transformer sizing

[dukeb2009]

Hello , im a little confused about transformers for some reason i can'tget it in my head so im hoping that that some of you here could help me.
Now say i want to install a 100 amp 240 single phase panel , and i have 480 as a supply and i know that if i calculate it out i come up with 25kva for the secondary . Now to find my primary current using this 25kva i come up with52amps , i hope im right so far this is where i get confused is this the current load on the(52amps) primary with the secondary loaded @ 100amps.... and reading table(450.3 ) tells me how to protect it.........i understand that thanks for any help.....

 

[augie47]

Welcome to the Forum.
You have it correct. On single phase its I (amps) = P(watts)/ E (volts)
25000 watts is 104 amps 240, 52 amps 480
Art 450 for Transformer protection, Art 240.21 for conductor protection.

this link might help:
http://www.electrician2.com/calculators/theorypea.html

 

[bob]

If you have a 25 kva transformer at 480 volts, the full load amps is
25 kva/0.48 volts = 52 amps. This is the primary current when the secondary load is 25 kva/0.24 volts = 104 amps. The primary protection is usually
125% of the primary amps or 1.25 x 52 amps = 65 amps. You could use a 60 amp or 70 amp breaker for the primary protection.

 

[dukeb2009]

Glad to be here , thanks for the replies....