New transformer installation

[rojay]

A recent request by a customer to add a subpanel has opened a can of worms. The panel that is to feed our sub is 225 amp 208/120v three phase. Sub is single phase 60 amp 240/120v. Upon opening the 225 amp panel, it was discovered that it currently is only serving three phase loads, and no neutral was brought to it. Panel is on the 7th floor of the building, and MDP several hundred feet away. The "solution" of adding a 15kva single phase transformer was suggested. 208 on the primary side and 120/240 at the secondary. This seems crazy to me. Does it make sense to add a transformer just to establish a neutral point?:huh:

 

[roger]

Does it make sense to add a transformer just to establish a neutral point?:huh:

With the information you have provided, yes it does make sense.

Roger

 

[augie47]

It certainly is one solution.
Keep in mind, however, that your will need to establish a grounding electrode fro the new transformer and perhaps 250.30(A)(4) will apply.
It might be easier to route a neutral from the existing transformer than to comply with 250.30

 

[rojay]

Establishing a grounding electrode for the new transformer was one of the major concerns that I had with this proposed solution. The equipment is located in the penthouse of a seven story building. Any structural steel that I've seen seems to be encased in concrete. If I was able to expose the steel, how do I know that its effectively grounded? There is a ground bar for some cell equipment up there. It runs all the way into the basement. It is fed with a #2 though. If this were a 3/0 would I be able to use it as a common grounding electrode as in 250.30a6? Does the fact that this is only a single separately derived system rule out this possibility anyway? Not really sure of what my most cost effective option would be .

 

[petersonra]

Establishing a grounding electrode for the new transformer was one of the major concerns that I had with this proposed solution. The equipment is located in the penthouse of a seven story building. Any structural steel that I've seen seems to be encased in concrete. If I was able to expose the steel, how do I know that its effectively grounded? There is a ground bar for some cell equipment up there. It runs all the way into the basement. It is fed with a #2 though. If this were a 3/0 would I be able to use it as a common grounding electrode as in 250.30a6? Does the fact that this is only a single separately derived system rule out this possibility anyway? Not really sure of what my most cost effective option would be .

250.30a6 does not seem to allow the common GEC to be a bus bar or you could potentially replace the #2 in the basement with a 3/0.. It does allow the metal frame of the building to serve as the common GEC.

 

[rojay]

250.30a6 does not seem to allow the common GEC to be a bus bar.

I believe 250.30A6c2 allows the connection to a ground bar. I was concerned that there may be an issue with the fact that this was a ground bar that is serving cell equipment. It really doesnt serve as the grouding electrode conductor for multiple separately derived systems as 250.30A6 allows.

 

[qcroanoke]

250.30a6 does not seem to allow the common GEC to be a bus bar or you could potentially replace the #2 in the basement with a 3/0.. It does allow the metal frame of the building to serve as the common GEC.

I would say the cell company will not allow you to connect to their ground bar. No real reason, they just probably wont.