480Y/277V (WYE) Service Entrance to 480V Delta Transformer -- Final Thread

[AsparkyBOI]

Hello all,

This is my first post here. I wanted to start off by saying thank you for the treasure trove of electrical information that has helped guide and elucidate me through several jobs over the years.

I'm an electrical engineer, and my team has a project where the customer wants several thousand amps of 208Y/120V electrical service. To save hassle with the electric utility, we are hoping to have this served by a 480V service and step the voltage down to 208Y/120V to serve the customer's loads. According to the POCO's service manual, only 480Y/277V is available.

Thus, to beat a dead horse, can we serve a 480V delta primary : 208Y/120V secondary transformer from a 480Y/277V electrical service?

This topic was brushed on in these two threads, but I was hoping to get an answer for this in a more general sense for the sake of education.

Feeding a Delta to Wye transformer with a Wye primary?

I have a customer that has a piece of equipment that was shipped with a 15kva 3-phase Delta to Wye transformer. The only power at the facility is Wye configuration. I have been told by some that I can still use the transformer and to just not run the neutral to the transformer. If this is true...

forums.mikeholt.com

working with wye and going to delta

I work at an old manufacturing plant which has 3ph 240v wye. We have a need for 3ph 480 volt to run a new piece of equipment. My plans were to reverse connect a 112.5Kva step down transformer that would give me the 240v to 480v but am concerned with the wye element of our current wiring...

forums.mikeholt.com

Here is a graphic of the connections, grounding, and bonding that I think are required. Please let me know if you have any feedback or need additional information.

https://imgur.com/a/3UcvwPI

I see that I can't post photos yet. I don't mean any disrespect by my attempt to circumvent the rules by using a link. It's to clarify my request, rather.

Take care,
AsparkyBOI

 

[infinity]

can we serve a 480V delta primary : 208Y/120V secondary transformer from a 480Y/277V electrical service?

Yes it's done everyday and is the standard for most 480 volt systems where you use step-down transformers for the 120 volt loads. I enlarged your sketch for easier reading:

 

[jim dungar]

can we serve a 480V delta primary : 208Y/120V secondary transformer from a 480Y/277V.

Yes
You ignore the fact a neutral exists. Just bring the 3 phase wires to the delta primary.
You may not need a local transformer disconnect if the feeder OCPD is padlockable.

 

[AsparkyBOI]

Yes it's done everyday and is the standard for most 480 volt systems where you use step-down transformers for the 120 volt loads.

Thanks for this. Shopping around for stepdown transformers on manufacturers' websites suggests this as well (it seems less common to find a WYE on the high side).

Does anyone have any feedback on the grounding/bonding? This gets into separately derived systems and what that means in practical terms. I think we have it figured out, but wanted to double check if possible.

Mike Holt's illustrated guides have also been helpful on our end indeed!

 

[infinity]

Does anyone have any feedback on the grounding/bonding?

The primary 480 side will have an EGC run with the feeder. The NEC requires a terminal for all of the bonding connections within the transformer for the system bonding jumper (SBJ) bonding the case to the X0, the primary EGC, and the supply side bonding jumper (SSBJ). The GEC can connected directly to the X0 or the terminal with a wire type SBJ. The secondary has a SSBJ run with the conductors to the secondary OCPD, it can be certain metal raceways and not a wire type SSBJ. The GEC will be run the building grounding electrode system (GES).
Code tables:
EGC-T250.122
SSBJ, SBJ-T250.102(C)(1)
GEC-T250.66

 

[AsparkyBOI]

Thanks Rob. I think that cleared it up. Here's an updated schematic.

I updated the transformer graphic to note "grounding terminal" and "x0".

I added the first OCPD on the secondary side and annotated the SSBJ.

Do you see any other immediately apparent concerns?

 

[infinity]

Do you see any other immediately apparent concerns?

It appears that you've called for a continuous EGC across the 480 primary disconnect. That is not required to be continuous unless you meant something else.

 

[AsparkyBOI]

Good point. I think better wording on my part would have been "electrically continuous"".

I took another gander at Mike Holt's illustration for (NEC 2020) 250.30(A)(1) and updated my drawing.



I think we're getting closer to a typical detail for this (480Y service entrance to 480V delta transformer) service application. Would you agree?

 

[infinity]

I think we're getting closer to a typical detail for this (480Y service entrance to 480V delta transformer) service application. Would you agree?

Everything looks correct. One other thing worth mentioning is that a primary disconnect may not be required for the transformer if it complies with 450.14.

450.14 Disconnecting Means.
Transformers, other than Class 2 or Class 3 transformers, shall have a disconnecting means located either in sight of the transformer or in a remote location. Where located in a remote location, the disconnecting means shall be lockable open in accordance with 110.25, and its location shall be field marked on the transformer.