7.5 HP Conveyor: Breaker, Overload, and Nuisance Tripping
A conveyor at a food plant is driven by a 7.5 HP, 208 V, 3-phase Design B motor. Nameplate data: 22.0 A, 1.15 service factor, 40°C ambient, 75°C terminations. The branch circuit is 8 AWG THHN copper in EMT from a combination starter containing a 70 A inverse-time breaker and a thermal overload relay set at 30 A; the motor disconnect is rated 30 A and is within sight. On cold mornings, roughly one start in ten trips the 70 A breaker, while the overload relay has never tripped. The maintenance supervisor wants to install a 100 A breaker because the motor draws high inrush.
1. Check the installed conductors, the 70 A breaker, and the 30 A disconnect against code minimums. Is anything undersized or oversized improperly?
Read the worked answer
Use the Table 430.250 full-load current for 7.5 HP at 208 V = 24.2 A; the 22.0 A nameplate value is for overload sizing only.
- Conductors (430.22): 125% × 24.2 = 30.3 A. 10 AWG copper at 75°C (35 A) is the minimum; the installed 8 AWG is oversized but fine.
- Breaker (430.52, Table): inverse-time breaker at 250% × 24.2 = 60.5 A. That is not a standard rating, so the next higher standard size, 70 A, is permitted by 430.52(C)(1) Exception No. 1.
- Disconnect (430.110): at least 115% × 24.2 = 27.8 A, so 30 A is the minimum and is compliant.
The 70 A breaker exceeding the 8 AWG conductor ampacity is acceptable in a motor branch circuit because 430.52 protection is short-circuit and ground-fault protection only; running overload protection comes from the overload relay.
2. Evaluate the 30 A overload relay setting.
Read the worked answer
Under 430.32(A)(1), a motor with a service factor of 1.15 must have overload protection selected to trip at not more than 125% of the nameplate current: 22.0 × 1.25 = 27.5 A.
A 30 A setting exceeds that maximum. The result is a motor that can run continuously in a sustained overload and burn up - precisely the failure the overload is installed to prevent. The overload relay has never tripped in service, which is consistent with it being set too high to do its job.
Correct action: set the relay (or select the heater/class) to no more than 27.5 A, sized from the 22.0 A nameplate figure, not the 24.2 A table value. Verify the relay's range brackets the setting and that the overload is of the correct class for the starting duty.
3. Should the supervisor's 100 A breaker be installed? What conditions and checks apply?
Read the worked answer
Not automatically, and not as a first step.
430.52(C)(1) Exception No. 2 does allow an inverse-time breaker to be raised to as much as 400% of FLC where the 250% device will not carry the starting current: 4.00 × 24.2 = 96.8 A, so a 100 A breaker is the absolute ceiling permitted. It is a last resort, not a default.
Before going there:
- Confirm the trip is genuine starting inrush - check for a failing breaker, loose lugs, low voltage during start, a mechanical bind, or too-frequent jogging.
- Verify the breaker enclosure, starter, and breaker terminals accept the conductors, and that the disconnect and controller remain suitable.
- Note the overload still protects the motor and conductors; the bigger breaker does not relieve that duty.
If 100 A still trips, 430.52 permits no larger device - the fault is in the motor, the driven equipment, or the supply, and must be found. Document the justification for the increase in the maintenance record.