# Transformer Installation Question: Three Wire Delta



## bsps (Jun 21, 2012)

Hello,

I have been doing mostly residential and am a bit rusty on the commercial. A bit of an interesting commercial project came up and I just want to get some clarification on a transformer install. 

The client is setting up a small machine shop with several 3 phase 460V machines. (lathe, mill, welders..). He has a dry type xfmr (25 kVA, 480V Delta-208Y/120) he would like to use from his old place but I am not entirely sure on the install as the new place is is serviced with 230V - 3 phase, 3 wire delta and the old place was 230V 4 wire wye. I did the install tonight but was concerned about a couple things.

Just to give an overview of the install, I fed the low side of the xfmer from the main splitter box (fed from the main 200amp fused disconnect) and through a 60 amp fused disconnect (at 230V of course) and as the client needs the lathe and mill operable first, out of the high side of the xfmr to a junction box and two fused disconnects for both the milling machine and lathe....Just one circuit for now.

The transformer case and all boxes are grounded. 

Upon testing, I am concerned about a couple things: 

1) It hums like mad.
2) I am not totally confident with matching the transformer to the service, has anyone had any experience with this configuration?
3) For the lowest voltage taps, (on the xfmr schematic about 420V), I am getting around 492V. I think his lathe is 440V and the mill is 460V. This is close to what I would normally do, percent wise and was expected, but again, what do you guys think?

As more machines are added, I plan on adding a 480V panel or perhaps a bus and use disconnects. 

Sorry for the long winded explanation, should have taken a photo!

Thanks in advance.


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## wildleg (Apr 12, 2009)

you could try the next lower taps to see if you can get closer to 477.

there's no way to figure out if it's properly sized from the information you provided (no load information), but 25kva is around 30 amps @480 ?


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## bsps (Jun 21, 2012)

Thanks for the reply Wild Leg,

As far as load I've got approx. 6 kVA on the secondary and for protection on the primary it works out to 62 amps. That's not taking into account the 125% according to NEC 450.4 so I will most likely bump it up.

Unfortunately, I was referring to the lowest taps.


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## micromind (Aug 11, 2007)

A couple of observations;

1) I'd be willing to bet that the old service was not a 230 wye, it was more likely a 120/208 wye. If so, the reason the transformer hums loudly is because the low voltage side is being overdriven. The hum is because the core is saturated and the very top and bottom of the voltage waves are a short circuit. I bet it draws a lot of current with no load, and runs hotter than normal. 

2) When a transformer is being backfed, the taps will have the opposite effect on high side voltage. If the taps are moved to a lower voltage, the actual voltage will increase. If they're moved to a higher voltage, the actual voltage will decrease. 

It's entirely possible that this transformer won't work with a 240 volt input. One way around this would be to install two (not 3, only two) buck-boost transformers on the 240 volt side and lower the input voltage. 

If a 208 volt transformer is supplied with 240 volts continuously, it'll very likely burn up in a fairly short time.


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## kaboler (Dec 1, 2010)

micromind, how many transformers have you worked on to know all this stuff? Or is it schooling?


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## bsps (Jun 21, 2012)

Micro,

You're totally right, I made a mistake in my original post, it was indeed a 208Y supply. 

I am going to get in there tomorrow to finish up some lighting and will check the current draw at no load. I like the idea of a buck-boost on there. I will also check around for another xfmr. Interesting thought on the short circuiting. Thanks for the observations. 

I was doing some more calculations and design for additional machines and was wondering what case is best for branch circuits (mostly motor loads). Do you guys prefer to use a 480V sub or use a splitter box and disconnects? The client has some disconnects kicking around but no sub and breakers. Obviously cost is an issue as well, so leaning towards case 2.


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## micromind (Aug 11, 2007)

kaboler said:


> micromind, how many transformers have you worked on to know all this stuff? Or is it schooling?


Hundreds.

All sizes and voltages, from small ones up to 75 MVA and 230 KV. 

I barely graduated high school, never been to college. Never even been an apprentice. 

I really have no idea where my electrical knowledge came from. Lots of independent study and years of experience I guess.


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## micromind (Aug 11, 2007)

bsps said:


> Micro,
> 
> You're totally right, I made a mistake in my original post, it was indeed a 208Y supply.
> 
> ...


I've done maybe a dozen or so backfed transformers like this one. Usually, I ground the B phase and use a 480 3ø panel. But there's no reason not to use fused disconnects.


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## bsps (Jun 21, 2012)

Yeah, I think that's the route I am going to go. I just tracked down a 30 kVA 230Y/208Y Auto that I might put in there. I like the buck and boost and would rather do that but this should work. Micro, why would you ground the B phase for those installs?


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## micromind (Aug 11, 2007)

bsps said:


> Yeah, I think that's the route I am going to go. I just tracked down a 30 kVA 230Y/208Y Auto that I might put in there. I like the buck and boost and would rather do that but this should work. Micro, why would you ground the B phase for those installs?


Pretty much every AC system is required to be grounded or have some sort of ground fault detection. 

Since a transformer is a separately derived system (no electrical connection from one side to the other), it's not allowed to float. 

Grounding one of the phases (usually B) satisfies this requirement, and it's a lot less expensive that a ground fault detector. 

True enough, grounding a phase results in 480 to ground on the other two phases, but the equipment being powered doesn't care. 

In reality though, I've seen far more of these systems float than properly grounded.

P.S. If you install the 30KVA transformer, and it is indeed a 4 wire wye, make sure that the center of the wye (H0 or X0, whatever the 230 volt side is labelled) isn't connected to anything. You may very well need to remove the bond strap so it isn't connected to ground. 

If it's grounded, you'll have a bolted ground fault, and it'll go BOOM when you turn it on.


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## bsps (Jun 21, 2012)

Ahh, exactly. I have a ground fault detector that I planned on installing. I understand what you are saying. And of course I will not connect the XO or HO, thanks for the heads up.


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