Motor Circuit Overload and Short-Circuit Protection Coordination
A 50 hp, 460 V, three-phase, 60 Hz induction motor with a 1.15 service factor drives a conveyor. Nameplate FLA is 62 A. The motor is fed from a combination starter with a thermal overload relay and a 150 A instantaneous-trip circuit breaker. The branch-circuit conductors are 4 AWG THHN copper in conduit. The controller is a NEMA-rated starter with a marked short-circuit current rating of 10 kA. Available fault current at the starter is 22 kA. The overload relay is set at 65 A. The operator reports nuisance tripping on hot days.
1. Verify the branch-circuit conductor and short-circuit protection sizing against Article 430.
Read the worked answer
Motor FLA from Table 430.250 for a 50 hp, 460 V motor is 65 A (not the nameplate 62 A - Table values are used for conductor and protection sizing). Branch-circuit conductors must be at least 125% of FLA: 65 × 1.25 = 81.25 A, so 4 AWG THHN at 75°C (85 A) is acceptable. The instantaneous-trip breaker may be sized up to 800% of FLA for a standard breaker (430.52(C)(1) Ex. 1 allows up to 1300% if the standard size does not hold starting current, but 800% is the general maximum): 65 × 8 = 520 A, so a 150 A breaker is well within the allowance - but it may nuisance trip if starting current is high. The 150 A breaker is acceptable for short-circuit protection.
2. Evaluate the overload relay setting and the nuisance tripping.
Read the worked answer
Overload protection is sized per 430.32: for a motor with a service factor of 1.15, the overload must trip at no more than 125% of nameplate FLA. 62 × 1.25 = 77.5 A. A 65 A setting is below that maximum and is therefore permitted, but it may be too close to the running current on hot days, causing nuisance tripping. The relay should be set to the motor nameplate FLA or up to 125% (77.5 A). Nuisance tripping on hot days also suggests ambient compensation is needed - verify the overload is ambient-compensated and that the starter enclosure ventilation is adequate. If the motor actually draws near 65 A, the 65 A setting leaves no margin; raise to 75–77 A.
3. The starter has a 10 kA SCCR but 22 kA is available. What must be done, and does the breaker change?
Read the worked answer
The equipment must be applied within its short-circuit current rating per 110.10 and 430.53. A 10 kA SCCR starter on a 22 kA system is a violation. Options: (1) replace the starter with one rated 22 kA or higher; (2) add a current-limiting device ahead of the starter (e.g., fuses or a current-limiting breaker) that coordinates to raise the assembly SCCR - the combination must be listed and marked; (3) increase impedance to reduce available fault current. Changing the 150 A breaker to a current-limiting type may raise the assembly rating only if the combination is listed. The reviewer should require a listed combination with a marked SCCR ≥ 22 kA.