Okay, Mark, let's break down this transformer protection scenario and try to make it less crazy-feeling. You're right, the NEC can lead to seemingly oversized breaker requirements for transformers.

Understanding the Problem

You have a 45kVA autotransformer stepping down 480V to 380V, supplying a 29A load. You're trying to size the primary and secondary circuit breakers and conductors. You're also questioning the need for secondary protection.

Key NEC Articles



NEC 450.3(B)

Overcurrent Protection. This is the primary article governing transformer protection.

NEC 240.4

Protection of Conductors. This tells you how to protect your wires based on their ampacity.

NEC 240.6(A)

Standard Ampere Ratings. This is where you find the standard breaker sizes.

NEC 240.21

Location in Circuit. This helps if you're running longer secondary runs and want to use tap rules.

Let's Work Through the Calculations


Transformer Full Load Amps (FLA)



Primary (480V)

45,000 VA / 480V = 93.75A

Secondary (380V)

45,000 VA / 380V = 118.42AImportant Note: Your calculation of 54A for the transformer rating might be based on a different KVA rating on the nameplate, or perhaps you were calculating the load it supports. We need to use the transformer's full kVA rating for breaker sizing.2.

Primary Overcurrent Protection (NEC 450.3(B))



General Rule

The primary overcurrent protection device shall be set at not more than 125% of the transformer's primary FLA if the primary current is 9 amps or more. If the primary current is less than 9 amps, then 167% is permitted. 480V primary falls into this category, so 125% is applicable.

Calculation

93.75A 1.25 = 117.1875A

Standard Breaker Size (NEC 240.6(A))

You can use the next higher standard breaker size. The next standard size higher than 117.1875A is 125A.

Therefore, the maximum primary breaker size is 125A. This is where the "crazy" numbers come from. It's not based on your load, but on the transformer's capacity.3. Secondary Overcurrent Protection (NEC 450.3(B))



General Rule

The secondary overcurrent protection device shall be set at not more than 125% of the transformer's secondary FLA if the secondary current is 9 amps or more. If the secondary current is less than 9 amps, then 167% is permitted. 380V secondary falls into this category, so 125% is applicable.

Calculation

118.42A 1.25 = 148.025A

Standard Breaker Size (NEC 240.6(A))

You can use the next higher standard breaker size. The next standard size higher than 148.025A is 150A.

Therefore, the maximum secondary breaker size is 150A.4. Conductor Sizing (NEC 240.4)



Primary Conductors

These conductors must be rated for at least 125% of the transformer's primary current. So, 93.75A 1.25 = 117.1875A. You would need conductors rated for at least 118A, which would require #1 AWG.

Secondary Conductors

These conductors must be rated for at least 125% of the transformer's secondary current. So, 118.42A 1.25 = 148.025A. You would need conductors rated for at least 149A, which would require #1/0 AWG.

Addressing Your Questions



Can I just protect the primary side with a 60A breaker and feed it with #6’s and protect the secondary side with a 40A breaker and feed it with #8’s? No, not according to the NEC for transformer protection. While this might work, it doesn't meet the requirements for transformer protection. The transformer could be overloaded before the breakers trip. The NEC protects against transformer failure, not just the load. Also, the primary conductor would be undersized. Do I even need secondary protection? I believe I do based on what I read. Yes, you DO need secondary protection. NEC 450.3(B) requires it unless you meet very specific exceptions (short primary run, built-in protection, etc.), which are unlikely in your case. A secondary fault could severely damage the transformer if there's no secondary protection. I came up with 135A max protection on the primary side and 85A on the secondary. I believe those are maximums so not sure if my thought process is the way to approach this. The calculated max primary breaker would be 125A. However, you have to protect your conductor as well. Therefore you may be limited by the conductor size. In a case like this you may be able to lower your breaker size to provide that protection if you want, but it is not required.

A More Practical Approach (and Where Your Thoughts Come In)

While the NEC allows for the larger breakers, it doesn't require you to use them. The key is to balance transformer protection with load protection and conductor protection.1.

Consider Your Load

You have a 29A load on the secondary. This is significantly less than the transformer's capacity.2.

Adjusting the Secondary Breaker Down (But Still Following NEC Rules)

You can use a smaller secondary breaker if it's properly sized to protect both the conductors and the load. This is where the conductor protection rules in 240.4 come in.
3.

Important Considerations Before Downsizing the Secondary Breaker



Inrush Current

The equipment you're powering likely has an inrush current when it starts up. You need to ensure the secondary breaker can handle this inrush without tripping. This is a critical point. Look at the equipment's nameplate or consult the manufacturer. You might need a breaker with an "inverse time" characteristic (like a "C" curve breaker) to handle inrush.

Future Load

Are you planning to add more load to this transformer in the future? If so, keep that in mind when sizing the breakers.

Putting it All Together (Revised Recommendation)


Primary Breaker

125A (Maximum Permitted, using #1 AWG)2.

Secondary Breaker



The key to this circuit is to pick a secondary breaker that is sized right so it doesn't trip upon startup! If it consistently trips upon startup, it is recommended to switch to a breaker that has an inverse time curve to handle inrush.

Important Notes



Local Codes

Always check with your local electrical inspector. They may have specific requirements or interpretations of the NEC.

Overcurrent Protection

Make sure all overcurrent protection devices are properly labeled to indicate the protected equipment.

Qualified Electrician

This is electrical work. It should be done by a qualified and licensed electrician who can properly size conductors, install equipment, and ensure safe operation. I am an AI; I cannot provide hands-on electrical advice.

In Summary:

The NEC's transformer protection rules prioritize protecting the transformer itself. This can lead to breaker sizes that seem large compared to the actual load. You can often use smaller breakers, especially on the secondary, but only* if you carefully consider conductor ampacity, load characteristics (especially inrush current), and potential future load. Always err on the side of safety and consult with a qualified electrician.I hope this helps to de-mystify the process! Let me know if you have any more questions. Flag for review