Delta Wye step up transformer

[garth8934]

I am hooking up a 208v delta to 480 wye transformer for 480v x-ray equipment. My question is; do I need to bond the H0 and just leave it unused?:?

 

[raider1]

The transformer should be grounded and bonded in accordance with 250.30.

Chris

 

[augie47]

I may be in error and Chris can correct me, but as I read 250.20(B) you do have the option of no using the HO terminal.
If yo don't you need to adhere to 250.21(B).
There are a variety of reason that most of us feel your should ground HO, but, as I say unless I'm mistaken, you do not have to. (assuming the equipment does not need the neutral)

 

[texie]

I am hooking up a 208v delta to 480 wye transformer for 480v x-ray equipment. My question is; do I need to bond the H0 and just leave it unused?:?

It would be a rare event to not need to have a grounded system, certainly not an xray machine. As was mentioned you need to carefully comply with 250.30.

 

[kwired]

Please clarify, is this H0 a high voltage or low voltage terminal?

Since this is a step up transformer the low voltage is the primary and the high voltage is the secondary.

Assuming there is no need for a 4th conductor on the delta 208 primary I kind of assume this H0 is the neutral of the 277/480 secondary.

If your load uses the neutral at all I think you have no choice but to ground the neutral as required in 250.26.

If your load does not use the neutral you still may have to ground it, but I think it can be more debatable to ground a phase or even use an ungrounded system if there is no use of the neutral as a circuit conductor.

If you don't use it and don't bond it, do we have to call it a 4 wire system or can it be treated like it is a 3 wire delta system?

This has come up before when someone wants to back up a 3 wire delta system with a wye connected generator.

 

[texie]

Well, I took the OP at his word that he has a 208 delta primary X 480/277Y secondary. That would mean to me that he means the H0 termininal is on the 480Y side. I would be willing to bet that the specs for the xray machine call for a connection to a solidly grounded 480/277Y system.

 

[kwired]

Well, I took the OP at his word that he has a 208 delta primary X 480/277Y secondary. That would mean to me that he means the H0 termininal is on the 480Y side. I would be willing to bet that the specs for the xray machine call for a connection to a solidly grounded 480/277Y system.

I kind of took OP for his word too, but just wanted clarification. Others have posted about similar things in past and actually did have a typical step down transformer and were operating in reverse. But I guess it is not too likely to even have a H0 terminal if it is a typical step down transformer. Maybe I should just keep my mouth (keyboard) shut.

 

[garth8934]

X-ray equipment does not require a neutral it is strictly 480v, I suspect the delta wye transformer was ordered because of cost considerations but I don't really know. Service to building is 208/120 3 phase, transformer is fed strictly 208v on X1, X2, and X3 and has H1, H0, H2, H3 outputs.

 

[texie]

X-ray equipment does not require a neutral it is strictly 480v, I suspect the delta wye transformer was ordered because of cost considerations but I don't really know. Service to building is 208/120 3 phase, transformer is fed strictly 208v on X1, X2, and X3 and has H1, H0, H2, H3 outputs.

Sounds like you have the right type of transformer. By the way, this would likely be a little more expensive transformer than a more common 480 delta X 208/120Y transformer due to volume of sales. The point I was trying to make about the neutral is that the machine likely needs to be fed by a 480 solidly grounded Y system even if it does not need a neutral. Many things are like this, such as some VFD's. Ungrounded and corner grounded 480 delta systems are getting even more rare.

 

[kwired]

Sounds like you have the right type of transformer. By the way, this would likely be a little more expensive transformer than a more common 480 delta X 208/120Y transformer due to volume of sales. The point I was trying to make about the neutral is that the machine likely needs to be fed by a 480 solidly grounded Y system even if it does not need a neutral. Many things are like this, such as some VFD's. Ungrounded and corner grounded 480 delta systems are getting even more rare.

I think the main reason for ungrounded would be to detect faults but have time to orderly shut down a process instead of killing the process immediately because there was a fault. I think that has always been the main reason to have an ungrounded system, and is and was likely was always not very good chance of a utility supplying you with an ungrounded system. Ungrounded systems are likely separately derived at the point of use in most cases.

The corner ground - you used to find and probably still can find utility supplied corner ground systems - most of them that still exist probably have some age on them.

New corner ground systems are most likely separately derived systems near point of use also. Most common application is likely back fed delta - wye transformer where there is no neutral terminal available to use on the secondary, leaving no choice but to ground a phase or go with ungrounded system. Grounding a phase is simpler and less cost.