480/277wye to 240/120 delta transformer selection

[chuckmanj]

I'm having a tough time finding a replacement 30kva dry transformer to go from 480/277 wye 3 phase to 240/120 delta 3 phase. Most choices are either 480delta 3 phase to 240 delta w/ 120V lighting tap or 480 delta to 240 delta. This is the second transformer to go bad (probably due to lightning). The first one had the usual H1,H2,H3 and X1,X2,X3,X0 connections with X0 going to the neutral bar in the sub panel and ground strap on transformer. The second (present) transformer, which just bit the dust, was a 480 delta to 240 delta configuration with the 120V midtap. The only difference that I could see is that there was an X4 terminal instead of X0. The subpanel neutral was connected to X4 in this case which doesn't seem correct to me. I was under the impression that X4 would be for the 120V midtap.

My question is, can I connect 480/277 wye to a 480 delta transformer and still power a subpanel that has a 240V 3 phase pump motor and (2) 120V circuits w/o any problems????
Any help would be greatly appreciated.

 

[Smart $]

I'm having a tough time finding a replacement 30kva dry transformer to go from 480/277 wye 3 phase to 240/120 delta 3 phase. Most choices are either 480delta 3 phase to 240 delta w/ 120V lighting tap or 480 delta to 240 delta. This is the second transformer to go bad (probably due to lightning). The first one had the usual H1,H2,H3 and X1,X2,X3,X0 connections with X0 going to the neutral bar in the sub panel and ground strap on transformer. The second (present) transformer, which just bit the dust, was a 480 delta to 240 delta configuration with the 120V midtap. The only difference that I could see is that there was an X4 terminal instead of X0. The subpanel neutral was connected to X4 in this case which doesn't seem correct to me. I was under the impression that X4 would be for the 120V midtap.

My question is, can I connect 480/277 wye to a 480 delta transformer and still power a subpanel that has a 240V 3 phase pump motor and (2) 120V circuits w/o any problems????
Any help would be greatly appreciated.

Typically, the center-tap terminal of delta-secondary stepdown transformer is marked X4. A transformer which has a terminal marked X0 should have wye-configured windings.

Not sure that you worded you main question correctly... because you can't power a 240/120V 3? 4W panel with any 480V to 480V transformer.

You can use either a wye or delta configured 480V primary. I am of the impression that you get better performance out of a delta configured primary. It will work just fine connected to a 480Y/277 3? 4W system... there is simply three primary line connections, i.e. no neutral connection on the primary side even if you have a wye configured primary.

The secondary of the transformer you choose needs to be configured for 240/120 3? 4W... with conventionally marked terminals—X1, X2, X3, and X4, the latter of which is supposed to be the center-tap of the winding directly between X1 and X2... but you should always check the transformer's documentation and/or nameplate diagram/chart to verify connections

 

[chuckmanj]

480/277wye to 240/120 delta transformer selection

Thanks Smart $.
Sorry about the scatter-brained question wording...it was late. What I meant to ask is If I have 480/277 wye coming into a dry transformer that is specified as a "480 delta primary to 240 delta secondary transformer" what will I have to do to ensure that I get 240 delta in my subpanel where A phase is 120v to ground, B phase is the "stinger" leg (208v to ground), C phase is 120v to ground and of course phase to phase is 240v.

Thanks.

 

[jim dungar]

Thanks Smart $.
Sorry about the scatter-brained question wording...it was late. What I meant to ask is If I have 480/277 wye coming into a dry transformer that is specified as a "480 delta primary to 240 delta secondary transformer" what will I have to do to ensure that I get 240 delta in my subpanel where A phase is 120v to ground, B phase is the "stinger" leg (208v to ground), C phase is 120v to ground and of course phase to phase is 240v.

Thanks.

You do not need to bring the neutral (277V) to the primary side of a transfromer. In fact very few, non-utility, transfromers would allow a primary neutral. So, just bring in the 3 primary phase conductors and use a standard 480V delta primary to 240/120 3PH 4W transformer.

 

[Smart $]

... It will work just fine connected to a 480Y/277 3? 4W system... there is simply three primary line connections, i.e. no neutral connection on the primary side even if you have a wye configured primary.

...

