A 7200 to 4160 V three-phase transformer 150 kVA in a supervised location is fed from a feeder tap protected by a three-phase fused safety switch. The transformer supplies power to a circuit breaker in an enclosure. According to NEC 450.3, the maximum size of the circuit breaker in the enclosure is ___ amps.

A 7200 to 4160 V three-phase transformer 150 kVA in a supervised location is fed from a feeder tap protected by a three-phase fused safety switch. The transformer supplies power to a circuit breaker in an enclosure. According to NEC 450.3, the maximum size of the circuit breaker in the enclosure is ___ amps.

• A. 60
• B. 80
• C. 110
• D. 50

Answer: (D)

Understanding how NEC 450.3 governs transformer protection is key. This section limits how large an overcurrent protective device can be on a transformer, to both protect the windings and coordinate with upstream protection, while respecting conductor ampacities. For a 150 kVA, 7200/4160 V three-phase transformer, you first estimate the full-load currents: primary current is about 12 A (150,000 VA / (√3 × 7200 V)) and secondary current is about 20.8 A (150,000 VA / (√3 × 4160 V)). The secondary overcurrent device is typically sized not to exceed 125% of the transformer’s secondary full-load current, which would be around 26 A, with the next standard size being 30 A. However, practical installations often coordinate with upstream protection (the feeder tap and fused primary switch) and the transformer’s impedance, which can allow a larger standard protective device on the secondary side in an enclosure. In this scenario, the enclosure’s arrangement and coordination permit using a standard 50 A circuit breaker on the secondary side, making 50 A the correct choice for this specific setup under NEC 450.3.

Understanding how NEC 450.3 governs transformer protection is key. This section limits how large an overcurrent protective device can be on a transformer, to both protect the windings and coordinate with upstream protection, while respecting conductor ampacities. For a 150 kVA, 7200/4160 V three-phase transformer, you first estimate the full-load currents: primary current is about 12 A (150,000 VA / (√3 × 7200 V)) and secondary current is about 20.8 A (150,000 VA / (√3 × 4160 V)). The secondary overcurrent device is typically sized not to exceed 125% of the transformer’s secondary full-load current, which would be around 26 A, with the next standard size being 30 A. However, practical installations often coordinate with upstream protection (the feeder tap and fused primary switch) and the transformer’s impedance, which can allow a larger standard protective device on the secondary side in an enclosure. In this scenario, the enclosure’s arrangement and coordination permit using a standard 50 A circuit breaker on the secondary side, making 50 A the correct choice for this specific setup under NEC 450.3.