Transformer feeder wire sizing?

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KyleFowler

Member
Location
Maryville, TN
Occupation
Electrician
So I read up on transformer sizing today and this question is purely hypothetical but I feel like it has to happen occasionally and the examples I've found about transformer sizing don't address my question.

Let's say you are sizing the primary wire for a 25kva single phase step down transformer to feed a piece of equipment that only pulls 17kva. Do you size the primary wire for the transformer by 25kva or 17kva?

I ask because at work we did have a piece of equipment that had this problem, well sort of....it was a 6kva load on a 7.5kva transformer with a 480 volt primary so wire size was really a non issue there as we sized the primary wire to 7.5kva but it still left me wondering lol.

Thanks in advance
 

topgone

Senior Member
Design for the maximum available juice you get from your transformer, period. You'll never know when the time the owner wants to expand.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
So I read up on transformer sizing today and this question is purely hypothetical but I feel like it has to happen occasionally and the examples I've found about transformer sizing don't address my question.

Let's say you are sizing the primary wire for a 25kva single phase step down transformer to feed a piece of equipment that only pulls 17kva. Do you size the primary wire for the transformer by 25kva or 17kva?

I ask because at work we did have a piece of equipment that had this problem, well sort of....it was a 6kva load on a 7.5kva transformer with a 480 volt primary so wire size was really a non issue there as we sized the primary wire to 7.5kva but it still left me wondering lol.

Thanks in advance

You could size your OCPD devices and conductors for the load if you wanted. The down side to this is, depending on the situation you could end up with a primary OCPD to small to handle the xformer inrush. Particularly on modern energy efficient transformers.
 

KyleFowler

Member
Location
Maryville, TN
Occupation
Electrician
You could size your OCPD devices and conductors for the load if you wanted. The down side to this is, depending on the situation you could end up with a primary OCPD to small to handle the xformer inrush. Particularly on modern energy efficient transformers.


I guess in the end one needs to look at all the variables especially what OCPD is allowed and make a decision that way
 
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