480delta to208/120 dry transformer connections

[higher power]

I have a 480v delta to 120/208 dry transformer 225kva . Power to the building is 277y (480 between phases). This has not changed since the original installation. The building was stripped of copper and now we are rewiring for a remodel. This transformer was in the building and was used to provide 208/120 power. the 277/480 was used for a/c and lighting. I can't see how the 277y was fed to this transformer to get 208/120 out since it was made to be 480 delta input. Is this possible, if not, then I'm wondering if the power company might have changed from 480 delta to 277y coming into the building, but I have seen the transformer label on the inside and it clearly shows 277y. Nothing appears to have been changed. The original building plans show this transformer was coming off of a main breaker and fed with 350mcm.

 

[roger]

The primary feeder is simply using phases A,B, and C, there is no neutral going to the transformer, hence the "Delta" to Wye

Roger

 

[infinity]

Although the transformer primary coils are connected in a Delta arrangement the supply to the building is a 3 phase, 4 Wire Wye. The correct nomenclature for that system would be 480Y/277.

 

[kwired]

I have a 480v delta to 120/208 dry transformer 225kva . Power to the building is 277y (480 between phases). This has not changed since the original installation. The building was stripped of copper and now we are rewiring for a remodel. This transformer was in the building and was used to provide 208/120 power. the 277/480 was used for a/c and lighting. I can't see how the 277y was fed to this transformer to get 208/120 out since it was made to be 480 delta input. Is this possible, if not, then I'm wondering if the power company might have changed from 480 delta to 277y coming into the building, but I have seen the transformer label on the inside and it clearly shows 277y. Nothing appears to have been changed. The original building plans show this transformer was coming off of a main breaker and fed with 350mcm.

Your answer lies within the question.

 

[ActionDave]

This transformer was in the building and was used to provide 208/120 power.

As common as a Stop sign at an intersection.

I can't see how the 277y was fed to this transformer to get 208/120 out since it was made to be 480 delta input. Is this possible,.... .

The transformer was not fed with 277V, it was fed with 480V; no neutral/grounded is brought to it, only the hots/ungrounded. The result is what is known as an a Separately Derived System. Check out NEC art. 250.30

 

[higher power]

thanks for the "separately derived" tip. I was trying to carry the system neutral through to Xo instead of letting the transformer create its own neutral Xo.

 

[higher power]

does the neutral from the derived system have to remain isolated from the 480y/277 neutral

 

[infinity]

does the neutral from the derived system have to remain isolated from the 480y/277 neutral

Well you wouldn't bring a neutral with the 480 primary side circuit so no you would not create a direct connection. However by virtue of the fact that both systems (480Y/277 and 208Y/120) having grounded neutrals they are connect together.

 

[kwired]

Like Rob said because they are both grounded, they are connected together, which means they will operate at same potential but they are completely separate systems that happen to be tied together at one point. One does not depend on the other to function in any way .