3 phase boost transformer questions

[Bosco]

I'm trying to hook up a 3 phase piece of equipment in a warehouse. The equipment is 480 volt (only) and was originally shipped with (but never used because the old warehouse had 480) a transformer (9kva) to boost the voltage. The transformer is a Delta/Delta with a 240 volt input/output. I'm assuming I would need a different transformer (208v input). I don't think this would be considered a seperately derived system Am I correct? There will be no grounded conductor (straight 480) so how should this transformer be grounded? Does the secondary remain floating? Anything else I should be concerned with?

Thanks

 

[augie47]

You could corner ground the output or have an ungrounded output.
250.21(b) would require a detector on an ungrounded system.

Opinion only (others may have better advice).. depending on where your 208 is actually running, what the taps are on your existing transformer, and how critical the 480 requirement is, the old transformer might work

 

[Bosco]

Thanks for the input (no pun intended). After looking at it further the equipment is marked 440 volt so I might be able to get away with the old transformer (Feeding 240 transformer with 208 would probably get me somewhere around 440). The right way to ground this still has me concerned. Would I need a seperate grounding electrode setup (again I don't think this is a seperately derived system) or do I ground the secondary with the incoming egc in some way?

Thanks again

 

[augie47]

If its a delta-delta transformer (as opposed to an auto-transformer), it is a SDS. Follow post #2 and 250.30(C) if the system remains ungrounded.
My opinion would be ground 1 phase and follow 250.32(B) to avoid having to mess with a detector.

 

[LarryFine]

The transformer is a Delta/Delta with a 240 volt input/output.

This sounds incorrect. Did you mean 240 in and 480 out?

 

[Bosco]

Well I have 3 phase 208 at the warehouse now. The transformer that was originally came with the large band saw is a 480 to 240 volt. I called the manufacturer that shipped the equipment and he said they ship those transformers with the equipment to boost the voltage to 480. So I assume this transformer can be used in either direction. The transformer has never been used because the old warehouse had 480 panels.

The equipment has a nameplate of 440 volt on it I was going to see if I could get something close to 440. feeding the transformer with 208 rather than 240 I thought I might get something close. If not I would order a 208 to 480 volt transformer. My concern was with the grounding system. I do not need
a neutral.

Quote from my 02 handbook

"Two of the most common sources of seperately derived systems in premises wiring are transformers and generators. An autotransformer OR step-down (I assume step-up also) that is part of the electrical equipment and does not supply premise wiring is not the source of a seperately derived system. "

So the fact that this transformer is only suppying the equipment. I thought it might not be considered a seperately derived and more as part of the equipment? I wasn't sure if I would need a grounding electrode system for this. If not how should it be grounded.

I hope I made sense and thanks for all opinions.

The transformer in question a Hammond mk009kd (9kva)

 

[cadpoint]

This transformer, Hammond mk009kd is a very tough item to find, I would call any of the suppliers that list that they sell it, but to outright get a spec. sheet on it seems non existence as a down loadable.

They list plenty of spec. sheets of Hammond projects just not your model!

Good Luck

 

[hardworkingstiff]

I believe if you feed that transformer with 208V, you will get 416V out. That's about 5.5% less than the nameplate rating you mentioned. I believe it will work OK. If you have over/under taps on the transformer you might be able to get much closer to 440V out.

As pointed out by augie, it is a SDS. Follow augie's advice.

 

[Bosco]

I would like to corner ground the secondary which leads me to my next question. I will be protecting the primary with 125% I will not need secondary overcurrent protection. I will feed the equipment directly off of the secondary of the transformer. I am going to need an EGC for the equipment where do I "land" the EGC in the transformer? Doesn't that make a redundant since "b" phase will intentionally be grounded?

If the transformer was designed to go both ways and had a center tap (between a phase, not corner tapping) on the high side would this be acceptable?

I hope I'm making sense....thanks

 

[LarryFine]

Bosco, treat the grounded conductor as you would a grounded neutral, including the insulation color. Many people forget thatm by the way.

You run separate EGC (and/or metallic conduit) and grounded conductors, and they get bonded together either in the transformer or the panel, but not both.

If the secondary that you use as a primary has a neutral terminal, do not connect it to the supply neutral. Leave it insulated (of course) and floating.

 

[Bosco]

Thanks Larry, I don't need a neutral at the equipment so I would just need a EGC and it would be tied to the grounded phase conductor in the transformer correct?

Thanks a bundle

 

[Bosco]

and the incoming EGC would need to be grounded to the transformer frame right?