Okay, let's break down the overcurrent protection (OCPD) requirements for your transformer installation, focusing on why a 125A breaker might be acceptable, even though the calculation suggests a larger size.

Understanding the NEC (National Electrical Code) and Transformer Protection

The NEC dictates how to protect transformers from overcurrent, and there are a few acceptable methods. The basic goal is to protect the transformer itself and the conductors connected to it. Here's the relevant NEC section you're likely working with: NEC 450.3.

Your Calculations (Primary Protection)

Your calculation for the primary overcurrent protection is correct in principle: Transformer KVA / Primary Voltage = Primary Current (FLA) 25,000 VA / 480V = 52.08 Amps (Primary FLA) 52.08 Amps 1.25 = 65.1 Amps The rule you cited is for transformers rated over 9 amps and primary protection ONLY, and permits the next standard size breaker (70 amp) for a 65.1 amp OCPD rating on the primary.Note: You incorrectly calculated the primary FLA.Important Note Regarding Your Original Calculation: You incorrectly used the secondary FLA in your calculation and also the 125% multiplier applies to transformers with primary overcurrent protection only.

Secondary Protection Secondary Breaker/Overcurrent Device Sizing (NEC 240.4): The 1 AWG copper wire is rated for around 110 amps, depending on insulation type and installation conditions. You cannot protect 1 AWG with a 125A breaker according to NEC table 310.16. You must protect it with a 110 amp breaker or less (assuming the lowest rated conductor). A 25 KVA transformer with a 240V secondary has a secondary FLA of 25000 VA / 240V = 104.17 Amps. The 125A breaker is only ~19% over the FLA which allows for some starting current for inductive loads, but this doesn't permit sizing beyond what is safe for the wire.

Why a 125A Breaker Might Be Acceptable (and why it likely isn't in your case)

The 125A breaker might be acceptable if the following both hold true:1. Coordination

The primary protection (your calculated 70 amp breaker) is properly sized according to NEC 450.3(B), and2.

Tap Rules are NOT being used

You are not tapping off of a larger conductor that requires overcurrent protection.

Why the 150A Breaker is Unlikely to be Compliant Wire Protection

As mentioned earlier, you cannot protect 1 AWG wire with a 150A breaker; even a 125A breaker violates this. You are required to use the lowest ampacity rating for your conductor and its terminations. Terminations can be labeled, and you must abide by that rating.

In Summary: Your Situation and Recommendations

Based on the information provided:1. Primary Protection

Recalculate your primary FLA. Use this value (52.08 amps) and size your primary breaker accordingly.
2.

Secondary Protection

The 125A breaker is too large. Downsize it to a 110 amp breaker to properly protect the wire.3.

Load Calculation

Verify your actual load demands. If your actual load is significantly less than the transformer capacity, you might be able to further reduce the breaker size, but you still need to protect the wire.

Crucial Considerations



Local Codes and Inspector

Always consult your local electrical codes and your electrical inspector. They have the final say on what is compliant in your specific jurisdiction.

Transformer Nameplate

The transformer's nameplate provides critical information about its characteristics and any specific protection requirements.

Qualified Electrician

This type of work should be performed by a qualified and licensed electrician. They can properly assess the entire installation and ensure that it meets all applicable codes and safety standards.*

Coordination Study: For more complex systems, a coordination study might be necessary to ensure that overcurrent protective devices trip in the correct sequence to minimize outages. Flag for review