A fused safety switch feeds a 480 to 208Y/120-volt, 3Ø, 30 kVA transformer that supplies power to a 3Ø 100-amp main breaker panelboard. The fuses in the safety switch are permitted to be not larger than how many amps?

A fused safety switch feeds a 480 to 208Y/120-volt, 3Ø, 30 kVA transformer that supplies power to a 3Ø 100-amp main breaker panelboard. The fuses in the safety switch are permitted to be not larger than how many amps?

• A. 80 A
• B. 1100 A
• C. 150 A
• D. 90 A

Answer: (D)

Sizing the primary protection for a transformer fed through a fused safety switch is about matching the protective device to the transformer's primary current while allowing for inrush and normal operation. First, find the transformer's primary full-load current. With a 30 kVA transformer at 480 V (three-phase), the primary current is I_pri = kVA × 1000 / (√3 × V_pri) = 30,000 / (1.732 × 480) ≈ 36 A. Transformers on the primary side are protected with fuses that may be sized to tolerate inrush and the transformer's characteristics, up to a practical maximum. In this setup, the allowed primary overcurrent device rating is effectively up to about 2.5 times the primary full-load current to accommodate inrush while still protecting the transformer. So: 36 A × 2.5 ≈ 90 A. The largest standard fuse size at this voltage that fits this limit is 90 A, which is why the fuses in the safety switch are permitted not to exceed 90 A. The transformer’s secondary current is about 83 A (30 kVA / (√3 × 208 V) ≈ 83 A), which aligns with using a 90 A primary fuse—providing protection without unnecessarily limiting normal operation. The other options either exceed practical protection limits for this setup (1100 A, 150 A) or fall short of the maximum allowed rating (80 A), whereas 90 A represents the correct maximum size permitted by the sizing rule for this transformer and voltage.

Sizing the primary protection for a transformer fed through a fused safety switch is about matching the protective device to the transformer's primary current while allowing for inrush and normal operation.

First, find the transformer's primary full-load current. With a 30 kVA transformer at 480 V (three-phase), the primary current is

I_pri = kVA × 1000 / (√3 × V_pri) = 30,000 / (1.732 × 480) ≈ 36 A.

Transformers on the primary side are protected with fuses that may be sized to tolerate inrush and the transformer's characteristics, up to a practical maximum. In this setup, the allowed primary overcurrent device rating is effectively up to about 2.5 times the primary full-load current to accommodate inrush while still protecting the transformer. So:

36 A × 2.5 ≈ 90 A.

The largest standard fuse size at this voltage that fits this limit is 90 A, which is why the fuses in the safety switch are permitted not to exceed 90 A. The transformer’s secondary current is about 83 A (30 kVA / (√3 × 208 V) ≈ 83 A), which aligns with using a 90 A primary fuse—providing protection without unnecessarily limiting normal operation.

The other options either exceed practical protection limits for this setup (1100 A, 150 A) or fall short of the maximum allowed rating (80 A), whereas 90 A represents the correct maximum size permitted by the sizing rule for this transformer and voltage.