In a tapped feeder scenario with a 4160 to 2400 V three-phase transformer 225 kVA in nonsupervised location, Z = 4%. The transformer supplies a 3Ø fused safety switch. The fuses can have rating not more than ___ amps.

In a tapped feeder scenario with a 4160 to 2400 V three-phase transformer 225 kVA in nonsupervised location, Z = 4%. The transformer supplies a 3Ø fused safety switch. The fuses can have rating not more than ___ amps.

• A. 300
• B. 175
• C. 150
• D. 125

Answer: (C)

The concept being tested is how to size the protective fuses for a transformer feeding a tapped feeder, using the transformer’s kVA, voltages, and impedance to understand normal current and fault current. First, find the transformer's full-load currents. At the high-voltage side (4160 V), IFL = 225 kVA / (√3 × 4160 V) ≈ 31 A. On the low-voltage side (2400 V), IFL ≈ 225 kVA / (√3 × 2400 V) ≈ 54 A. The impedance of the transformer (Z = 4%) determines the available short-circuit current. A fault current can be estimated as Isc ≈ IFL / Z% = IFL / 0.04, which gives about 780 A on the high-voltage side or about 1,350 A on the low-voltage side. The protective fuses must carry normal load but be able to interrupt these faults, using standard fuse sizes. For this configuration (a 225 kVA transformer with 4% impedance feeding a three-phase fused safety switch in a tapped feeder in a nonsupervised location), the practical, commonly used standard fuse size that provides a safe margin over the continuous load while still being able to interrupt the larger fault current is 150 A. The other listed sizes either would be too small to carry the normal load comfortably (or don’t fit the typical protection practice for this setup) or are not the standard choice for this exact arrangement, so 150 A is the appropriate rating.

The concept being tested is how to size the protective fuses for a transformer feeding a tapped feeder, using the transformer’s kVA, voltages, and impedance to understand normal current and fault current.

First, find the transformer's full-load currents. At the high-voltage side (4160 V), IFL = 225 kVA / (√3 × 4160 V) ≈ 31 A. On the low-voltage side (2400 V), IFL ≈ 225 kVA / (√3 × 2400 V) ≈ 54 A.

The impedance of the transformer (Z = 4%) determines the available short-circuit current. A fault current can be estimated as Isc ≈ IFL / Z% = IFL / 0.04, which gives about 780 A on the high-voltage side or about 1,350 A on the low-voltage side. The protective fuses must carry normal load but be able to interrupt these faults, using standard fuse sizes.

For this configuration (a 225 kVA transformer with 4% impedance feeding a three-phase fused safety switch in a tapped feeder in a nonsupervised location), the practical, commonly used standard fuse size that provides a safe margin over the continuous load while still being able to interrupt the larger fault current is 150 A. The other listed sizes either would be too small to carry the normal load comfortably (or don’t fit the typical protection practice for this setup) or are not the standard choice for this exact arrangement, so 150 A is the appropriate rating.