Sizing Transformers'

Overcurrent Protection

The charts below are based on the Article 450 of the National Electrical Code as well as tried-and-true methods commonly used in the field. These charts are used to size the overcurrent protection for the transformer on both the primary side and the secondary side. The overcurrent protection is typically a circuit breaker.

Overcurrent Protection for Three Phase Transformers
208 Volts Circuit Breaker	Transformer KVA	480 Volts Circuit Breaker
110 AMPS	30 KVA	50 AMPS
175 AMPS	45 KVA	70 AMPS
300 AMPS	75 KVA	125 AMPS
400 AMPS	112.5 KVA	175 AMPS
600 AMPS	150 KVA	225 AMPS
800 AMPS	225 KVA	350 AMPS
1200 AMPS	300 KVA	450 AMPS

Single Phase Transformers are not used as often as three phase transformers. Where three phase transformers are common in commercial distribution applications, single phase transformers are used for specialty applications or small commercial buildings. An example of this would be a 480v,1ø circuit supplying power, via a step-down transformer, to a 240v,1ø wood lathe in a cabinet-making shop.