How to calculate transformer heat loss

[StephenJcan]

I am trying to figure out what the heat gain would be for a 250kVA transformer.
I have done several searches here and thru google and have not found corroborating info on the calculation.
One site calculates the heat gain as:

Heq = PeqK1K2 (W)
where
Heq = heat transferred from electrical equipment (W)
Peq = electrical power consumption (W)
K1 = load coefficient
K2 = running time coefficient

and others use a rule of thumb between 1% and 5% of rated load.

Can anyone point me to a calculation that I could reference? I have searched thru my ASHRAE handbooks but have not been able to find anything.

 

[lilliput1]

What you need to find out is the efficiency of the transformer and how heavily loaded it is. Then consider the transfrmer like a motor in the space but the driven equipment as being out of the space. The heat gain to the space therefore is:

watts heat gain = ((1/eff)-1)x(Kw input) x 1000

Efficiency ranges from 90% for small transformers to 98% for large transformers. You have to get manufacturer's data. Efficiency may vary w/ % loading.

Transformers typically can tolerate a maximum of 104°F but check with manufacturer. If you exhaust high over the equipment & supply air low, by stratification you may be able to reduce air supply required. But it is better to be safe than designing it too close.