In a 7200 to 4160-volt, 3Ø, 150 kVA transformer installed in a supervised location and supplied from a feeder tap protected with a 3Ø fused safety switch, the maximum size fuses permitted in the safety switch is:

In a 7200 to 4160-volt, 3Ø, 150 kVA transformer installed in a supervised location and supplied from a feeder tap protected with a 3Ø fused safety switch, the maximum size fuses permitted in the safety switch is:

• A. 35 A
• B. 30 A
• C. 80 A
• D. 40 A

Answer: (B)

The key idea is how protection on a transformer feeder is chosen. For a 150 kVA transformer with a 7.2 kV high-voltage primary, the full-load current on the primary is about 12 A (I = S / (√3 × V) ≈ 150,000 / (1.732 × 7,200)). When the feeder tap is protected by a 3-phase fused safety switch, the fuse size is limited by the switch’s own rating. Standard fused safety switches of this type are designed to accept up to 30 A fuses, so that is the maximum size permitted for the unit. Fuses larger than that would exceed the switch’s rating and aren’t allowed, even though the transformer’s actual full-load current is only about 12 A.

The key idea is how protection on a transformer feeder is chosen. For a 150 kVA transformer with a 7.2 kV high-voltage primary, the full-load current on the primary is about 12 A (I = S / (√3 × V) ≈ 150,000 / (1.732 × 7,200)). When the feeder tap is protected by a 3-phase fused safety switch, the fuse size is limited by the switch’s own rating. Standard fused safety switches of this type are designed to accept up to 30 A fuses, so that is the maximum size permitted for the unit. Fuses larger than that would exceed the switch’s rating and aren’t allowed, even though the transformer’s actual full-load current is only about 12 A.