Conveyor Motor Branch Circuit: Nameplate vs. Table Current
A machine shop installs a 25 hp, 480 V, three-phase, design B squirrel-cage motor on a conveyor. Nameplate data: 32.0 A, service factor 1.15, code letter G. The branch circuit is 100 ft of 8 AWG THWN-2 copper in 1 in. EMT, fed from a combination starter containing a 75 °C-rated inverse-time circuit breaker and Class 10 overload relays. The ceiling ambient at the starter is 40 °C. A journeyman sized the conductors and the breaker from the 32.0 A nameplate value, then set the overload dial at 32 A. On startup the breaker tripped twice, so he proposed replacing it with a 100 A breaker and dialing the overloads up to 40 A to 'make it hold.'
1. Size the branch-circuit conductors and state whether the installed 8 AWG THWN-2 copper is acceptable, showing the ampacity adjustments.
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Conductors are sized from the Table values, not the nameplate.
- Motor full-load current from Table 430.250 for a 25 hp, 480 V, 3-phase motor = 34 A. (The table value is used for conductors, disconnects and short-circuit protection; the nameplate value is used only for overloads.)
- Minimum conductor ampacity, 430.22(A): 34 A × 1.25 = 42.5 A.
- 8 AWG Cu THWN-2: 50 A at 75 °C, 55 A at 90 °C.
- Ambient correction for 40 °C from the 90 °C column (Table 310.15(B)(2)(a)): 0.91. 55 A × 0.91 = 50.1 A.
- Termination limit, 110.14(C): the 75 °C ampacity (50 A) is the ceiling because the breaker is 75 °C rated.
Adjusted ampacity 50 A ≥ required 42.5 A, so the 8 AWG THWN-2 is acceptable - but only because the run is copper and the ambient correction leaves it above 42.5 A. 8 AWG at 60 °C (40 A) would have failed, which is a common trap when someone reads the wrong column.
2. Set the overload relay and determine the maximum permitted inverse-time breaker rating. Is the proposed 100 A breaker legal?
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Overload (430.32(A)(1)) - uses nameplate current: service factor 1.15 means the trip setting may not exceed 125% of nameplate current. 32.0 A × 1.25 = 40 A maximum; the dial may not go higher. Dialing to 40 A is at the top of the range, but a Class 10 device should normally be set closer to nameplate so the motor is actually protected - the conveyor tripping is a mechanical/start-time problem, not a reason to raise overloads.
Short-circuit and ground-fault protection (430.52(C)(1)): inverse-time breaker at 250% of 34 A = 85 A. 85 A is not a standard rating (240.6), so Exception No. 1 permits the next higher standard size = 90 A maximum.
A 100 A breaker exceeds 90 A and is not permitted under the standard rule. It could only be justified by 430.52(C)(1) Exception No. 2 (listed motor-circuit protector or engineering supervision) after verifying the controller, disconnect and conductor withstand - not as a field guess. The correct fix is to look at why the 90 A (or smaller) device trips: incorrect trip curve, voltage drop during start, or a mechanical overload.
3. The disconnect is a non-fused 60 A safety switch. Is that acceptable, and what other requirements govern its location?
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Rating: 430.110(A) requires the disconnecting means to have an ampere rating of at least 115% of the motor full-load current: 34 A × 1.15 = 39.1 A. A 60 A switch is well above that, so the rating is fine; a 40 A switch would also be acceptable.
Location (430.102): the disconnect must be within sight of the motor and within sight of the driven machinery, and within 50 ft (the controller disconnecting means). 'Within sight' means visible and not more than 50 ft away. If the switch cannot be seen from the conveyor drive, a second disconnect or a lockable controller within sight is required.
Other items to check:
- 430.107 - the disconnect must be readily accessible.
- 430.109 - must be a listed type (motor-circuit switch, molded-case switch, etc.).
- The equipment grounding conductor from the starter to the motor must be sized per 250.122 from the 90 A protection: 8 AWG Cu.
- Overload protection is separate from the disconnect; the 60 A switch provides no overload protection.