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?

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    \$\begingroup\$ Welcome to EE.SE. If you add your location into your profile page and post you may get some sensible answers to suit your local electrical regulations. This is an international site. \$\endgroup\$
    – Transistor
    Sep 6, 2016 at 16:58
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    \$\begingroup\$ Currently, is your 480V delta equipment grounded to earth? \$\endgroup\$
    – user57037
    Sep 7, 2016 at 16:56
  • \$\begingroup\$ Yes -- is your 480V equipment designed for a corner grounded (2 hots and the 3rd phase is a neutral-of-sorts) or an ungrounded (3 hots, no neutral) delta? \$\endgroup\$ Oct 7, 2016 at 2:07

1 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.

  • \$\begingroup\$ No problem. Not sure I adequately answered your question, but there is a reason grounding is the most confusing topic in the NEC. \$\endgroup\$ Sep 10, 2016 at 0:20
  • \$\begingroup\$ You answered perfectly. My reason for the question was that in another forum, someone said that one corner of the 480 delta output would have to be grounded. I'm not in need of a common for the output - won't be using 277 for anything, so that comment about grounding one corner of the delta completely went over my head. Can you explain what that would have been about? \$\endgroup\$ Sep 12, 2016 at 22:51
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    \$\begingroup\$ The comment about corner grounding is because in a Delta system, you must EITHER ground one corner, OR provide a ground fault monitoring system. Corner grounding the Delta is by far a lot cheaper. You should be aware however, that Delta power feeds to things like VFDs are risky to the VFDs. Most of them will have MOVs and some add Common Mode Filters, all of which are designed around expecting a Wye power supply where the voltage reference to ground is 58% of the line to line voltage. \$\endgroup\$
    – JRaef
    Oct 6, 2016 at 19:26

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