How to calculate total 3-phase current using single-phase transformers 1

[stockman1]

Overview: In an industrial building, I am upgrading our current 3-phase 480VAC 100A panel to a 200A panel. The transformers,7200:480 step-down, suppling the power off the utility pole, are configured open-delta (also called Wild Leg) using (2) 25KVA transformers.

Question: What size and how many single-phase transformers do I need, for a delta:grounded Y system, 480VAC, 3-phase, 200A service?

 

[BJC]

An open delta system is not necessarly a "wild leg" transformer. On of the transformer would have to be center tapped for that to happen.
Most of the wild leg sytems I have seen are 240 delta with the center tapped transformer supplying 120 volt. Utilities u here in the land of slugs and moss will not supply a building with a wild leg system. They will service an existing one. A 200 amp 480 volt service is 166KVA so you can use 3 50kva units. The utility will most likely use 3 37.5 KVA units.

 

[rbulsara]

Since you want a grounded wye service, you would need three transformers, so do what BJC said.

For the records, the V connected transformer will provide 86.6% its rated capacity or 57.7% of a 3-transformer bank of the same size.

For example, the two 25kVA units are only good for 43.3 kva which is 57.7% of 75kVA.

Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

I don't know if I got it right; you intend to replace the existing transformers with another set of transformers to supply an electrical panel with a 200A main breaker? What BJC just said will do.
But you will have to specify 3 X 37.5kVA, 7200/277V transformers to get 480V-3phase grounded Y! Your full-load amps will be 135A and 200A breaker will be fine.

 

[stockman1]

Thanks for the response.

Could you please show me in detail, step-by-step, how you calculate the total available current using (3) single-phase transformers? Do you overdrive these transformers? The field consultant for the utility is planning to install (3) 25KVA transformers, however, I am not sure if they will provide enough power.

 

[burnt2x]

Open-delta kVA = 57.7% of Delta kVA = 0.577 X 3 X 25kVA = 43.3kVA
Full-load amps for 3 X 25kVA = 75,000VA/(1.732 X 480) = 90.2Amps; Breaker size = 1.7 X 90.2 = 153A, say 150A, 200A is too large.

 

[waross]

For a wye connection, the rated full load current of each transformer will be 25000 VA / 277 V = 90 Amps per phase.
The utility will probably be using your billing history to size the transformers and will allow short time overloads on their transformers.
Are these transformers utility owned? If so, it's not really your problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

stockman:
200A service is only good for 80% of its rating, 160A. Utility co.'s routinely size their transformers below design loads.


I am now confused as to are you upgrading utility transformers or just need a 480/277 grounded wye service?

I would not worry about what utility co. does, they will replace them if they have to. Just ask for what you need that is a 200A, 480Y/277V service.


Rafiq Bulsara

http://www.srengineersct.com

 

[stockman1]

rbulsara:
Yes, we are changing our utility service to a grounded-Y system on the 480 side to meet arc-flash requirements.
Yes, I believe that the utility service needs to be increased in sized for our 200A panel.
Yes, we are upgrading our 480VAC panel from a 100A to a 200A panel.

Few more questions:
waross: you calculated 90A per phase. What does that mean for total 3-phase current? 3 x 90A = 270A?

Anybody: when calculating 3-phase current, are the calculations done using L-L 480VAC or L-N 277VAC?

burnt2x: I will be installing a 200A main breaker on my new 480VAC panel. So I need my utility transformers sized accordingly to meet my new panel size (fyi: we are adding additional loads). When you are sizing your breakers, why do you multiply the current by 1.7 (90.2 x 1.7 = 153.34)?

Anybody: what are the NEC requirements for sizing breakers?

 

[BJC]

stockman1
Just curious, don't take this the wrong way but were yoou an electronics or computer engineer and now find yourself in another world?? Calculating power in 3 phase circuits something you get in the first ( or early second year) if you were in a power circulium.
If you were not into power in school I would suggest a Schaums Outline on the subject. tehy are cheap books and well worth the money. They explain vector analysis of three phase systems very well.

 

[rbulsara]

stockman:
So what do all your yes answers mean?
Are you utiltiy co or a user?

BJC is too polite. You have gotten all the answers, if you have new questions, you need to hire some consultant.

Repeating your ignorance will not get you new answers.

Rafiq Bulsara

http://www.srengineersct.com

 

[stockman1]

I appologize for my ignorance.
I went to school for electronics engineering years ago.
I work for a utility, however a different department takes care of the transformer work.
I am only trying to gain a better understanding of the design criteria. I could hire a consultant but I thought this was too simple of a question.

BJC: I appreciate your response. I was just hoping to see an explaination as to how you came up with your numbers.

 

[davidbeach]

faq238-1287

Happy reading.

 

[BJC]

Stockman 1
I wasn't trying to pick on you. I have worked with and helped people with the same background as you.
Most utilities have standards on how to hook up transformer etc. If your working for one you should have a copy. I would still recommend the Schuams outline series and the IEEE red book. The NEC code digest may also be helpful
For load calculations some of the books written for electricans are better than some of the ones written for engineers. This is especially true when calculating loads for NEC criteria. Find a good technical bookstore and look before yo buy,

 

[stockman1]

BJC:
Thanks for the tips. The field consultant of our utility has suggested using (3) single-phase 25KVA transformers. When I asked how many amps will be feeding my new 200A panel, they were unable to tell me. Thus I will dig deeper to find the answer.

 

[burnt2x]

"The field consultant of our utility has suggested using (3) single-phase 25KVA transformers. When I asked how many amps will be feeding my new 200A panel, they were unable to tell me. Thus I will dig deeper to find the answer."

You should understand that the amps feed depends on the amps draw! Your load will determines that! As for the max amps allowable from your 3 X 25 kVA, the answer is 90.2A.

 

[davidbeach]

Utilities will regularly "under size" their transformers based on years of experience. If you've told the utility that you want a 200A service, don't worry about what size transformers they put in. The utility typically won't up size an existing transformer until it is loaded to 150% or more for extended periods of time.

 

[BJC]

Stockman1
3x25 KVA = 75 KVA

KVA = ExIx 3^.5
Solving ofor I

I=25,000/(480 * 1.732)= 90 amps.
90 amps is the full nameplate load for the transformers you have. That may be enought for your connected load.
Check sec 450 of the NEC. the main breaker can be 125% of full load current.
I would by a 225 or maby even a 250 amp panel and go with a 250 and a 110 or 125 anp main. The proce may not be that much more tahn a 125.
Here's a good referenece to transformer connectionss.

http://www.cooperpower.biz/Library/pdf/R201902.pdf

 

[waross]

If these are Utility transformers, NEC does not apply.
Use a 200 amp panel with 200 amp breaker, and 200 amp service conductors to the point of utility connection. The utility will decide what size transformers and service drop conductors to use based on their own criteria.

Bill
--------------------
"Why not the best?"
Jimmy Carter