for overload sizing).
Branch-circuit conductors (430.22):
- 125% x 34 A = 42.5 A
- 8 AWG Cu at 75 °C = 50 A ✓ - acceptable because the lugs are marked 75 °C.
- Caution: if the lugs were not marked for 75 °C, 110.14(C) would force the 60 °C column (40 A), and 8 AWG would be too small; 6 AWG would be needed. This is the most common error in motor branch circuits at this size.
Overload (430.32(A)(1)): Service factor 1.15 (not less than 1.15) allows sizing at 125% of the nameplate current: 32 x 1.25 = 40 A maximum.
- The 34 A setting is legal but leaves only 1 A of margin over the measured 33 A running current - it is set too close to normal running current.
Branch-circuit short-circuit and ground-fault protection (430.52, Table 430.52):
- Inverse-time breaker: 250% x 34 A = 85 A → next standard size 90 A is permitted by Exception No. 1 to 430.52(C)(1).
- The 90 A breaker is correct as installed and does not need to match the 50 A conductor ampacity - 240.4(G) and 430.52 recognize that the overload device, not the breaker, protects the motor and its conductors from overload.
2. Evaluate the supervisor's proposal to raise the overload to 45 A and install a 100 A breaker.
Read the worked answer
Raising the overload to 45 A is not permitted. 430.32(A)(1) caps the setting at 125% of nameplate current (40 A) for a motor with a 1.15 service factor. 45 A is 141% of nameplate. Provisions for going to the next higher size exist only for cases where the motor cannot start or accelerate (check 430.32 as adopted in Kentucky and the overload manufacturer's listing), not to cover a running overload on a fully loaded conveyor.
Raising the breaker to 100 A is not permitted either. 100 A / 34 A = 294%, exceeding the 250% table value, and the single permitted step up to the next standard size (90 A) has already been used.
What the data actually says: The motor is drawing 33 A on a 34 A FLC motor - normal loading. The trip is a setting problem plus a possible mechanical problem, not an overcurrent-protection problem. Correct actions:
- Set the overload to a value that protects the motor but rides through normal load - up to the 40 A maximum, per the overload manufacturer's table.
- Investigate the mechanical cause of the last-hour trips: belt tension, bearing wear, product accumulation, conveyor binding, or voltage unbalance on the three phases.
- Record the change and the measured running current.
The supervisor's plan would leave the motor unprotected between 40 A and 45 A running current and would remove the fault-current protection margin that 430.52 was designed to provide.
3. What does the 45 ft distance between the starter and the motor require, and what rating must a local disconnect have?
Read the worked answer
Disconnect location (430.102(B)): A disconnecting means must be provided within sight of the motor. 'Within sight' means visible and not more than 50 ft - but visibility is the key word, and a starter in another room or around a corner is not within sight even at 45 ft. Options:
- Install a local disconnect at the motor, or
- Use the exception permitting the disconnect to be out of sight only if it is capable of being locked in the open position (lockable means a permanent means of locking, not a chain and padlock improvised on a breaker handle). For a conveyor, a local lockable disconnect is also the practical lockout/tagout point.
Disconnect rating (430.110): The disconnecting means must have an ampere rating at least 115% of FLC = 34 x 1.15 = 39.1 A. A 60 A, 3-pole, horsepower-rated switch or a 60 A non-fused disconnect is typical and acceptable. The disconnect must also be rated for the horsepower (not just the amperes) - a 60 A general-duty switch not marked with a 25 hp rating at 460 V would be a violation even though the amperes look adequate.
Do not forget 430.102(A): the controller itself must be located within sight of the motor, or be capable of being locked open, for the same reason.