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Sizing Transformers' Overcurrent Protection
The charts below are based on 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
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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 | 112.50 KVA | 175 AMPS |
| 600 AMPS | 150 KVA | 225 AMPS |
| 800 AMPS | 225 KVA | 350 AMPS |
| 1200 AMPS | 300 KVA | 450 AMPS |
Overcurrent Protection for
Single Phase Transformers
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240 VOLTS
CIRCUIT BREAKER | TRANSFORMER
KVA | 480 VOLTS
CIRCUIT BREAKER |
| 90 AMPS | 30 KVA | 50 AMPS |
| 140 AMPS | 45 KVA | 70 AMPS |
| 2250 AMPS | 75 KVA | 125 AMPS |
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