jtester said:
You should note that most sizes above 250 single phase and 2500 3 phase are often considered substation and not distribution transformers. They might not be as readily available, particularly in a padmounted configuration for instance.
Jim T
C57.12.00 is standard general requirements for liquid-immersed, distribution, power, and regulating transformers. Of which, a padmounted transformer is a distribution transformer. In accordance with C57.12.80, the term "substation transformer" is not considered standard terminology for power or distribution transformers. Perhaps you were meaning to have said, unit substation transformer; which, would include articulated unit substation, integral unit substation, primary unit substation, and secondary unit substation.
A distribution transformer is for transferring electrical energy from a primary distribution circuit to a secondary distribution circuit or consumer's service circuit. The definition has nothing to do with size or type of transformer. C57.12.10 is for transformers from 750 through 100,000 KVA with no mention of function differentiation.
The long and short of it, is that transformer are based predominanantly on function and type, and not necessarily on size. I can get a power transformer that is 2500KVA or larger, that is not a unit substation transformer. I can also get a unit substation transformer that is smaller then 2500KVA, and it is not a pad-mounted transformer, but used for distribution.
Yes, pad-mounted transformers are usually 2500KVA and smaller in size, but a 5000 KVA transformer, can still be a distribution transformer, of which it may be a primary or secondary unit substation transformer type.