Okay, let's break this down and address the wire sizing issue. You're right to question the 12 AWG. It sounds like there's a misunderstanding somewhere.

Here's why you likely need larger wire (and why 6 AWG is probably closer to correct, but we need to verify)



Transformer Current Calculation

You're stepping up from 208V to 460V. The current on the 208V side will be higher than the current on the 460V side, even though the load (the air handler) is 15A at 460V.

Approximate KVA Calculation The load is 460V 15A = 6900 VA or 6.9 KVASince transformers are rated in KVA, and you know the transformer is 15 KVA. It will be able to power the A/H but we will have to see if the wire can handle the voltage. 208V Primary Current Calculation

KVA = (Volts Amps) / 1000 -> Amps = (KVA 1000) / Volts Amps = (6.9 kVA 1000) / 208V Amps ≈ 33.2 A

Ampacity of 12 AWG

12 AWG wire, depending on the insulation type and installation method (conduit, open air, etc.), is typically rated for around 20-25 amps. You're already exceeding its ampacity based on the calculated primary current.

NEC (National Electrical Code)

The NEC requires conductors to be sized based on the calculated load, not just the nameplate rating of the equipment. It also considers factors like termination temperature ratings of the equipment.

Why 6 AWG (Probably, but Verify)

6 AWG copper wire typically has an ampacity around 55 amps (depending on insulation type and installation method, check the NEC table 310.16). This should be sufficient for the 33.2 amp calculated primary current, but you need to confirm that all factors are accounted for (see important verification steps below).

Here's What You Need To Do (and tell your foreman/engineer to do)


Precise Calculations



Transformer KVA

Is the 15 kVA rating the actual KVA the transformer is providing? Transformer manufacturers also have tables for current rating.

Primary Voltage

Is your actual primary voltage exactly 208V? If it's lower (e.g., 200V), the current will be higher. Use the lowest voltage you will expect in this scenario.

NEC Table 310.16

Carefully consult NEC table 310.16 to determine the ampacity of the wire based on:

Insulation Type

(e.g., THHN, THWN, XHHW)

Installation Method

(e.g., in conduit, in free air, direct burial)

Ambient Temperature

If the location is hot, you'll need to derate the ampacity of the wire.

Termination Temperature Ratings

Check the temperature ratings of the transformer terminals and the breaker terminals. The wire ampacity must be appropriate for the lowest temperature rating. (e.g., if the breaker is only rated for 75°C, you can't use the 90°C ampacity of the wire). This often limits you to the 75°C column in NEC table 310.16.
2.

Overcurrent Protection (Breakers/Fuses)

The overcurrent protection (breaker or fuses) needs to be sized appropriately to protect the transformer primary and the conductors. The NEC has specific rules for transformer protection. It's generally higher than the conductor ampacity. Make sure your breaker is properly sized. NEC 450.3 covers transformer overcurrent protection.
3.

Grounding

Make sure you have proper grounding from the transformer to the A/H and back to the source. This is a safety issue.4.

Documentation



Transformer Nameplate

Take a picture of the transformer nameplate. It will contain important information like KVA rating, primary/secondary voltages, and impedance.

Wire Type

Document the insulation type of the wire you used (THHN, etc.).

In Summary:

You're right to be concerned. 12 AWG is almost certainly too small for the 208V primary side of the step-up transformer with a 15A, 460V air handler. You need to do a thorough calculation based on the primary current, the wire type, installation method, temperature ratings, and the NEC. 6 AWG is probably sufficient, but you absolutely need to confirm with a qualified engineer and proper NEC calculations. Do not submit an as-built until this is resolved. This is a safety issue, and potentially a code violation. Using an undersized wire can lead to overheating, insulation damage, and potentially a fire. Flag for review