To determine the required amp rating breaker to use, follow these simple steps:
1. Click calculator here, or on the main navigation bar on the left.
2. Input rated KVA and primary voltage of your transformer and click calculate.
3. Take the resulting amperage rating and multiply it by a minimum of 1.2 (recommended 1.4) do determine the amperage required.
4. Breakers are made in standard ratings, you should purchase a breaker rated equal to or higher than your requirements.
When reverse feeding a transformer the breaker required will be larger due to reduced primary voltage. Follow the same procedure to determine your breaker requirements.
NOTE: In case of discrepancy, provisions of NEC 450-3 and/or other local/national codes shall prevail over the above general guidelines.