You're right to question this. It's generally not allowed to use 12 AWG wire on a 25-amp breaker supplying the primary of a 15 kVA transformer, and here's why

NEC Fundamentals
NEC 240.4(D)

This is the key section. It's the "small conductor rule." Unless you meet an exception, 12 AWG copper wire is typically limited to a 20-amp overcurrent protective device (OCPD).

NEC 450: Transformers NEC article 450 covers the installtion and protection of transformers.

Why the 25A Breaker is Problematic

The primary concern is overcurrent protection of the conductor. A 25-amp breaker exceeds the ampacity rating of 12 AWG copper wire, which is typically rated for 20 amps (or sometimes 25 amps under specific conditions, but we'll get to that). If an overload or short circuit occurs, the 12 AWG wire could overheat before the 25-amp breaker trips, creating a fire hazard.

Calculating Primary Current

Let's calculate the full-load primary current of your 15 kVA transformer, assuming a primary voltage of 480V (a common voltage for this size transformer): I = kVA x 1000 / V I = 15 kVA x 1000 / 480V I ≈ 31.25 ampsIf we assumed a primary voltage of 240V, then the full load primary current would be: I = kVA x 1000 / V I = 15 kVA x 1000 / 240V I ≈ 62.5 ampsEven though your transformer's primary current draw will likely be less than 25 amps under normal operating conditions, the OCPD must protect the conductors from overcurrent regardless of the load.

Permissible Transformer Primary Overcurrent Protection

NEC 450.3(B) covers the overcurrent protection requirements for the primary side of transformers. The size of the primary overcurrent protection device must be in accordance with Table 450.3(B):

Primary Protection

For transformers over 600 volts, the primary overcurrent protection device rating cannot exceed 125% of the transformer's rated primary current. For transformers under 600 volts (like yours), it's generally 125% for circuit breakers.

Secondary Protection

Is typically not required if primary protection is within the specifications.So, based on our first calculation above (assuming 480v primary), a breaker of 31.25 amps x 1.25 =

39.06 amps maximum would be permitted by NEC 450.3. However, based on our second calculation (assuming 240v primary) a breaker of 62.5 amps x 1.25 = 78.12 amps maximum would be permitted by NEC 450.3.

Possible (But Unlikely) Exceptions

There are very specific, rare exceptions to 240.4(D), but none that are likely to apply here. For example, some motor circuits have exceptions that allow the OCPD to be sized larger than the conductor ampacity, but that's not the case for a transformer primary.

Best Practice and Recommendation1. Re-evaluate the Primary Voltage

Make sure you've correctly identified the transformer's primary voltage. The primary current calculation changes significantly based on voltage.2.

Correct Conductor Sizing

Based on the calculated primary current and the 125% overcurrent protection rule in 450.3(B), select conductors that have an ampacity equal to or greater than the full rated load of the transformer. Then, choose an OCPD that protects the conductors but doesn't exceed 125% of the transformer's primary current rating.3.

Consult a Qualified Electrician

The correct sizing of conductors and overcurrent protection for transformers is crucial for safety. Have a licensed electrician review the installation to ensure it meets all applicable codes and standards.

In summary, unless you have a very specific exception that applies (and it's unlikely), using 12 AWG wire on a 25-amp breaker for the primary of a 15 kVA transformer is a code violation and a safety hazard. It needs to be corrected. The exact wire size will depend on the primary voltage of the transformer. Flag for review