Transformer sizing

Status
Not open for further replies.
E

Electspec2002

Member
Location
Lexington, SC
Guys I am currently trying to replace a 60 amp 3 phase 120/208 panel my space is limited. They wish to install a 100 amp 18 or so circuit panel and I am trying to figure out my transformer size. The engineer that is involved in project is telling me I need a 45 kva but I have done a little research and am wondering why I can't use a 30 kva. Does anyone have suggestions?
 
Electspec2002 said:
Guys I am currently trying to replace a 60 amp 3 phase 120/208 panel my space is limited. They wish to install a 100 amp 18 or so circuit panel and I am trying to figure out my transformer size. The engineer that is involved in project is telling me I need a 45 kva but I have done a little research and am wondering why I can't use a 30 kva. Does anyone have suggestions?
Click to expand...

The information that you have given is sketchy, but it is typical to feed a 100A panel with a 30KVA transformer. and 45KVA to feed a 125A panel. The physical size of the transformers isn't a whole lot different. A 30KVA transformer is only good for 83 amps at full load, and it would have a problem starting a big motor load as start up, but in a typical office situation you don't draw that much.
 
Electspec2002 said:
Guys I am currently trying to replace a 60 amp 3 phase 120/208 panel my space is limited. They wish to install a 100 amp 18 or so circuit panel and I am trying to figure out my transformer size. The engineer that is involved in project is telling me I need a 45 kva but I have done a little research and am wondering why I can't use a 30 kva. Does anyone have suggestions?
Click to expand...
Why not just do what the engineer wants? You will make more money and he gets what he wants? win-win.
 
Problem

Problem

The problem with doing what the engineer is asking is from my calculations if I install a 45 kva transformer my ocpd would have to be rated at 175 amps on secondary and that would require a bigger area for the panel than what I have to work with. Since I am replacing a 60 amp panel the area is only about 20" wide by 34" tall. Anything is better than what they had, they were feeding that 60 amp panel with a 9 kva transformer.
 
The transformer size needed depends on the load. The secondary breaker can be sized for less than the transformer full load current.
 
There are so many possibilities that there is no "right" answer. Since there is an engineer involved I would suggest you discuss it with him as you may not have the full picture. There are only a few basics you are required to address: Assure to transformer will carry the load, assure the transformer has proper protection by sizing the primary or secondary overcurrent device and size the conductors correctly. Beyond that, there are so many variables including projected future loads, the engineer's input is pretty much mandatory since he is involved.
 
Where did 175 amps come from?

45kVA / 208 Volts / 1.73 = 125 Amps.

I do lots of 45 kVA transformers feeding 125 Amp panels in projects we build.

And yes, you could use a 100 Amp or a 60 Amp breaker after the transformer, it does not care. The breaker is to protect the downstream wiring. The primary side breaker protects the transformer.

Frank DuVal
 
Status
Not open for further replies.
Top