Delta and wye transformer

[Jemstone85]

I have a 277/480Y service in this building and need 230 volt 3 phase for a piece of equipment. Can I use an existing transformer that is a 480 delta 3 wire step down to a 120/240 delta 4 wire and just don't use the supplied neutral on the secondary. Or should I buy a new transformer that's 277/480 to 120/240 3 phase?

 

[augie47]

IMO, the best route would be to use the transformer you have and bond/ground the neutral per 250.30

 

[Jemstone85]

IMO, the best route would be to use the transformer you have and bond/ground the neutral per 250.30

That's what someone else suggested. Thanks

 

[Jemstone85]

That's what someone else suggested. Thanks

I have always had the correct transformer and didn't know if the delta windings would respond to a 277/480 wye supply the same.

 

[retirede]

I have always had the correct transformer and didn't know if the delta windings would respond to a 277/480 wye supply the same.

As long as the primary windings are fed with the proper voltage, it doesn’t care about the source configuration.

 

[augie47]

Word of caution: Just ignore the neutral on the 277v side (occasionally someone will connect it).

 

[GeorgeB]

I have a 277/480Y service in this building and need 230 volt 3 phase for a piece of equipment.

It's worth asking what the loads will be. If there is a VFD, you probably want wye out. I believe that 480 delta:240/138 wye is a catalog item with most manufacturers. Of course, the 138 won't be used.

 

[Jraef]

I have a 277/480Y service in this building and need 230 volt 3 phase for a piece of equipment. Can I use an existing transformer that is a 480 delta 3 wire step down to a 120/240 delta 4 wire and just don't use the supplied neutral on the secondary. Or should I buy a new transformer that's 277/480 to 120/240 3 phase?

120/240 delta 4 wire and 120/240 3 phase are the same thing. You can't get 120V without the 4th wire, so if you wanted just 3 wire, it would just be 240V 3 phase. You don't HAVE to use the neutral (4th wire) though, so the transformer you already have is what you need, notwithstanding the issue raised by GeorgeB.

And just to be pedantic...
"120/240V" would be how you describe a single phase service. To differentiate and avoid confusion, when describing a 3 phase service you always put the higher voltage first. So it would be 240/120 3 phase 4 wire. Not everyone knows this or does it, but officially, that's the way it is.

 

[Flicker Index]

120/240 delta 4 wire and 120/240 3 phase are the same thing. You can't get 120V without the 4th wire, so if you wanted just 3 wire, it would just be 240V 3 phase. You don't HAVE to use the neutral (4th wire) though, so the transformer you already have is what you need, notwithstanding the issue raised by GeorgeB.

And just to be pedantic...
"120/240V" would be how you describe a single phase service. To differentiate and avoid confusion, when describing a 3 phase service you always put the higher voltage first. So it would be 240/120 3 phase 4 wire. Not everyone knows this or does it, but officially, that's the way it is.

I was aware it's written that way by convention.. but is that ANSI or IEEE? Do you happen to have the standard # that defines the nomenclature?

 

[kwired]

As long as the primary windings are fed with the proper voltage, it doesn’t care about the source configuration.

IOW, if the source is corner grounded or even ungrounded, as long as you input 480 volts line to line on all three lines the secondary output will still be the same.

 

[kwired]

I have a 277/480Y service in this building and need 230 volt 3 phase for a piece of equipment. Can I use an existing transformer that is a 480 delta 3 wire step down to a 120/240 delta 4 wire and just don't use the supplied neutral on the secondary. Or should I buy a new transformer that's 277/480 to 120/240 3 phase?

Chances are the neutral that is made available can be left floating and you would be capable of using this as corner grounded or ungrounded if you desired on the secondary. Most would probably ground the neutral though unless there were good reason to need otherwise, which is more likely to be the ungrounded application if anything has a definite need.

 

[robertd]

Word of caution: Just ignore the neutral on the 277v side (occasionally someone will connect it).

What happens if you connect it?

 

[winnie]

What happens if you connect it?

It depends on the specifics of the situation.

If you have a 480/277 wye service feeding a transformer with a 480V delta primary winding, there is no place to connect the 480/277 neutral. The grounding/bonding of the secondary will actually create a connection to the grounded/bonded neutral of the service. This is a normal setup with the correct sort of transformer, no problem.

A common situation is where a wye:delta transformer is 'reverse fed', for example a 208/120 wye secondary is being used as the primary. If you connect the service neutral to this wye _primary_ then you can have problems. In such a setup current circulates to try to force the supply voltages to be balanced. This can easily overload the transformer and circuit, without even supplying a load.

Jon

 

[kwired]

What happens if you connect it?

If you have a three wire primary there is nothing to connect it to without displacing one of the phase conductors.