Transformer Size

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rwreuter

Senior Member
i met with the electric company at my new house, (being built) and they told me they were going to install a 15Kva transformer.

i was wondering if that is the proper size for a 3400 square foot house with gas heat. also, i will be building a 20 x 40 detached garage/work shop that will be powered from that service.

any opinions?
 

iwire

Moderator
Staff member
Location
Massachusetts
i met with the electric company at my new house, (being built) and they told me they were going to install a 15Kva transformer.

i was wondering if that is the proper size for a 3400 square foot house with gas heat. also, i will be building a 20 x 40 detached garage/work shop that will be powered from that service.
So your figuring the power company is clueless about what is needed to supply your home?

Don't worry they do know what they are doing.:)
 

rwreuter

Senior Member
So your figuring the power company is clueless about what is needed to supply your home?

Don't worry they do know what they are doing.:)
no, i don't think that at all. i am just wondering.

in fact i would not have questioned it all if i wasn't standing there when they said they were going to place a 25KVA (because they told me they installed a 25KVA in my neighbors house) there and the other guy changed it to 15KVA.

so i figured i would ask. to be honest, and i know this sounds silly, i didnt' know there were different size transformers for houses, i thought one size fits most houses unless they are huge.

plus, doing the basic math on it:

15KVA = 125amps
25KVA = 208amps

i will have a 3400 heated square foot house and around a 1000 square foot detached garage, not to include the 700 square foot attached garage.

i understand that for the majority of homes with a 200amp panel installed they will be lucky if they ever see 80amps being used at once.

just curious is all.
 
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iwire

Moderator
Staff member
Location
Massachusetts
Here is my understanding of it take it for what it's worth, I am not a power company guy. :)

First the NEC load calculations are intended to always result in a service that can without any doubt carry the load. Because of this you end up with services that are 'over sized'. You do a calculation, it comes out to 151 amps. This number is already much higher then what the actual load will normally be. Then the electrician is always going to move up to the next standard size. So here we would end up 200 amp service on a home that may never really draw more than 100 amps.


Second you have the power company, they have many years of real usage history. They come pretty close to knowing what the real load of home will be. They also treat transformers differently then we do. They need to keep the transformers sized close to capacity to avoid the lost power from installing large transformer everywhere. They also do not mind if a transformer is run above it's stated capacity for short periods of time.
 

kbsparky

Senior Member
Location
Delmarva, USA
WE are doing a ballpark lighting job, with 86 kVA of continuous load. POCO installed a 45 kVA transformer. :confused:

I figured they would have at least used a 75 kVA unit? That thing ought to be smokin' after a few hours of overloaded use, don't you think?

Someone once told me that they were actually capable of handling twice their rated loads? If that is the case, then they are just under the wire (pun intended).
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well i put a 400 amp service on my house and on the pole sits a 15kva .

Were the only one on this transformer .

That said the pole type transformers are oil filled in open air up on that pole also they are not like you everyday dry types there built with thicker wire and insulation tuff stuff.

They can handle 3 or 4 homes if needed .
 

rwreuter

Senior Member
good info from all as it helps me better understand. i have seen pole transformers blow up and it is not a pretty sight and it is rather inconvienant to say the least.

just wanted to get a feel from others who have had experience with them, especially commercial electricians as they are common in large industrial and commercial buildings. i figure they would know the ratio between the transformer and the service.

if it has worked for you it will work for me, plus like iwire said and i stated in a previous post most houses never ever even get close to the amp rating of the panel.

i tell many people building a house a 320amp (2 200amp panels) isn't needed, what you need is a panel with more breaker spaces. from my understanding the nec lifted the size max to somewhere around 60 spaces now. that is a good thing.
 

muzafferz

Member
Iwire is right, their fuse is set way high and they do not mind to run transformer hot, especially in winter when outdoor is cold, transformers are designed based on 105 degree outside temp.
 
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