Transformer size

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erickelec

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Houston, Texas, Harris
The load analysis for a church building is 1200A 120/208V 3⌀ , what would be the appropriate transformer size (kVA) should this service require on the power pole.

Note- The energy company installed 3 transformers on the power pole each of them of 75kVA with a total of 225kVA.

In my opinion it should be 3 transformers of 150kVA, due to the formula for a 3⌀ transformer being ((Volts X Amps X 1.732)/1000)=kVA.

Am i correct on the size of the 3 transformers needing to be 75kVA each, or am I wrong bc we transformer kVA shall be calculated based on the actual running load and not on connected load or rated load?
 
For a number of reasons POCOs use entirely different methods for calculating the required transformer(s) than we do under the NEC. Your sizing disparity is very normal.
 
I can see NEC load calculation being higher than what POCO will provide for transformers. I can also see there being some added capacity for possible future loads but POCO typically will only look at what is there now, or if it is definitely known there will be more load in very near future they may allow for that.

They also do factor in peak demand along with how much cool down period may be expected when they do have marginally sized transformers.

Schools, churches, or other places that have regular periods of reduced loading are perfect to apply those cool down periods into their calculations. If it were a place with fairly constant 24 hour loading they won't make such adjustments or at very least will do so at different factors depending on expected characteristics of the loading.

I upgraded a school to 1200 amp service one time, my load calculation probably was only 600 to 800 amps (208/120). POCO used a 225 kVA transformer. I had one 600 amp feeder off this for HVAC loads that was being added. I bet that HVAC feeder seldom saw more than 300 amps even though nameplates and NEC calculations said it was over 400 (don't remember what actual was but being over 400 is reason we went with 600)
 
We use experience based sizing for most of these.
We have several churches, businesses, homes, etc., and demand data for them.
Transformers have losses associated with them, and these are non revenue making kWh being used.
If we can size the transformers to be loaded at 100%, rather than 25 or 50% loaded, then the system losses aren’t so great.

150kVA isn’t a transformer size in a single unit. 167 is the closest. It’s really too big to hang three of them on a pole anyway. Bigger than a 300kVA bank it really needs to be pad mount.

numbers...

167kVA is ~191W no load losses and ~1900W load losses
75kVA is ~147W no load losses and ~671W load losses
3-167s are ~6000 watts and cost ~$300 per year in no load and load losses realized by the POCO.
3-75s are ~2400 watts and cost ~ $225 per year in no load and load losses.
based on ~50% load-no load.
 
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