3 phase transformer question

Mrdave714

Member
Location
California
Occupation
Electrical Contractor
40 years in, but I don't do a lot of 3 phase work.

I've got a commercial project. 3 existing services. 2 of them are 1Ph 240/120. The 3rd service is 3Ph 240v 3-wire corner grounded.

The 3P 240v service is supplying a 3ph 208Y120 transformer.

I'm thinking of combining all 3 of the services into a single 3ph 4 wire 240/120 service. (if POCO can provide)

Question is: Can I use the existing transformer on the new 4 wire service or is it only good for 3-wire corner ground supply?

My 2nd question is as I'm new here, how do I attach a photo of transformer faceplate for reference so people can answer ???

Thanks in advance for any help!
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The transformer will be fine with any 240V three phase source, and will be just as happy with the high leg delta as the corner ground delta.

If I were combining services, I'd consider changing to 208V 3 phase service (most common) and then using a transformer only for loads requiring 240V. But there may be reasons that you need the center tapped 120/240V single phase that you've not mentioned.

At the top of the box where you enter your post there are little icons for things to do. The 'chain' icon lets you add a link (say to an image hosting website), the picture icon lets you add an image.

20220930_053303.jpg
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
If your service is at the 240V level, the transformers probably belong to the utility.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As I read the OP, the utility is providing 120/240V single phase services, and a separate 240V corner grounded delta service. Internally to the facility there is a customer owned 240V delta to 208/120V wye transformer. The OP wants to change what the utility is providing to 240/120 high leg delta and keep the internal customer owned transformer.

-Jon
 

Mrdave714

Member
Location
California
Occupation
Electrical Contractor
As I read the OP, the utility is providing 120/240V single phase services, and a separate 240V corner grounded delta service. Internally to the facility there is a customer owned 240V delta to 208/120V wye transformer. The OP wants to change what the utility is providing to 240/120 high leg delta and keep the internal customer owned transformer.

-Jon
Yes this is correct. I want to combine the three services into one 3 phase 4wire and keep the customer owned 208Y120 transformer

here's a picture of transformer plate. wasn't sure is this transformer only works on 240V corner grounded delta, or can it be used on the new 4-wire 3 phase.

CC Transformer.jpg
 

Mrdave714

Member
Location
California
Occupation
Electrical Contractor
for clarity, there are both 240v and 120v 1phase loads on the 2 existing 1 phase services, and 3 phase 208v and 1 phase 120v loads on customer transformer. I don't want to change the existing transformer or get new transformer.
I Thought a 240/120 three phase service and retain existing 208y120 customer transformer sounded like best option if poco can provide.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
That is a standard delta-wye transformer and will work just fine with a 240/120 high leg delta supply.

A suitably sized 240/120 high leg service will best fit supplying this transformer as well as supplying the existing 120/240V loads as a single service.

However 240/120 high leg is much less common than 208/120 wye service. 208/120 will directly supply your existing 208/120 loads, and can also supply any 120V loads on the 120/240 single phase stuff. Where 208 gets ugly is if you have loads that _must_ have 240V, or if you have shared neutral circuits with derating issues. (Take a single phase shared neutral MWBC and move it to a 208/120 service and the neutral counts as a 'current carrying conductor', increasing the number of CCC in a conduit, for example.) The benefit for you is _eliminating_ a customer owned transformer with its losses.

-Jon
 

Mrdave714

Member
Location
California
Occupation
Electrical Contractor
Thanks for the help. I wanted to be sure I can reuse customer transformer before I speak with poco about what they can supply for 4 wire service (if any)
Definitely have equipments that is both 240v and 208v only.

Will have to reassess options once poco tells me what 3 phase service they are willing to supply.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
One thing to be aware of, the 240/120 3 ph 4 wire service will likely have a limit to the amount of single phase l20V load it can handle, so combining the two existing single phase services may be more problematic than the utility will be willing to support. They will let you know after you apply, but just be ready with a plan B.
 
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