permisible load on transf accor. to codes

[raynh]

Can any body please tell me what is the permissible load that can be connected to a power transformer (250KVA)for an ambient temp. which does not exceed 45deg. cen, indoor type according to the codes, the transformer is new .

 

[alehman]

What codes? Where are you located?
Oil insulated or dry?

40C is the standard rating for dry type. If the temp exceeds the rating, you should consult the manufacturer for applicable de-rating.

 

[raynh]

Thanks Alehman, My question was how much can I load a new transformer of 250kva according to B.S or Canadian code, can I load it till 80or 90% of the transformer rating? For both oil and dry type

 

[7anoter4]

Hi raynh
For steady state of load 80% may be suitable [ if you don't expect any future development].
For a short time [hours] the transformer could be overloaded without any damage [how much depend on manufacturer indications].
But you have to check if when starting the most powerful motor the voltage drop through the transformer [ loaded 80% ] is not too high so the voltage at the
other still consumer terminals does not fall under the permissible .
A thumb rule is the transformer KVA > 3*maxMOTOR KW
Regards

 

[raynh]

Hi 7anoter4

Thank you for yr reply, and I fully agree with you, however my load is lighting and HVAC and no further extension expected in the building, the 80% you mentioned , is it according to any international code? or it is subjected to the designer estimate?

Kind Regards

 

[7anoter4]

Hi raynh
I don't know any code stating this. It is only my experience. In my opinion even 90% load is not so bad. But a slight reserve of 20% is better, I think. Nobody knows when a sudden extension -HVAC may be- will occur.
Regards

 

[alehman]

The NEC requires electrical equipment to be applied within it's ratings. If the transformer is rated 40C ambient and the room temp is 45C, the transformer capacity must be adjusted per the manufacturer's recommendations.