X-former neutral

[eager2learn]

We have a 3ph 208v circuit feeding a new transformer. The transformer was installed to pick up a neutral also rated at 208v on the secondary side. The transformer is 45kva delta wye. The existing amperage on the primary side is 175a. Can we utilize the existing feeder on the primary side and extend that circuit to an outside load that needs a neutral. If we extend the primary side through the transformer with double barrel mechanical chair lugs can we utilize the neutral on the secondary side of the transformer with the primary side conductors extending to the outside load. The outside load is a sub panel rated at 150a. The secondary side of the transformer feeds a120/208 100a panel. I hope this makes sense. All in all, is it legal to utilize the neutral on the secondary side with the primary side conductors in the same raceway considered it’s all in the transformer. Thank you in advance.

 

[infinity]

If I'm understanding this correctly you have a 3-wire circuit feeding the primary of a transformer and you want to tap off that circuit to feed another load that requires a neutral which you'll get from the secondary of the transformer? Why wouldn't you feed the other load from the secondary?

 

[kingpb]

Recap:
45KVA
208-208Y/120V Dy
Rated amps is 125A

Assuming that the impedance is 6% or lower and their is secondary protection not greater than 125% (156A), the 175A breaker/circuit feeding the delta side of the transformer would be OK.

Forget the neutral issue. Loading on XFMR feeder circuit: if breaker is 175A and you want to feed a 150A panel and a 100A panel (secondary side of transformer); you could potentially have 200A of continuous load on that circuit. Do the sub-panels have mains? I assumed the earlier at least the 100A does. I think a load analysis is in order. As far as neutral, it sounds like your looking for a way to not upsize/replace the transformer, for whatever reason. In that regard, my logic alone says it's not a good way to accomplish your goal. I have never been in favor of cutting corner installations, except in a temporary emergency situation.

 

[jim dungar]

No, you can not use a secondary side neutral, from a standard isolation transformer, to supply a load tapped off of the primary side feeder even though they have the same lines line voltage.

Might your transformer be a "grounding" autotransformer? Then you could do what you want as as long as the neutral has enough capacity.

 

[eager2learn]

If I'm understanding this correctly you have a 3-wire circuit feeding the primary of a transformer and you want to tap off that circuit to feed another load that requires a neutral which you'll get from the secondary of the transformer? Why wouldn't you feed the other load from the secondary?

The transformer is undersized. The additional load is a 150a storage facility. The load on the secondary side of the transformer is rated at 125A. They want to pick up the feeder voltage before it goes through the transformer but want to pick up a neutral for the additional load (storage) on the secondary side. Is there a code reference stating that you can’t

 

[eager2learn]

No, you can not use a secondary side neutral, from a standard isolation transformer, to supply a load tapped off of the primary side feeder even though they have the same lines line voltage.

Might your transformer be a "grounding" autotransformer? Then you could do what you want as as long as the neutral has enough capacity.

It’s there a code reference?

 

[jim dungar]

It’s there a code reference?

No.

My answers are based on the physics of the transformer connections.