Calculating the Maximum Circuit Breaker Size for a 75 KVA Transformer

Calculation of Maximum Circuit Breaker Size

The maximum size circuit breaker permitted by Note 1 to NEC Table 450.3(A) for protecting the primary of a 4160 to 480-V, 3-phase 75 KVA transformer is determined by first calculating the primary current of the transformer using the formula:

Primary Current (Iprimary) = (Transformer VA Rating) / (Primary Voltage x √3)

For a 75 KVA transformer, this calculation would be:

Iprimary = (75,000 VA) / (4160 V x √3) ≈ 10.4 A

According to the NEC, the maximum size breaker for a transformer with an impedance of 1.7% is 250% of the primary current if the transformer is in a non-supervised location:

Maximum Breaker Size = 2.5 x Iprimary = 2.5 x 10.4 A ≈ 26 A

As none of the options provided (109.2 A, 130.0 A, 144.8 A) match the calculated value, it seems there may be an error in the given options or question details. However, as per standard safety regulations, continuous use of a circuit breaker is limited to 80% of the breaker current capacity. Therefore, the correct answer should be a circuit breaker that is appropriately sized based on the primary current, multiplied by 250% as per NEC guidelines, and not exceeding the safety margin of 80% continuous use.