Motor Branch Circuit and Overload Protection for a Conveyor
An industrial plant is installing a 25 hp, 460 V, 3-phase, 60 Hz conveyor motor with a nameplate full-load current of 32 A and a service factor of 1.15. The motor is across-the-line started and is protected by a magnetic-only circuit breaker at the motor control center, with a separate thermal overload relay in the starter. The branch circuit conductors run 150 ft in rigid metal conduit with three current-carrying conductors in an ambient of 40 degrees C. Terminations are rated 75 degrees C. The electrician must select the branch-circuit conductors, the short-circuit and ground-fault protective device, and the overload relay setting, and explain the difference between the two levels of protection.
1. Size the branch-circuit conductors and the short-circuit/ground-fault protective device.
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Use Table 430.250, not the nameplate, for conductor sizing: a 25 hp 460 V motor has an FLC of 34 A. Branch-circuit conductors must be at least 125% of FLC per 430.22: 34 x 1.25 = 42.5 A, so select a 45 A conductor. Correct for 40 degrees C ambient and 75 degrees C terminations: a 6 AWG copper THWN at 75 degrees C is 65 A, derated for 40 degrees C ambient (0.88 per Table 310.15(B)(2)(a)) to about 57 A, which exceeds 42.5 A. For short-circuit protection, an inverse-time breaker is sized per Table 430.52 at 250% of FLC: 34 x 2.5 = 85 A, so a standard 90 A breaker is permitted (standard size per 240.6). The breaker protects the circuit from short circuits and grounds, not overloads.
2. What overload relay setting should be used, and why is it different from the branch breaker?
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The overload relay protects the motor from sustained overcurrent and is set from the nameplate current, not the table FLC. Per 430.32, for a motor with a service factor of 1.15, the overload device is set at no more than 125% of nameplate FLC: 32 x 1.25 = 40 A. If the relay cannot carry the motor without tripping, 430.32(C) allows increasing to 140% (44.8 A), but the setting must not exceed the nameplate-based value otherwise. The branch breaker at 90 A is for short-circuit and ground-fault protection only and is intentionally much larger than the running current; the overload relay is the running protection. Document the nameplate current and the selected setting on the starter.
3. The motor trips the overload on hot days. What should the electrician investigate before increasing the setting?
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First verify actual running current with a clamp meter against nameplate 32 A; a high reading suggests mechanical binding, belt tension, bearing wear, or voltage imbalance. Check voltage imbalance across phases: even 2-3% imbalance can raise motor current significantly. Verify the overload relay is the correct class and that ambient compensation is set for the enclosure temperature. Confirm conductor derating and terminations are correct. Only after ruling out mechanical and voltage causes should the electrician consider the 430.32(C) allowance to raise the setting, and then only to the maximum permitted. Increasing the setting to mask a mechanical problem risks motor failure.