Bonding a Delta Y transfromer

[tobyk]

I have building with a 277/480V service.
from the main 277/480V Distribution Panel (PanelA)
i am feeding a Delta-Y Step down transfromer for 120/208V Lighting loads. I am running a Equiptment grounding conductor with the Supply Conductors to the transfromer. In the Transfromer i will bond to the Transfromer Housing and to the Equiptment Grounding conductor running to my first 120/208V distribution Panel(Panel B.)
A Grounded Nuetral Conductor will also Feed Panel B. for the centertap of the low side.
As Far as i understand I need to bond the Center tap of the Y side to the Equiptment ground at some point. Correct?
Does it matter if i Bond at the transfromer or at panel B.
normaly the nuetral and ground are bonded only at the frist point of disconnect to a building. but i guess in a situation like this we have an exception. Correct?

 

[bennie]

Re: Bonding a Delta Y transfromer

Connect the equipment ground conductor, from the service, to the X-O of the transformer.

If there is metal conduit to the first panel, float the neutral.

 

[Ed MacLaren]

Re: Bonding a Delta Y transfromer

tobyk,

What you are suggesting (sketch) seems OK . The guys with NEC expertise will advise if this isn't NEC compliant.

Ed

 

[iwire]

Re: Bonding a Delta Y transfromer

Originally posted by tobyk:
As Far as i understand I need to bond the Center tap of the Y side to the Equiptment ground at some point. Correct?

You have to Bond XO to the equipment grounding conductor but you also must run a Grounding electrode conductor to building steel or back to the water service within 5' of the water entrance, whichever is closer.

Take a look at all of 250.30 Grounding Separately Derived Alternating-Current Systems, it is not that long and tells you all you need to know for grounding this transformer.

Also take a look at 250.104(A)(4)

Bonding of Piping Systems and Exposed Structural Steel, Metal Water Piping, Separately Derived Systems.

Does it matter if i Bond at the transformer or at panel B.

It is allowed to do either, I find it easer to do it in the transformer and float the neutral at the panel

normally the neutral and ground are bonded only at the first point of disconnect to a building. but i guess in a situation like this we have an exception. Correct?

Not so much an exception, but a requirement, the neutral (XO) of this transformer must be bonded to the grounding system.

The bonding is the most important aspect of installing a transformer like this.

[ April 12, 2004, 01:28 PM: Message edited by: iwire ]