Neutral required on the LINE side of 3 buck-Boost transformers from 208-240

[RICHRICH36]

I have a strange situation in which on a strictly "theory" point I can not understand the need for the neutral on the line side. I am wiring three buck boost transformers to boost from 208 to 240. There is no neutral required on the load side (which you can not get with buck boost's anyway). I have attached a copy of the wiring diagram. Note that the ground and neutral look to be bonded at the transformer but Acme point blank said that this is the building bond and do not under any circumstance bond at the transformer.

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[iwire]

I think that is the wrong diagram for a 3 phase load. I think that is mis-drawn diagram for a 3 phase four wire load.

I have done a lot of buck boosts and have never used 3 transformers for straight three phase loads.

For a straight three phase load you only need two buck or boost transformers.

 

[RICHRICH36]

Maybe I should have told you that these are 7.5 kva which will handle a load of 468amps. I will have a little over a 400amp load. You are correct in the fact that this could be done with two transformers but it was specified to use three.

 

[iwire]

You are correct in the fact that this could be done with two transformers but it was specified to use three.

And that is the diagram they gave you so I guess you are good to go.

 

[LarryFine]

I have a strange situation in which on a strictly "theory" point I can not understand the need for the neutral on the line side.

Each BB is supplied line-to-neutral, so they're really stepping each 120v up to approx 139v. The output is still a Wye even though your load is Delta.

There is no neutral required on the load side (which you can not get with buck boost's anyway).

That's not true; your line-side neutral could easily be brought out to the load, giving you a 240Y/139 source.

Bob's suggestion ignores the neutral completely, wiring the BB's as if both the source and load are Delta.

 

[RICHRICH36]

Yes that is the diagram and I am sure it will work but I was wondering if there is anyone that can tell me from a "theory" point of view why I need this neutral on the line side? I could leave well enough alone but it bothers me not knowing the purpose of the neutral.

 

[RICHRICH36]

Larry, I stand corrected on the neutral on the load. You are correct. Your Y output makes sense. I am going to print the diagram and trace it out.

 

[xformer]

one needs the feeder neutral run to the loads due to the fact that a voltage potential exists between feeder grounded conductor and a reestablished grounding point... hopefully i have worded that correctly.

 

[LarryFine]

Unlike a "real" 3ph Wye-primary transformer, which must have the neutral terminal (H0 (or X0, if reversed)) left floating, a trio of BB's (three 1ph units) needs the primary neutral reference in order to properly energize the secondaries.

 

[LarryFine]

one needs the feeder neutral run to the loads due to the fact that a voltage potential exists between feeder grounded conductor and a reestablished grounding point... hopefully i have worded that correctly.

I think you did, and, if so, it's incorrect. An SDS requires a GEC, not a neutral, despite their being bonded at the service.

 

[iwire]

Bob's suggestion ignores the neutral completely, wiring the BB's as if both the source and load are Delta.

Actually the 'source' can be, and often is Wye. we just choose not to bring the neutral to the load.