Distribution Transformer Loading
Distribution Transformer Loading
(OP)
I have run across Anthony Pansini's text and it gives recommended loading for distribution transformers. I believe that some of the recommended kVA ratings seem high. (Just my opinion) For example loading a 167kVA padmount transformer for up to 370kVA load under Table 6-13. I understand that they are using Table 6-12 electric space heating.
What are your opinions?
See attachment for reference
What are your opinions?
See attachment for reference






RE: Distribution Transformer Loading
You also have to define your terms. Are you talking about connected load, peak demand load, average load or what? Winter versus summer makes a difference as well.
There is no question that utilities routinely undersize their distribution transformers if you look at the estimated peak demand loads. It is just more economical to replace the occasional transformer that fails due to overload than to size all transformers to carry the peak load without overloading.
RE: Distribution Transformer Loading
Some utilities load transformers high enough to produce some loss-of-life on an economic basis, considering transformer losses and replacement costs.
370 kVA seems a bit high for a 167 kVA transformer, particularly if any load growth is anticipated, but I don't know what the criteria is used by Pansini.
RE: Distribution Transformer Loading
RE: Distribution Transformer Loading
RE: Distribution Transformer Loading
As the temperature drops, the demand increases but the transformer is better able to withstand the overload in the cold ambient temperature.
The tables can not be construed to indicate that a 370KVA load may be fed from a 167KVA transformer under all conditions.
Peak loading as suggested by the tables would only be experienced for a short time during the year and not at all during mild winters. A few days per year of overload during extremely cold weather will not raise the transformer temperature very much. Transformer life is shortened by sustained, elevated temperatures but in this example the reduction in the life of the transformer will be marginal.
Most utilities will have their own loading tables which will take into account local conditions such as local building methods and local weather patterns. These tables may then be modified by a formula that takes into account the cost of a larger transformer vs the cost of the shortened life of the transformer. This adjustment may be calculated or it may be based on past experience.
Bill
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"Why not the best?"
Jimmy Carter
RE: Distribution Transformer Loading
RE: Distribution Transformer Loading
JIM
RE: Distribution Transformer Loading
RE: Distribution Transformer Loading
RE: Distribution Transformer Loading
You misunderstood the point I was trying to make. At an ambient temperature of 30°C, the 80°C hottest-spot rise a 65°C rise unit (bear in mind there are 55°C rise units) gives a hottest spot winding temp. of 110°C, and at this temp the transformer still has a normal life expectancy. However the transformer kVA rating is based on an ambient temp. of 30°C.
RE: Distribution Transformer Loading
DPC, sometimes I assume too much when asking for opinions, thanks for your input.
I am usually conservative when sizing transformers as I believe many are oversized, therefore inefficient, but when reading through this text, I noticed the higher than expected values which led me to post.