Using a 3 phase step down transformer in reverse

Question

I have a 30kVA transformer with primary 480V delta to 208V Wye secondary. I am relocating to a building that has 208 service, and would like to use the above transformer in reverse, so that I can run my 480V equipment without other conversion. This transformer used in reverse to produce 480V delta output will not have a ground or common terminal on the output side. My question is: What then are the grounding requirements for the machinery that the transformer would be servicing, and, are there any other wiring requirements for the transformer itself?

Answer 1

I will assume you are in the US and therefore under the NEC. Based on that assumption, my answer is: There is a difference between grounding and bonding, and this is one of the most confused sections of the NEC. Grounding is intentionally tieing something to earth, whereas bonding is making everything electrically continuous... so we create a grounding system, and then bond all metallic parts that may become energized under a fault condition to that grounding system (called the grounding electrode system, or GES). Having said this, your particular equipment will probably still be bonded, but not have a neutral wire, which is normally tied to the ground at the service entrance or the separately derived system.

In my mind, your system would be a "separately derived AC system" as defined by the NEC, and article 250.30 talks about grounding such. Section E of that article is ttiled "Ungrounded Systems" (so you can already see where the NEC gets very confusing at best, we are talking about grounding an ungrounded separately derived systme, hench the widespread confusion!) This section has three subparts which define the rules for the grounding electrode conductor, grounding electrodes, and the bonding path. Most of these rules just reference other parts of the NEC.

I think in short, you will install a grounding electrode system (rebar if accessible, water pipe if metallic and accessible, and building steel; if none of these are available, you will have to install ground rods, rings, plates or something else - typically some sort of combination of all these items will be used) and then run a grounding conductor from the transformer case to this electrode system. Then all metallic parts will be bonded together and to this system to effectively give you a grounded, ungrounded system.

The NEC really needs to change the verbage to bonded, not grounded, but they don't, from my understanding, for legacy reasons.