Transformer Question

D

Dan120

Member
Location
Florida
Occupation
Journeyman Electrician
Hello everyone, I have a transformer question. I have a wire were the maximum ampacity is 20A, If am running a 240v line to a transformer were it will be stepped down, to 120/240 what is the maximum size transformer that could be installed?
Would it be a 5KVA or it would have to be smaller? because of only being able to load the wire to 80%?
 
The 80% would not factor unless you are dealing with a continuous load on the transformer.
Mathematically, a 5kva would be allowable but you might have a problem with your breaker tripping on inrush.
I think the next "standard" size is 3 kva.
 
  • Like
Reactions: MTW
A transformer is not a load; it does energy conversion. Code provides you with _maximum_ ratings for the OCPDs connected to the transformers, but not _minimum_ ratings.

This means that you would be perfectly code legit to install a 10kVA transformer.

The supply circuit is only rated for 20A, so you can only supply 4.8 kVA of load, and you would be limited to 3.8 kVA of continuous load.

As @augie47 notes you might have inrush problems that cause the 20A breaker to trip, but that 'trip on inrush' issue depends heavily on things like the available short circuit current at the transformer; if there is enough circuit impedance (voltage drop) then the inrush is reduced and not a problem.
 
  • Like
Reactions: MTW
Dan120 said:
Hello everyone, I have a transformer question. I have a wire were the maximum ampacity is 20A, If am running a 240v line to a transformer were it will be stepped down, to 120/240 what is the maximum size transformer that could be installed?
Would it be a 5KVA or it would have to be smaller? because of only being able to load the wire to 80%?
Click to expand...
How is it a step down? It sounds like you are just adding a neutral so that you can run 120V loads. Not that that has to do with anything. :D
 
ggunn said:
Or a 100kVA transformer, for that matter, as long as you protect your conductors. :D
Click to expand...

In a pinch I once used an oversized transformer; I think the primary OCPD was something like 60% of transformer full load current. And it was reverse fed. I didn't have any inrush tripping issues, but it was a very long circuit fed from a less than stiff service.
 
Ok, thanks. I am running a very long run, I need 120v at the end, and the reason for the transformer was so I could run smaller wire at 240v.
 
You must log in or register to reply here.
Top