transformer sizing

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papalarge

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Hi everyone! First timer lol!
Here it goes. My boss wants two panels installed. One is a 208-120v 3 phase 250a panel w/ 250a main breaker. The other is a 240v 3 phase 225a panel w/
225a main breaker. I told my boss to get two 75kva transformers. He wants to get two 112.5kva transformers. So I showed him the calculation,
secondary current 208a x 1.25 = 260a, 250a main breaker 250a panel. Secondary current 180a x 1.25 = 225a, 225a main breaker 225a panel. This is what was taught to me by an instructor in college. Who is correct?
 
Maybe I'm misunderstanding this, but it looks like a 75kva x-fmr will barely cover your antispated load of 208 amps. I know you can get more then that out of the transformer (125%), but it is generally not a good design. I would think the 112.5 would be a better option.
 
Looks to me like they would work. You can always put in a larger transformer, you just won't get the use of the larger size. Make sure it isn't needed because of the loads, e.g. motors connected to it.
 
papalarge said:
Here it goes. My boss wants two panels installed. One is a 208-120v 3 phase 250a panel w/ 250a main breaker.
Click to expand...

How were the sizes of the mains determined. Was a load calculated? If not I think that needs to be the starting point.
 
transf.

transf.

I'd look at it like this:

1. as Mike said, calculate the load..see how close you are "pushing the envelope"

2. remember the 208/120 load might not be balanced.

4. are $$ an issue? how about $$$ vs possible downtime ??

3. If the BOSS says 112 and you talk him into a 75 and it does fail....................................
Well, some of life's lessons are rough.

Its not always just the math.
 
For both of those applications I would use 100 kva transformers minimum. Using the the 1.25 (or .8) you assume you will never load the panels above 80%, that is the purpose of the factors - remembering that the purpose of the NEC / NFPA is fire prevention & safety. Using the .8 factor as you are applying it would load the transformer beyond FLA. I always provide full kva for the panel(s) that are attached - this allows for growth, potential "mistakes" or over loads, motor starting, etc...and not running the transformer at capacity provides for longer life (less heat) and better voltage regulation - if 112.5 is the next standard size, buy them.
 
Be careful with sizing a dry type distribution transformer. One can expect the lifetime to be about 30 years with an average load of 35% based upon industry standards according to my applicaiotn notes.
At 100% load anticipate the life to be severely reduced to about 3-5 years.
This is based upon an avg. amb. temp of 30degC w/40degC max.
For each 10degC above 30degC the transformer life is reduced by 50%.
So don't get tunnel vision just on the kva of the transformer.
 
Templdl, Those are interesting statistics. Where did you find those, are they published by a manufacture?
 
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