Nuisance Tripping on a 10 hp Motor Branch Circuit
A 10 hp, 208 V, three-phase continuous-duty motor drives a conveyor in a small shop. The motor nameplate reads 28 A; the supply is 208Y/120 V, and the branch circuit is THHN copper in EMT with 75°C terminations. The existing circuit uses 10 AWG conductors and a 60 A inverse-time circuit breaker. The overload relay is set to 30 A and the motor trips after about 20 minutes under normal load. The electrician who installed it says the 60 A breaker is fine because the motor starts without tripping, and the 10 AWG is fine because the nameplate is only 28 A. Ambient is about 30°C; terminations are 75°C. The motor is not a listed assembly with specific overload markings.
1. What is the minimum branch-circuit conductor size for this motor, and is the existing 10 AWG copper acceptable?
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The branch-circuit conductors are sized from the motor full-load current in Table 430.250, not from the nameplate current. For a 10 hp, 208 V, three-phase motor, the table value is 30.8 A. Under 430.22(A), branch-circuit conductors for a continuous-duty motor must have an ampacity of at least 125% of the FLC: 30.8 A × 1.25 = 38.5 A. A 10 AWG copper conductor with 75°C insulation has an ampacity of 35 A, which is below 38.5 A, so it is undersized for this motor branch circuit. An 8 AWG copper conductor at 75°C has an ampacity of 50 A and satisfies the 38.5 A minimum. The nameplate current does not determine branch-circuit conductor size; it is used for overload protection. The 30°C ambient does not require correction, but the conductor ampacity must still be at least 38.5 A before any adjustment factors.
2. What is the maximum permitted inverse-time circuit breaker rating for short-circuit and ground-fault protection, and is the 60 A breaker acceptable?
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Short-circuit and ground-fault protection is sized under 430.52. For an inverse-time circuit breaker on a standard motor, the maximum is 250% of the motor FLC from Table 430.250. Using 30.8 A, 250% gives 77 A. The next standard rating above 77 A is 80 A, but 430.52(C)(1) Exception No. 2 permits the next higher standard rating only where the calculated value does not correspond to a standard rating and the higher rating does not exceed the allowed percentage. A 60 A breaker is below 77 A, so it is acceptable from a maximum-rating standpoint. The fact that the motor starts without tripping does not alone prove the branch-circuit protection is correct, but the 60 A inverse-time breaker is not a violation here. The nuisance tripping is more likely caused by the overload relay setting or by the undersized conductors.
3. How should the overload relay be set, and what is the likely cause of the tripping?
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Overload protection is based on the motor nameplate current, not the Table 430.250 FLC. Under 430.32(A)(1), a separate overload device for a continuous-duty motor must be selected to trip at no more than 125% of the nameplate current for a motor with a marked service factor of 1.15 or a temperature rise of 40°C, or 115% otherwise. Here the nameplate is 28 A, so 125% is 35 A. The existing 30 A setting is below that, so it is likely to trip on normal overload even when the motor is healthy. The relay should be set to 35 A or the next standard size not exceeding the permitted value, following the manufacturer's instructions and 430.32. The undersized 10 AWG conductors also create excessive voltage drop and heating, but they do not change the overload sizing rule. Correct both the conductor size and the overload setting, then check actual running current against nameplate.