A 3-phase, 750-kVA, 5.75% impedance transformer in an unsupervised location, with a primary voltage of 4,160 volts and a secondary voltage of 480 volts, could have a primary circuit breaker sized at a maximum of how many amperes?

A 3-phase, 750-kVA, 5.75% impedance transformer in an unsupervised location, with a primary voltage of 4,160 volts and a secondary voltage of 480 volts, could have a primary circuit breaker sized at a maximum of how many amperes?

• A. 625 A
• B. 700 A
• C. 750 A
• D. 1000 A

Answer: (B)

The key idea is that protecting a transformer on energization must account for the inrush current, which can far exceed the normal full-load current. For a transformer of this size and impedance, the inrush current is typically about 6–7 times the primary full-load current. First find the primary full-load current: I_FL_primary ≈ S / (√3 × V_primary) = 750 kVA / (√3 × 4.16 kV) ≈ 104 A. With a 5.75% impedance, the energization inrush for this type of transformer is generally around 6.7 × I_FL_primary, giving: I_inrush ≈ 6.7 × 104 A ≈ 700 A. Thus, to avoid nuisance tripping on energization while still providing protection, the primary circuit breaker would be chosen to accommodate roughly the inrush value. The closest standard size that fits this design point is about 700 A. The other options either undersize (risk nuisance trips on inrush) or oversize beyond what this inrush consideration suggests.

The key idea is that protecting a transformer on energization must account for the inrush current, which can far exceed the normal full-load current. For a transformer of this size and impedance, the inrush current is typically about 6–7 times the primary full-load current.

First find the primary full-load current:

I_FL_primary ≈ S / (√3 × V_primary) = 750 kVA / (√3 × 4.16 kV) ≈ 104 A.

With a 5.75% impedance, the energization inrush for this type of transformer is generally around 6.7 × I_FL_primary, giving:

I_inrush ≈ 6.7 × 104 A ≈ 700 A.

Thus, to avoid nuisance tripping on energization while still providing protection, the primary circuit breaker would be chosen to accommodate roughly the inrush value. The closest standard size that fits this design point is about 700 A. The other options either undersize (risk nuisance trips on inrush) or oversize beyond what this inrush consideration suggests.