The total connected load applied to most transformers in the range of 1500 kVA would exceed the nameplate rating of the transformer. This is normal. There is considerable diversity in the load attached to a transformer. Depending on the type of load, i.e., electric heat, motors, induction furnaces, etc. there will be an approximate amount of the connected load that will be seen on the transformer at any given time. As an example, lets assume you have a process that requires 200 kW of lighting load that will stay on 24 hours a day. Heating load of 500 kW in total throughout the building but controlled in each area by separate thermostats. Since not all areas of the building will require heating capacity of the same levels or at the same time, it is reasonable to assume there will not be 500 kW of heating load on all the time. Since we know that not all heating load will be on 24 hours a day we need to figure out how much heat will come on and how often. A great deal of this type of work has been done in the past and average figures have been measured. For some types of space heating the demand at any one time is about 50% of the connected load.
To size the transformer then we have:
Lighting load – 200 kW ( no diversity - it’s on 24 hours/day)
Heating load - 250 kW ( only 50% of total at anytime normally)
Total transformer load = 200 + 250 = 450 kW. Assume a power factor of 90 % and we end up with a total load of about 450 kW/0.90 = 500 kVA.
The transformer should be a 500 kVA unit. The overload ability of the transformer for a short duration would allow you supply a greater load assuming a return to normal loading say within a day or so.
Hope this helps.
