Okay, let's break down the transformer primary protection rules and apply them to your specific situation. The NEC (National Electrical Code) has specific rules for transformer overcurrent protection to prevent damage to the transformer itself.

Understanding the NEC Rules

The NEC allows for overcurrent protection of transformers at either the primary or the secondary. If you protect the secondary, the primary overcurrent protection is allowed to be higher. Primary Protection Only (NEC 450.3(B))

If you're only providing overcurrent protection on the primary side, the maximum overcurrent protection is generally 125% of the transformer's primary current rating for transformers over 9 amps. (For transformers rated 9 amps or less, 167% is allowed).

Primary and Secondary Protection (NEC 450.3(B) exception)

If you have overcurrent protection on the secondary side, you are allowed a higher percentage on the primary side.

Your Specific Scenario (15 kVA, 480V, Single-Phase) Primary Current

15,000 VA / 480V = 31.25 Amps

125% Primary Overcurrent Protection

31.25 Amps 1.25 = 39.06 Amps. The next standard breaker size up is 40 amps.

Addressing your Question

You mentioned having secondary protection at 125%. With this said, if your secondary protection is set at 125% and your primary is a maximum of 250%, you would be compliant.

Primary Current

15,000 VA / 480V = 31.25 Amps

250% Primary Overcurrent Protection

31.25 Amps 2.50 = 78.125 AmpsThis means you can protect the primary at 78 amps. Therefore, upsizing from a 45 amp to a 50 amp breaker is no issue.

Important Considerations1. Conductor Ampacity

This is the most critical point. The conductors feeding the primary side of the transformer MUST be sized with an ampacity that is equal to or greater than the overcurrent protection device. In your example of using a 50A breaker on the primary side, you would need to use wire rated for at least 50A. The 125% doesn't apply to conductors2.

Secondary Overcurrent Protection

Verify that your secondary overcurrent protection is in place and correctly sized. Its purpose is to protect the secondary conductors and the transformer itself from overload conditions on the secondary side.3.

Coordination

Consider coordination between the primary and secondary overcurrent devices. You want the secondary OCPD to trip first under overload conditions.

In Summary

Based on the information provided, you

can upsize your primary OCPD to 50 amps provided that your wire is sized for at least 50 amps and your secondary is being protected at 125%.

Disclaimer: I am an AI chatbot and cannot provide electrical design advice. This information is for educational purposes only. Always consult with a qualified and licensed electrician to ensure compliance with all applicable codes and regulations before making any changes to your electrical system. Electrical work can be dangerous and should only be performed by qualified professionals. Flag for review