A 7200 to 4160 V three-phase transformer 150 kVA in a supervised location is fed from a feeder tap protected with a three-phase fused safety switch. The transformer powers a circuit breaker in an enclosure. The maximum size of fuses for the safety switch is ___ amps. Z = 7%.

A 7200 to 4160 V three-phase transformer 150 kVA in a supervised location is fed from a feeder tap protected with a three-phase fused safety switch. The transformer powers a circuit breaker in an enclosure. The maximum size of fuses for the safety switch is ___ amps. Z = 7%.

• A. 30
• B. 35
• C. 80
• D. 70

Answer: (B)

The concept here is sizing the primary protection for a transformer using its full-load current and impedance to coordinate with fault currents. First determine the transformer’s primary full-load current: I_FL = S / (√3 × V_primary) = 150 kVA / (1.732 × 7.2 kV) ≈ 12 A. The transformer's impedance of 7% means the prospective primary fault current is roughly I_SC ≈ I_FL × (100 / Z%) = 12 A × (100/7) ≈ 172 A. You want a fuse on the feeder tap that will carry normal load but will clear a fault without nuisance tripping. A practical rule used in this context is to size the primary fuse at about 3 times the transformer’s full-load current, which gives a balance between tolerating inrush and providing protection: 3 × 12 A ≈ 36 A, with the closest standard value being 35 A. Thus, the maximum fuse size is about 35 A. The other options would be either too small to accommodate inrush without nuisance trips, or too large and risk allowing excessive current during overloads, even though the fault current is high (around 172 A) due to the 7% impedance.

The concept here is sizing the primary protection for a transformer using its full-load current and impedance to coordinate with fault currents. First determine the transformer’s primary full-load current: I_FL = S / (√3 × V_primary) = 150 kVA / (1.732 × 7.2 kV) ≈ 12 A. The transformer's impedance of 7% means the prospective primary fault current is roughly I_SC ≈ I_FL × (100 / Z%) = 12 A × (100/7) ≈ 172 A. You want a fuse on the feeder tap that will carry normal load but will clear a fault without nuisance tripping. A practical rule used in this context is to size the primary fuse at about 3 times the transformer’s full-load current, which gives a balance between tolerating inrush and providing protection: 3 × 12 A ≈ 36 A, with the closest standard value being 35 A.

Thus, the maximum fuse size is about 35 A. The other options would be either too small to accommodate inrush without nuisance trips, or too large and risk allowing excessive current during overloads, even though the fault current is high (around 172 A) due to the 7% impedance.