Delta- Delta transformer

[EL Schulte]

I have a 480v pri 240v sec Delta-Delta transformer. I'm questioning how to bond it. The transformer diagram has nothing on Grounding the B phase (x2). But I'm 90% sure that is what I need to do. Before I do this I want to be 100% sure. Any help would be grateful

 

[EL Schulte]

I have a 480v pri 240v sec Delta-Delta transformer. I'm questioning how to bond it. The transformer diagram has nothing on Grounding the B phase (x2). But I'm 90% sure that is what I need to do. Before I do this I want to be 100% sure. Any help would be grateful

Will the transformer Name plate have a Grounded B phase secondary diagram on it if it is capable of being grounded?

 

[GoldDigger]

Will the transformer Name plate have a Grounded B phase secondary diagram on it if it is capable of being grounded?

Probably not.

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[ActionDave]

I only have been around one corner grounded system, but as far as what the NEC says you pick one phase and ground it, B phase is customary but I don't think it's required. After that treat the grounded phase just like a neutral and follow 250.30 and 200.

 

[Jraef]

You don't actually HAVE TO ground one phase, you DO have to do that OR you must add ground fault monitoring and protection to the secondary side; one or the other. So the transformer nameplate diagram is not likely to show that, it's an installation decision on your part. It's a lot less expensive however to just use a corner grounded secondary and yes, B phase is the conventionally accepted one to ground, but it's not dictated.

 

[EL Schulte]

Thank you this helps. One more question. This transformer will be supplying a panel for 3 phase 240v equipment only. I can use 3 pole breakers right.

 

[kwired]

Thank you this helps. One more question. This transformer will be supplying a panel for 3 phase 240v equipment only. I can use 3 pole breakers right.

As long as the breakers are 240 volt and not 120/240 volt rated, but most 3 pole are not rated 120/240 AFAIK.

The transformer will not care which leg you ground, as long as there is only one leg grounded, more then one leg grounded is short circuit.

Make sure panel is rated for grounded phase system as well though. If so it may say "B" must be the grounded phase.

 

[bob52]

Delta - Delta transformer

Delta - Delta transformer

Once you ground one of the Phases ( corner grounding ) you can no longer use 3 pole breakers , as you cant have ocp on a grounded conductor

 

[GoldDigger]

Once you ground one of the Phases ( corner grounding ) you can no longer use 3 pole breakers , as you cant have ocp on a grounded conductor

You can have OCP on a grounded conductor as long as it has a common trip with the ungrounded conductor(s).
That option is generally not available with fuses.

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[jumper]

Once you ground one of the Phases ( corner grounding ) you can no longer use 3 pole breakers , as you cant have ocp on a grounded conductor

240.22(1) seems to say that a 3 pole common trip breaker is allowed.

Edit: Took too long to find code section. Goldy got it.

 

[petersonra]

You can either ground it or not. It is a design decision. There are ramifications both ways.

personally, I doubt it makes all that much difference one way or the other, although I have to say grounded 3 phase systems seem to be more common these days than they once were.

 

[Jraef]

When I worked at a steel mill in the late 70s we were 480V ungrounded delta, because the attitude was "Damn the torpedoes, full speed ahead!" when it came to operating that plant. We had ground lights in the electric shop that told us if there was a ground fault, somewhere. Then we jumped in golf carts to go out and try to find it. The usual sign was a production manager running around like a chicken with it's head cut off, flagging us down because his part of the plant was down and he was going to miss quota and thus, his bonus.