Numbers on power transformer 'cans'

Out of curiosity...
The power transformer 'cans' on poles (in the US) often have numbers
on them, such as '25', '37.5' '50+'. Given the larger 'cans' tend to
have a larger number I assume it is a power rating of some sort. Is it?
If I guess it's the rating in kVA, it seems a bit small, one marked '25'
often powers several houses where one house with a fully maxed-out 100A
service (or 200A half-loaded) would need its own 25kvA transformer.
Reply to
Michael Moroney
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Oil-insulated transformers for utility application have non-continuous overload capacity beyond a basic "ohm's law" calculation. It works well in most applications.
--s falke
Reply to
s falke
---------- That is the KVA rating. Overload ratings (time limited) are higher. As to the load on a household, a 100A service is rarely loaded to capacity as there is a wide diversity between loads in the home. Not all loads are on at the same time and few loads are or near the breaker rating. It is doubtful whether your individual household load on a 100A service is anywhere near 100A at any time. In addition, if your load was "maxed out" you would experience nuisance breaker tripping all the time. The service rating is really a generous one and allows for transient loads such as motor starting without excessive voltage dips (you don't want your computer to crash when the air conditioner turns on). Your actual load may peak in the 5KVA region and your neighbor's load may peak about the same but at a somewhat different time. The size of the transformer is based on this load diversity and the experience of the utility.
-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer
Reply to
Don Kelly
Smallest I've seen are 5 and 1.5 and those were on rural coop systems... the 5 being used for small residences (not farms - those usually got 10 at a minimum) and the 1.5 being used for the smallest loads - phone terminals or electric fencers etc.
Dave
Reply to
Dave Dahle

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