The primary fuses and circuit breakers can be sized at approximately 125%, or the next standard ampere rating, for the typical 480-volt to 208/120-volt, 15 kVA to 45 kVA dry-type transformers used in commercial installations because these transformers typically have low primary inrush currents. True or False?

The primary fuses and circuit breakers can be sized at approximately 125%, or the next standard ampere rating, for the typical 480-volt to 208/120-volt, 15 kVA to 45 kVA dry-type transformers used in commercial installations because these transformers typically have low primary inrush currents. True or False?

• A. True
• B. False
• C. Only for 50 kVA units
• D. Not specified

Answer: (A)

The protection on a transformer's primary is chosen to survive the energization inrush while still reacting to faults. For typical 480 V to 208/120 V dry-type transformers in the 15 kVA to 45 kVA range, the primary inrush is relatively modest, so sizing the primary fuses or circuit breakers at about 125% of the transformer's full-load primary current is a sensible rule of thumb. You compute the full-load current from the transformer's rating and primary voltage (for a 3-phase system, I = kVA / (V × √3)), multiply by 1.25, and then pick the next standard device size. For example, a 15 kVA unit on a 480 V primary has a primary full-load current around 18 A, so 125% is about 22.5 A, and a 25 A device would be used. This approach balances allowing typical inrush with protection against faults, and it aligns with common NEC/LUL practices for this range of transformers. If the inrush were much higher or the application different, a different protection strategy might be needed, but in this typical case the statement holds true.

The protection on a transformer's primary is chosen to survive the energization inrush while still reacting to faults. For typical 480 V to 208/120 V dry-type transformers in the 15 kVA to 45 kVA range, the primary inrush is relatively modest, so sizing the primary fuses or circuit breakers at about 125% of the transformer's full-load primary current is a sensible rule of thumb. You compute the full-load current from the transformer's rating and primary voltage (for a 3-phase system, I = kVA / (V × √3)), multiply by 1.25, and then pick the next standard device size. For example, a 15 kVA unit on a 480 V primary has a primary full-load current around 18 A, so 125% is about 22.5 A, and a 25 A device would be used. This approach balances allowing typical inrush with protection against faults, and it aligns with common NEC/LUL practices for this range of transformers. If the inrush were much higher or the application different, a different protection strategy might be needed, but in this typical case the statement holds true.