Re: Transformer capacity

Spir Georges GHALI

Spir Georges GHALI said:

farang said:

I have a oil filled 3 phase transformer 315 kva on a pole pripary voltage is 33 kv secondary is 220/380.

What would be the rated output in amps per phase. Here everyone brings in 3 phase to thier pannel there are hardly any 3 phase loads, maybe a pool pump motor or two. The rest are 1 phase A/C and lighting loads.

Our ampient temperature is 35C

What can I safely draw in current per phase on this transformer?




Dear Farang ;

Each transformer hase an ” S : Apparent Power ” in kVA, either for single phase with neutral or for 3 phases with or witout neutral  (depending of the connection).

The formula for Single phase + neutral is : S = V x I        where :

     – S : apparent power in kVA 

     – V : the nominal voltage between phase & neutral without load.

     – I : the nominal current for the transformer

and for 3 phase + neutral, the formula is : S = 1.73 x V x I

So. using one of these formula accordingly the the kind of transformer, we can calculat the nominal current for any one, noting that the ” I – nominal current ” for 3 phases with or without neutral means the value of current for each phase, in this case, when we have single phase loads we will distribute them to the 3 phases of the transformer trying to have approximetly an equivalant installed power on each phase.


Normally, we try to have the maximum load on any transformer not more than 80-85% of the apparent power.



Dear Farang ;

I', very sorry for this typing mistake, but in 3 phases we use the formula :

– S = 1.73 x U x I

where ” U ” is the voltage between 23 phases.