25 HP Conveyor Motor: Conductor, Disconnect, Overload, and a Tripping Breaker
A 480-volt, three-phase, 25-horsepower, NEMA Design B conveyor motor is being installed in a warehouse. Its nameplate reads 32.0 amperes full-load current at 460 volts, service factor 1.15, code letter G, 40 degrees C ambient, insulation class F. The motor is fed from an MCC 60 feet away with three THWN-2 copper conductors and a green equipment grounding conductor in EMT. The combination starter contains a 90-ampere inverse-time circuit breaker already installed in the MCC, and the overload relay is set at 34 amperes. A non-fusible safety switch within sight of the motor serves as the disconnecting means. The conveyor starts fully loaded, and the breaker trips during acceleration on roughly one start in five. The same MCC section also feeds an existing 10-horsepower exhaust fan on its own branch circuit; its nameplate reads 13.5 amperes, and the Table 430.250 value for 10 horsepower at 460 volts is 14 amperes. The maintenance supervisor wants to quote the job as simply putting in a bigger breaker.
1. Size the branch-circuit conductors, the disconnecting means, and the overload protection, and determine whether the existing 90-ampere breaker is permitted.
Read the worked answer
Motor full-load current. 430.6(A)(1) requires Table 430.250 values for conductor and short-circuit protection sizing: 25 horsepower at 460 volts = 34 A. Nameplate current is used only for the overload relay (430.6(A)(2)).
Branch-circuit conductors (430.22(A)). 125% x 34 = 42.5 A. A THWN-2 conductor on 75 degrees C terminations must have a 75 degrees C ampacity of at least 42.5 A: 8 AWG copper at 50 A works; 10 AWG at 35 A does not. Confirm the EMT fill per Chapter 9, Table 1 - three 8 AWG THHN conductors fit a 1/2-inch EMT, so 3/4 inch is generous but compliant.
Disconnecting means (430.110(A)). Ampere rating not less than 115% of full-load current: 1.15 x 34 = 39.1 A, so a 40-ampere switch is the minimum, and it must also have a horsepower rating not less than the motor's (430.109). Mounting it within sight of the motor satisfies 430.102(A), and 430.102(B) is satisfied for the controller by the same switch.
Overload protection (430.32(A)(1)). With a service factor of 1.15, the maximum setting is 125% of nameplate current: 1.25 x 32.0 = 40 A. The existing 34-ampere setting is 106% of nameplate and is within the permitted maximum, though it is on the high side; if nuisance overload trips occur, 430.32(C) allows a documented increase, but not beyond the next permitted step without justification.
Branch short-circuit protection (430.52(C)(1)). For an inverse-time breaker the maximum is 250% of full-load current: 2.50 x 34 = 85 A. The next higher standard rating (240.6) is 90 A, and 430.52(C)(1) Exception No. 1 permits using it. The 90-ampere breaker is permitted - the installer did not overfuse the branch circuit.
2. The 90-ampere breaker still trips during acceleration. What are the permitted options, what must be verified first, and how does a change affect the MCC feeder protection?
Read the worked answer
First, verify the trip is really a starting-current problem. Check voltage at the motor while starting (60 feet of 8 AWG at 34 A drops only about 3 volts, roughly 0.5%, so voltage drop is unlikely to be the cause), verify all three phase currents are balanced, confirm the overload relay is a proper class for the starting duty, and confirm the conveyor is not jammed. Code compliance does not fix a mechanical overload; a motor that is asked to accelerate a seized conveyor will trip any breaker you install.
Permitted upsizing. 430.52(C)(1) Exception No. 2 permits increasing the inverse-time breaker above 250% of full-load current, up to a maximum of 400% (4.00 x 34 = 136 A), where the breaker will not hold the starting current and the overload relay provides the overload protection. Standard sizes within that envelope would be 100 A or 125 A. This is legal because the branch conductors (8 AWG, 50 A) are protected against short circuit and ground fault by the breaker, while overload protection is provided separately by the relay. Document the change: the exception is written for exactly this situation, but an inspector will want to see why the increase is necessary and that 136 A is not exceeded.
Do not exceed 400% and do not use a larger breaker as a substitute for diagnosing the overload setting. If the motor is actually running overloaded, the correct fix is the mechanical load or a larger motor, not a bigger breaker.
Effect on the MCC feeder. Under 430.24, the feeder conductors must have an ampacity of 125% of the largest motor plus the sum of the others: (1.25 x 34) + 14 = 56.5 A, which means 6 AWG copper (65 A at 75 degrees C), not 8 AWG. Under 430.62(A), the feeder short-circuit protection cannot be less than the largest branch-circuit device rating plus the sum of the other motor full-load currents: 90 + 14 = 104 A, or a 110-ampere standard rating. If the branch breaker is raised to 125 A, the feeder device becomes 125 + 14 = 139 A, or a 150-ampere standard rating, and the feeder conductor ampacity and the MCC bus rating must both be rechecked. That knock-on effect is why a bigger branch breaker is never a quote-and-go change.