Service transformer size

[Sungate]

I am involved in design of the site servise transformer, dry type.
We have single, two phases, 3 phases loads as a heaters, lights, motors, HVAC, battery chargers, panels, receptacles ect.
If calculated total load is 100kwt, what is the size of transformer should we choose?
That type of load is not switched on all of time, there should be some kind of diversity coefficient, lets say 0.9, so transformer could be 90kwt only.
Or the total load cannot exceed the rating of transformer? Alternatively, , we can consider 80% load on the transformer, so it could be 125kwt transformer?

Another question is if we need to take into consideration the impedance of transformer Z% , or it is for short circuit current calculation?
I am not so familiar with these issues, so appreciate if anybody can help.

Thanks

 

[waross]

I am sure that you do not have any 2 phase loads. Phase to phase loads are not 2 phase loads, they are single phase loads.
You must base your calculations on the heaviest loaded phase.
Beware adding three phase and line to line loads to line to neutral loads because of the phase angle difference.
You must select your service equipment based on the Avaiable Short Circuit Current as calculated from the Z% of the transformer.
Size your service in accordance with the NEC or CEC and with consideration of special conditions that may INCREASE the demand. That will give you your allowable demands (or the pseudo reciprocal diversity).
Size your transformer to the main service breaker.
There are a lot of conditions and exceptions. You also need an understanding of the overall system. Consider using a consultant familiar with this aspect of engineering.

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

 

[Sparky1963]

Refer to NEC 220 for service and feeder calculations. Power Companies usually size their transformers for 50%-60% of connected load. However, they are not governed by the NEC.