If the primary full-load current is 104.21 A, the calculated primary circuit breaker size (104.21 A × 6.0) is approximately which amperage?

If the primary full-load current is 104.21 A, the calculated primary circuit breaker size (104.21 A × 6.0) is approximately which amperage?

• A. 520 A
• B. 625 A
• C. 700 A
• D. 104 A

Answer: (B)

Sizing the primary circuit breaker uses multiplying the transformer’s primary full-load current by a factor to cover inrush and startup currents. Here, that factor is 6.0. Multiply 104.21 A by 6.0 to get 625.26 A. Since we’re asked for an approximate amperage and the options are standard values, the closest practical match is about 625 A. Therefore, the primary breaker size is around 625 A.

Sizing the primary circuit breaker uses multiplying the transformer’s primary full-load current by a factor to cover inrush and startup currents. Here, that factor is 6.0. Multiply 104.21 A by 6.0 to get 625.26 A. Since we’re asked for an approximate amperage and the options are standard values, the closest practical match is about 625 A. Therefore, the primary breaker size is around 625 A.