In a setup with a 7200 to 4160 V three-phase transformer, 150 kVA, in a supervised location, the feeder tap is protected with a three-phase circuit breaker in an enclosure. The transformer has 7% impedance and powers a three-phase fused safety switch. According to NEC 450.3, the fuses in the switch may be no larger than ___ amps.

In a setup with a 7200 to 4160 V three-phase transformer, 150 kVA, in a supervised location, the feeder tap is protected with a three-phase circuit breaker in an enclosure. The transformer has 7% impedance and powers a three-phase fused safety switch. According to NEC 450.3, the fuses in the switch may be no larger than ___ amps.

• A. 50
• B. 70
• C. 125
• D. 45

Answer: (D)

The key idea is how NEC 450.3 guides fuse sizing for a transformer feeding a switch: you size the fuse on the transformer’s low‑voltage side so it will protect the transformer from faults but still tolerate normal operation and inrush. Compute the transformer’s secondary full‑load current: I2 = S / (√3 × V2) = 150 kVA / (√3 × 4.16 kV) ≈ 20.8 A. The transformer has 7% impedance, so the maximum short‑circuit (fault) current on the secondary is roughly: Isc ≈ I2 × (100 / z%) = 20.8 A × (100 / 7) ≈ 297 A. Fuses for a transformer secondary in this configuration are chosen to coordinate with this fault current while carrying the normal load (about 20.8 A). Among standard fuse sizes, 45 A is the largest size that properly coordinates with the transformer’s impedance and the switch rating for this arrangement. Therefore, the fuses in the switch may be no larger than 45 amps.

The key idea is how NEC 450.3 guides fuse sizing for a transformer feeding a switch: you size the fuse on the transformer’s low‑voltage side so it will protect the transformer from faults but still tolerate normal operation and inrush.

Compute the transformer’s secondary full‑load current:

I2 = S / (√3 × V2) = 150 kVA / (√3 × 4.16 kV) ≈ 20.8 A.

The transformer has 7% impedance, so the maximum short‑circuit (fault) current on the secondary is roughly:

Isc ≈ I2 × (100 / z%) = 20.8 A × (100 / 7) ≈ 297 A.

Fuses for a transformer secondary in this configuration are chosen to coordinate with this fault current while carrying the normal load (about 20.8 A). Among standard fuse sizes, 45 A is the largest size that properly coordinates with the transformer’s impedance and the switch rating for this arrangement. Therefore, the fuses in the switch may be no larger than 45 amps.