A feeder is tapped and a circuit breaker in an enclosure protects the tap. The transformer is 4160 to 2400 V, three-phase, 225 kVA with 4% impedance, located in a nonsupervised location, and it supplies a three-phase main breaker panelboard. Under NEC 450.3, the maximum size of the circuit breaker in the CB enclosure is ___ amps.

A feeder is tapped and a circuit breaker in an enclosure protects the tap. The transformer is 4160 to 2400 V, three-phase, 225 kVA with 4% impedance, located in a nonsupervised location, and it supplies a three-phase main breaker panelboard. Under NEC 450.3, the maximum size of the circuit breaker in the CB enclosure is ___ amps.

• A. 200
• B. 175
• C. 80
• D. 150

Answer: (A)

NEC 450.3 governs how large the overcurrent protection can be for transformers, including the protection used on a feeder tap that supplies a transformer or a panelboard. In this setup, a feeder is tapped to the 4160 V primary of a 225 kVA transformer (4160 to 2400 V, three-phase, 4% impedance) and a circuit breaker in a separate enclosure protects that tap. The location is nonsupervised, and the transformer is feeding a three-phase main breaker panelboard. First, find the primary full-load current: I_primary = 225 kVA / (√3 × 4.16 kV) ≈ 31 A. The NEC 450.3 rule for this kind of tap using a transformer of this size and voltage arrangement specifies a maximum overcurrent device rating on the primary tap, and for a 225 kVA transformer at 4160 V the allowed maximum primary OCPD is 200 A. The 4% impedance is provided so you could compute short-circuit current and check coordination, but it does not change the ceiling set by the code table for this configuration. Thus, the largest circuit breaker permitted in the CB enclosure protecting the tap is 200 A. The other options fall short of this code-stated limit and are not the maximum allowable.

NEC 450.3 governs how large the overcurrent protection can be for transformers, including the protection used on a feeder tap that supplies a transformer or a panelboard. In this setup, a feeder is tapped to the 4160 V primary of a 225 kVA transformer (4160 to 2400 V, three-phase, 4% impedance) and a circuit breaker in a separate enclosure protects that tap. The location is nonsupervised, and the transformer is feeding a three-phase main breaker panelboard.

First, find the primary full-load current: I_primary = 225 kVA / (√3 × 4.16 kV) ≈ 31 A. The NEC 450.3 rule for this kind of tap using a transformer of this size and voltage arrangement specifies a maximum overcurrent device rating on the primary tap, and for a 225 kVA transformer at 4160 V the allowed maximum primary OCPD is 200 A. The 4% impedance is provided so you could compute short-circuit current and check coordination, but it does not change the ceiling set by the code table for this configuration.

Thus, the largest circuit breaker permitted in the CB enclosure protecting the tap is 200 A. The other options fall short of this code-stated limit and are not the maximum allowable.