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Wiring a transformer backwards - NEC Compliant? (240V to 480V single phase)

Wiring a transformer backwards - NEC Compliant? (240V to 480V single phase)

Old 05-12-12, 05:12 PM
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Question Wiring a transformer backwards - NEC Compliant? (240V to 480V single phase)

Hello all,

Location: USA - Southern West Virginia - Raleigh County

My electrical knowledge lacks when it comes to commercial high voltage. My experience is with residential, and light commercial.

My father is opening up a restaurant, and the grill line is a 480V single phase unit. It came form the Ford Motor plant's cafeteria in Detroit circa 1960s. It consists of 3 appliances. One is (2) 8" cook-top "eyes" (unknown wattage, probably around 1kw or less each), a flat top griddle that is 9kw, and a grill totaling 8.1kw. All 480V single phase, totaling just under 20kw total.

All of the local businesses on the nearby poles are operated on standard 120/240 single phase power. The cost for the power company to come and install a new pole and transformer for 480V service would be over $15K. That was just not an option. The engineer that came out advised us to purchase a transformer and wire it backwards to boost 240V to 480V.

We sized and purchased a GE 9T21B9106 25kva single phase general purpose transformer. From the factory it's 480V primary (H1-H4) and 120V (X1-X3 or X2-X4) or 240V (X2-X3) on the secondary. Our solution was to wire it 240V on the primary, and get 480V on the secondary.

Is this even copacetic with the NEC regulations? I couldn't find anything in article 450 that says you can't wire a transformer backwards to boost voltage. My limited education tells me a transformer doesn't care what way it's working, as long as the conductors and over current protection are within specs. Obviously sufficient labeling and warnings will have to be applied, although honestly I'm not sure what specifically will have to be applied.

Here's the other item that I think may be an issue, or may not. This transformer does not have terminals for the connections, but wire leads. It is using 6AWG on the primary, and what appears to be 2 or 3AWG on the secondary (no visible markings on the leads). Obviously 6AWG is not sufficient for the 100A, however is there any provisions or exclusions for the wire leads since this is only 12inches from the transformer coils? I mean, the wires wrapping around the core certainly aren't 6AWG or more.

From my calculations (that I may be incorrect on) I'm using a 100A breaker on the primary (now to be) 240 input, and the secondary output (now 480) will be comprised of 4 fused dis-connects for each cooking appliance (disconnects rated at 600V 30A each, not sure what size fuses we will be putting in for each one yet). My plan is to use 2AWG to feed from the 100A breaker to the transformer. Going with 2AWG (good for 130A in THHN correct?), incase we need to go higher than 100A breaker size. The transformer will be located less than 15ft (wire length) from the panel feeding it. The cook top will be approximately 35 ft (wire length) from the transformer.

If you need any further information please let me know. I've tried to cover all my bases, and provide all relevant information.

Relevant Info:

Transformer Spec Label:


Last edited by poor_red_neck; 05-12-12 at 05:29 PM.
Old 05-13-12, 06:37 AM
Tolyn Ironhand's Avatar
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First off, I don't know the rules in WV, but most states require a licensed electrician to do the work in a commercial/retail space. Also, there are very tight codes that need to be followed in a restaurant setting or the fire and health dept. will shut you down in a heartbeat.

About your transformer:

Your plan is sound except you have a few things wrong. If you were to hook up the 240v feed to the primary side, the transformer will step down the voltage and you will end up with about 120v on the secondary. You need to hook up the 240v line to the secondary which will then give you 480v on the on the primary.

You will need to feed the secondary (240v) with #3 copper for 100 amps and the primary #6 copper. you will need to run a ground too. You will need to run the primary into a 480V, 60 amp fused disconnect and some 50 amp fuses. 600 volt equipment is quite expensive and you will need to go to an electrical supply house for that.
Old 05-13-12, 07:17 AM
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It is using 6AWG on the primary, and what appears to be 2 or 3AWG on the secondary (no visible markings on the leads).
The transformer's primary leads become the secondary and the transformer's secondary leads become the primary. You just feed it backwards. It's not that unusual and you do need to change the labelling on the transformer.

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