Transformer Wiring

[rwade0700]

We recently were called in to replace a 45 KVA transformer that is used strictly for an elevator. The elevator is a 460 volt 3 phase unit. The supply to the building is a 120/208 wye system. We replaced the xfmr with the same type that had been working there since 1995, it was a 480 volt Delta primary and 208y/120 secondary. I do get 486 volts across phases for an output but the individual phases to ground are 235v, 259v and 305v. When I put an ammeter on the ground it reads a steady 9 amps. We went through all grounding connections and tightened or repaired them. Input on each leg to ground is 121v per leg and 210 v across phases. There are (3) 3/0 copper conductors and (1) #4 copper grounding conductorfeeding this xfmr. The different output voltages and the 9 amps to ground concern me. Any input?

 

[nakulak]

We recently were called in to replace a 45 KVA transformer that is used strictly for an elevator. The elevator is a 460 volt 3 phase unit. The supply to the building is a 120/208 wye system. We replaced the xfmr with the same type that had been working there since 1995, it was a 480 volt Delta primary and 208y/120 secondary. I do get 486 volts across phases for an output but the individual phases to ground are 235v, 259v and 305v. When I put an ammeter on the ground it reads a steady 9 amps. We went through all grounding connections and tightened or repaired them. Input on each leg to ground is 121v per leg and 210 v across phases. There are (3) 3/0 copper conductors and (1) #4 copper grounding conductorfeeding this xfmr. The different output voltages and the 9 amps to ground concern me. Any input?

you have a floating ground on the output side. the elevator might not care (unless it needs the 277), but hopefully you have a properly bonded case and egc ?


edit: come to think of it, your derived neutral on the 480 side must not be bonded properly ? (looks like you have a wye-wye ?)

 

[augie47]

Do you have anything connected to XO on the 208 side ?
As far as your phase to ground readings on the 480v side since you have a delta winding for the 480 you will have varying voltages to ground unless you elect to ground one phase.

 

[rwade0700]

We have the grounding conductor to the xfmr and the grounding conductor going out of the xfmr to the elevator disconnect both on XO. There are no neutrals present. I really don't know about grounding one phase of the 480 side. Would this equalize the output on the high side?

 

[augie47]

There should be NO connections to XO.
You are wiring the transformer in reverse feed and basically using it as if it were delta/delta.
The connection to XO on the 208 side is probably why you are getting the ground readings.

There is no requirement to ground one leg of the 480 output.
If it were a new installation 250.21(B) would be a requirement.

 

[ceb58]

There should be NO connections to XO.
You are wiring the transformer in reverse feed and basically using it as if it were delta/delta.
The connection to XO on the 208 side is probably why you are getting the ground readings.

There is no requirement to ground one leg of the 480 output.
If it were a new installation 250.21(B) would be a requirement.

Augie is right there should be no connection on xo. You would be installing a corner grounded delta system. The grounding conductor should be bonded to the center leg of the 480v side (H2) then the grounding conductor is bonded to the frame of trans. then to any building steel and bonded to the disco. In the disco there should be a "no blow" fuse on the grounded leg.

 

[rwade0700]

Apparently there should be NO connections on XO and bring the ground wire from the source CB panel to the frame of the xfmr and to the elevator disconnect. The next part sounds like a short, bonding these grounds to H2 though I know that there is a tap to ground on a Delta system , this will not cause a short? What is a "no blow ' fuse? I would like to let everyone know that this is a Siemens transformer and after several calls to their tech help I only got a "we'll get back to you" that was last week. Thanks for your help.

 

[kwired]

Apparently there should be NO connections on XO and bring the ground wire from the source CB panel to the frame of the xfmr and to the elevator disconnect. The next part sounds like a short, bonding these grounds to H2 though I know that there is a tap to ground on a Delta system , this will not cause a short? What is a "no blow ' fuse? I would like to let everyone know that this is a Siemens transformer and after several calls to their tech help I only got a "we'll get back to you" that was last week. Thanks for your help.

There is no short because there is nothing to short to until something is grounded. Your transformer secondary has no reference to ground at all. Once you ground one of the phases then you have a ground reference, and you will have 480 volts from each of the other phases to ground.

Any transformer can have any of its taps grounded and not have a short condition. It is usually desirable and even code required in most cases to ground a common tap like the center of a wye or the mid point of a phase. With a three wire delta your choices are one of the three phases and the result of grounding any one of them is basically the same. You only have a short when a second phase becomes grounded.

Grounding the XO on the primary side will only cause unnecessary current to flow in this case and may be why the original transformer failed. I assume that is why you were replacing it?

 

[rwade0700]

Thanks, I think that you mean that I will have 480 volts between phases not to ground? I can see how a xfmr secondary has no reference to ground, that makes sense. Yes that is why we were replacing it. We had rewired it the way it was previously was wired but then I found the odd voltages and current to ground.

 

[ceb58]

It is basically a piece of pipe with a UL listing. There is a code article on it but I cant remember it right now. What it is for is if there is a fault between the grounded leg and an ungrounded one you will not loose the ground reference.

 

[CFL]

Thanks, I think that you mean that I will have 480 volts between phases not to ground? I can see how a xfmr secondary has no reference to ground, that makes sense. Yes that is why we were replacing it. We had rewired it the way it was previously was wired but then I found the odd voltages and current to ground.

With one of the phases, the other two phases will read 480v to ground. The grounded phase will obviously read 0v to ground.

 

[kwired]

Thanks, I think that you mean that I will have 480 volts between phases not to ground? I can see how a xfmr secondary has no reference to ground, that makes sense. Yes that is why we were replacing it. We had rewired it the way it was previously was wired but then I found the odd voltages and current to ground.

If you ground one phase you will have 480 volts between phases - one of them happens to be grounded so the other two phases will have 480 volts to ground, the grounded phase is the same potential as ground so your volt meter from this phase to any grounded object will read very low just like any other grounded conductor on other systems to ground.

I recommend disconnecting the neutral from the primary XO terminal and leave that terminal float, that may be one reason the previous transformer failed, and your 9 amps on the neutral that makes no sense will be gone. You still need an equipment grounding conductor installed with the primary, but only bond it to the transformer case and not XO.