40 hp Motor Branch Circuit, Overload, and Short-Circuit Protection
A 40 hp, 480 V, 3-phase, 1,750 rpm TEFC motor drives a conveyor in a paper mill. The nameplate reads 50 A, 1.15 service factor, 40°C ambient, 75°C rise, code letter G. The branch circuit is 150 ft of 6 AWG THHN copper in EMT from a 480 V MCC. Terminals are rated 75°C. The starter has a Class 20 overload relay with a 45-65 A range, currently set at 55 A. The engineer specifies a 150 A inverse-time circuit breaker for short-circuit and ground-fault protection. The motor is not hermetic, not a fire pump, and not multi-speed. Available fault current at the MCC is 22 kA. The electrician asks whether the conductor, overload setting, and breaker are acceptable, and whether voltage drop is a concern.
1. Is the 6 AWG THHN conductor acceptable for the motor branch circuit? Show the sizing and any ambient correction.
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Use the motor FLC from Table 430.250, not the nameplate, for branch-circuit conductors: 40 hp at 480 V is 52 A. 430.22 requires 125% of FLC: 52 × 1.25 = 65 A. 6 AWG Cu THHN has a 90°C ampacity of 75 A and a 75°C ampacity of 65 A. With 75°C terminals, the termination limit is 65 A, so 6 AWG is exactly at the minimum before ambient correction. If the actual ambient at the conductors is 40°C, apply the correction factor: the 75°C ampacity becomes about 57 A, or even using the 90°C column the corrected ampacity is about 68 A but still limited by the 75°C termination; the installation is marginal and likely noncompliant. Use 4 AWG Cu (75°C ampacity 85 A; corrected about 75 A) or reduce the ambient. Verify the equipment grounding conductor and the MCC terminals.
2. Is the Class 20 overload relay set at 55 A acceptable? How should it be set?
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Overload protection is based on the motor nameplate, not the table FLC. With a 1.15 service factor, 430.32(A)(1) permits the overload device to be selected at no more than 125% of nameplate current: 50 × 1.25 = 62.5 A. The Class 20 relay range 45–65 A can be set up to 62.5 A. A setting of 55 A is not a code violation because it is below the maximum, but it is likely to cause nuisance tripping on a conveyor with a high-inertia start. Set it to 60 A or 62.5 A if starting current requires, but do not exceed 62.5 A without the applicable 430.32 exception. Confirm the relay is listed for the motor and that the actual nameplate current is 50 A.
3. Is the 150 A inverse-time breaker acceptable, and is voltage drop a concern?
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Short-circuit and ground-fault protection for a standard inverse-time breaker is 250% of FLC under 430.52: 52 × 2.50 = 130 A. 130 A is not a standard rating; 430.52(C)(1) Exception No. 1 permits the next higher standard rating, 150 A, if the 250% value does not correspond to a standard size, and 150 A is 288% of FLC, within the 400% ceiling. The breaker must also have an interrupting rating at least equal to the 22 kA available fault current. Voltage drop: for 150 ft of 6 AWG Cu at 50 A, 480 V three-phase, VD ≈ 1.732 × 50 × 0.491 × 150 / 1000 ≈ 6.4 V, about 1.3%; that is acceptable for most industrial motors, although the NEC does not mandate a specific voltage-drop limit. The 150 A breaker is acceptable only if the conductor and starter are protected as required and the breaker is listed for the available fault current.