Yes, it is most likely a code violation and definitely bad engineering practice. Here's why: NEC 450.3(B) Secondary Protection

The National Electrical Code (NEC) generally requires secondary overcurrent protection for transformers rated 600V or less to be no more than 125% of the transformer's secondary current rating when the secondary current is less than 9 amps. In your case, the calculated secondary current of the transformer is 83.3 amps (as you correctly calculated). 125% of 83.3 amps is approximately 104.13 amps. A 150A breaker far exceeds this value.

Violation

Using a 150A breaker when the code would likely limit it to around 104A means the breaker won't trip under overload conditions that could damage the transformer. This is a clear violation of the NEC.

Bad Engineering Practice

The primary purpose of the secondary breaker is to protect the transformer from overloads on the secondary side. A 150A breaker is unlikely to trip quickly enough to prevent the transformer from overheating and potentially failing if the secondary load exceeds its rating but is less than 150A. It also creates a potential fire hazard.

Standard Breaker Sizes

Standard breaker sizes are often used, so it might be tempting to go to the next standard size up from the 125% calculated value. However, the specific* code sections and local amendments are critical, and simply going to the next size up is not generally permitted unless specifically allowed by the code.

In summary: Using a 150A breaker on the secondary of this transformer is both a code violation and poor engineering practice. You should replace it with a breaker sized according to NEC 450.3(B) which would likely be a 100A breaker. Always consult the latest version of the NEC and local amendments to ensure compliance. It's also recommended to consult with a qualified electrician or electrical engineer for any questions. Flag for review