480 volt wye to delta terminations

[Ron Dougherty]

A building has a 480v Wye service. Mail panel has a 70A. breaker terminated to a 50 kva step down transformer to 120/208v (the transformer input is (3 - hot wires and ground), no neutral. everything is working fine on the secondary. We have a new 480 volt wye generator unit and want to terminate it to through an automatic transfer switch, 3 - hots and no neutral?, - Is a Floating Wye considered a Delta or Floating Delta? Is this proper or safe? - am I missing something? I Got rotation meter.>>

 

[ptonsparky]

Supply the Main panel from the generator via the xfer switch. Both Utility & Gen are 480 Wye. By all means include the neutral. It is required. Leave everything else alone.

Look at the Wye and Delta symbols & you can see they are not the same. (No a floating Wye is not consider Delta of any kind.)

 

[LarryFine]

We have a new 480 volt wye generator unit and want to terminate it to through an automatic transfer switch, 3 - hots and no neutral?

The neutral does not need to be switched, but should be connected, giving you a non-separately-derived supply, which is almost always less complicated.

 

[Ron Dougherty]

There is nowhere to terminate the neutral to.? The existing transformer has only 3 hots and a ground wire, No neutral going to transformer

 

[LarryFine]

There is nowhere to terminate the neutral to.? The existing transformer has only 3 hots and a ground wire, No neutral going to transformer

Right, the transformer primary itself doesn't have a neutral, but the building does.

The the generator neutral should be connected to the utility neutral, usually in the ATS.

 

[Birken Vogt]

I have a customer with 3 services set up like this. They have no neutral and I suspect you don't either. The wye point at each generator is grounded to the bare ground and carried back to the transfer switch ground. I believe this is the way it should be. Since there are no L-N loads anywhere, it all goes to delta input transformers, no current flows on the "neutral" so it is OK this way.

 

[LarryFine]

A building has a 480v Wye service.

a 50 kva step down transformer to 120/208v

We have a new 480 volt wye generator ...

Something seems to be suggesting neutrals to me.

 

[ptonsparky]

I have a customer with 3 services set up like this. They have no neutral and I suspect you don't either. The wye point at each generator is grounded to the bare ground and carried back to the transfer switch ground. I believe this is the way it should be. Since there are no L-N loads anywhere, it all goes to delta input transformers, no current flows on the "neutral" so it is OK this way.

They have a neutral, a grounded conductor, just no neutral load. It is still required.

 

[Jpflex]

A building has a 480v Wye service. Mail panel has a 70A. breaker terminated to a 50 kva step down transformer to 120/208v (the transformer input is (3 - hot wires and ground), no neutral. everything is working fine on the secondary. We have a new 480 volt wye generator unit and want to terminate it to through an automatic transfer switch, 3 - hots and no neutral?, - Is a Floating Wye considered a Delta or Floating Delta? Is this proper or safe? - am I missing something? I Got rotation meter.>>

I don’t know what a “mail panel” is but if you are feeding a 50KVA step down transformer (assuming conversion to a single phase system as you did not specify?), then why are you terminating the service at 70 amperes?

The primary current of the step down transformer will be 50,000 KVA / 480 v input = 104 i amperes

The OCPD for primary of step down transformer (as far as I know) will be 250% x 104 amperes = 260 i ampere breaker (size to next size lower breaker for standard breaker) for inrush current but not 70 ampere breaker

A 3 phase system such as delta/ delta separately derived system transformer which does not have a system bonding jumper or no ground conductor, bonding connected between the transformer secondary windings and cabinet/ EGC system, is considered an UNGROUNDED SYSTEM even though the cabinets are to be bonded together and connected to a ground rod. This is a grounded cabinet for transformer but not a grounded system

This set up requires ground detectors to be installed instead of a grounded system (such as delta/ delta) Since you do not have the system grounded it is LESS SAFE than a grounded system because a ground fault, line to ground will not trip a breaker. This is why ground detectors are required to set an alarm or light to alert of this fault.

 

[kwired]

There is nowhere to terminate the neutral to.? The existing transformer has only 3 hots and a ground wire, No neutral going to transformer

There is no loads utilizing said neutral. It is still the grounded conductor and ultimately needs to tie into the service grounded conductor and the grounding electrode system. If there were neutral loads and this were non SDS transfer system you would be running separate neutral and grounding conductors and to be technically correct with the requirements you still should be in most cases. The neutral goes to the service and main bonding jumper, the EGC is coming back but ties only to the generator frame and not directly the wye point of the windings.

 

[ptonsparky]

I don’t know what a “mail panel” is but if you are feeding a 50KVA step down transformer (assuming conversion to a single phase system as you did not specify?), then why are you terminating the service at 70 amperes?

The primary current of the step down transformer will be 50,000 KVA / 480 v input = 104 i amperes

The OCPD for primary of step down transformer (as far as I know) will be 250% x 104 amperes = 260 i ampere breaker (size to next size lower breaker for standard breaker) for inrush current but not 70 ampere breaker

A 3 phase system such as delta/ delta separately derived system transformer which does not have a system bonding jumper or no ground conductor, bonding connected between the transformer secondary windings and cabinet/ EGC system, is considered an UNGROUNDED SYSTEM even though the cabinets are to be bonded together and connected to a ground rod. This is a grounded cabinet for transformer but not a grounded system

This set up requires ground detectors to be installed instead of a grounded system (such as delta/ delta) Since you do not have the system grounded it is LESS SAFE than a grounded system because a ground fault, line to ground will not trip a breaker. This is why ground detectors are required to set an alarm or light to alert of this fault.

I believe it is a 3 phase transformer so divide by 1.73. 70 amp primary protection is small but if works, it works

 

[Birken Vogt]

They have a neutral, a grounded conductor, just no neutral load. It is still required.

I guess you could bring a 14 awg neutral from the service to the transfer switch and generator if you wanted to say this. Neutral load = 0 so it's the minimum size wire. Would be less safe than grounding with the heavier ground wire though, in case of a fault that neutral would vaporize.

I am still not convinced it goes against code to do it the way it is, though. They are a sizeable water district and all the work was done by a large contractor that does impeccable work and I am sure it was drawn that way by an engineer and bid out and all that. And they have 3 of them all done at different times.

 

[LarryFine]

I guess you could bring a 14 awg neutral from the service to the transfer switch and generator if you wanted to say this. Neutral load = 0 so it's the minimum size wire. Would be less safe than grounding with the heavier ground wire though, in case of a fault that neutral would vaporize.

Actually, the minimum size would be the required EGC size for the service or feeder size.