You do not need to bring the neutral (277V) to the primary side of a transfromer. In fact very few, non-utility, transfromers would allow a primary neutral. So, just bring in the 3 primary phase conductors and use a standard 480V delta primary to 240/120 3PH 4W transformer.

Thanks Smart $.
Sorry about the scatter-brained question wording...it was late. What I meant to ask is If I have 480/277 wye coming into a dry transformer that is specified as a "480 delta primary to 240 delta secondary transformer" what will I have to do to ensure that I get 240 delta in my subpanel where A phase is 120v to ground, B phase is the "stinger" leg (208v to ground), C phase is 120v to ground and of course phase to phase is 240v.

Thanks.

If you happen to get a wye-delta xfmr, the following diagram represents the typical connections. Note there is no H0 connection.

Also note that to maintain the same rotation as the primary and have B phase as the high leg, the connections will have to be:

X1 to C?
X2 to A?
X3 to B?
X4 to N&G​

 

[chuckmanj]

Thanks Jim.

Just to make sure that I'm totally clear on this before I order it. By standard 480 delta primary to 240/120 3ph 4wire secondary you mean the transformer with or w/o the 120V center tap??? Will the transformer w/o the 120V center tap have 4 wire secondary connections and if so, where does the 4th wire usually connect to (i.e. neutral)?

Sorry to be so anal about this.

 

[chuckmanj]

Smart $:

I saw that one from Square D but cost and availability prohibit it as an option. If possible I just wanted to use a Delta primary to Delta secondary transformer if it will work w/o any problems.

You guys are great and thanks for all the input!

 

[Smart $]

Thanks Jim.

Just to make sure that I'm totally clear on this before I order it. By standard 480 delta primary to 240/120 3ph 4wire secondary you mean the transformer with or w/o the 120V center tap??? Will the transformer w/o the 120V center tap have 4 wire secondary connections and if so, where does the 4th wire usually connect to (i.e. neutral)?

Sorry to be so anal about this.

The notation "240/120 3? 4W" actually indicates a center-tapped delta configuration. It should be configured as in the diagram I posted above.

If the secondary were wye configured, the notation would be 208Y/120 3? 4W.

 

[Smart $]

Smart $:

I saw that one from Square D but cost and availability prohibit it as an option. If possible I just wanted to use a Delta primary to Delta secondary transformer if it will work w/o any problems.

You guys are great and thanks for all the input!

Actually I recommed using a delta-delta xfmr... just couldn't find a diagram for it as easy as I could find the wye-delta one.

Regardless, the secondary should be configured exactly the same on a delta-delta...

 

[LarryFine]

The subpanel neutral was connected to X4 in this case which doesn't seem correct to me. I was under the impression that X4 would be for the 120V midtap.

They're the same thing. Then neutral IS where the 120v mid-tap is supposed to go. That's how you get 120 between the neutral and either of its two hots.

 

[LarryFine]

What I meant to ask is If I have 480/277 wye coming into a dry transformer that is specified as a "480 delta primary to 240 delta secondary transformer" what will I have to do to ensure that I get 240 delta in my subpanel . . .

Just connect the Delta 480v primary to the three lines. Ignore the supply's neutral.

 

[LarryFine]

By standard 480 delta primary to 240/120 3ph 4wire secondary you mean the transformer with or w/o the 120V center tap???

If you want 120v, you must get the tap.

Will the transformer w/o the 120V center tap have 4 wire secondary connections and if so, where does the 4th wire usually connect to (i.e. neutral)?

No, which is why you'd have no 120v without it.

The tap gets treated just like an incoming service neutral. Bond it and use it as you would a 120/2430v 1ph neutral.

 

[chuckmanj]

Got It! Makes perfect sense now.
Thanks again for the help.

 

[LarryFine]

Bond it and use it as you would a 120/2430v 1ph neutral.

That's supposed to be "120/240v 1ph." :roll:

 

[zog]

Just connect the Delta 480v primary to the three lines. Ignore the supply's neutral.

So what do you do with the neutral?

 

[LarryFine]

So what do you do with the neutral?

:grin:​

 

[zog]

Larry, you know how to make a point Just asking becasue with all the transformers I have tested I have never seen it, just wondering what you sparkies do with it, this thread got me thinking.

 

[Jeffrey Jay]

This forum rocks!