Wye Source for Delta Primary

[JuliaTruchsess]

My client is bidding on inclusion of their equipment as part of a larger installation, and I'm "helping" spec/source some of the ancillary equipment such as transformers. The POCO delivers (presumably) MV delta to a transformer whose secondary is 480Y/277. This goes to a service-entrance-rated 4-pole ATS whose other source is a separately-derived generator. From the ATS, they run the 480Y/277 to various equipment around the site, which is outside our/my scope of involvement.

My client's equipment (ITE) requires 208Y/120. 408 delta / 208 wye transformers are obviously very common, and I'm pretty sure I could just hook one up to the line legs of the 480 wye (right?), but the question is: what to do about neutral on the secondary side of my 208Y/120? And even if I could find a 408 wye primary, this would still be the question. Can/should I just run a solid neutral tying the 480Y system's N to the 208Y's N? The alternative would seem to be creating another SDS at the 208 secondary, but having two different neutrals running around the same facility does not sound like a good idea.

 

[LarryFine]

what to do about neutral on the secondary side of my 208Y/120?

You must bond the derived neutral to a grounding electrode system and an equipment grounding conductor system.

And even if I could find a 408 wye primary, this would still be the question. Can/should I just run a solid neutral tying the 480Y system's N to the 208Y's N?

No, you should leave any primary neutral connection floating, and connect the transformer as a delta load.

but having two different neutrals running around the same facility does not sound like a good idea.

Not an issue. Each system has its own neutral, which happen to be tied together via each system's GEC.

 

[kwired]

Whatever you tie to ground on that secondary is a grounded conductor. NEC wants it to be the neutral on a three phase wye system, but in theory as long as you only ground one conductor you can ground any conductor. Only thing the grounded conductor on the primary does for you with this situation is give your primary a fault current path if there is a ground fault, there is no use of primary grounded conductor as a circuit conductor otherwise. You do want primary and secondary grounded conductors bonded together at some point - and they will be through properly installed grounding electrode system and main/system bonding jumpers this gives both primary and secondary systems the same "ground reference".

 

[electrofelon]

I recall some discussions here in the past about using a wye-wye and bonding the two neutrals together. The advantage would be that it would not be an SDS so you would not need a GES connection. Disadvantages are you need a 4th primary conductor, low voltage Wye-wyes are not very common, and it's "weird". I have never heard of anyone doing it for low voltage (most utility transformers are that way).

 

[kwired]

I recall some discussions here in the past about using a wye-wye and bonding the two neutrals together. The advantage would be that it would not be an SDS so you would not need a GES connection. Disadvantages are you need a 4th primary conductor, low voltage Wye-wyes are not very common, and it's "weird". I have never heard of anyone doing it for low voltage (most utility transformers are that way).

OP has a delta-wye transformer. There is no reason to bring a neutral with the supply conductors, nothing utilizes it. An EGC to bond to the transformer frame is still required, to assure low impedance path should there be a ground fault on primary conductors.

The secondary grounded conductor does get bonded to the primary grounded conductor via a GEC.

 

[JuliaTruchsess]

Thanks all, for the clarifications!

 

[Tony S]

A situation I’ve come across many times. Plants with 660V or 550V supply and machines needing 433V or 380V. All star points earthed to the main earth bar and nest. The neutrals for each voltage kept separate once they were out in the plant environs. It was only with 433V and 380V the neutral was used.