Calculating Secondary Current on XFMR

[NEC User]

I have a single phase 3KVA transformer (primary 480V and secondary 120V/240V) that only feeds one convenience receptacle on the secondary side. For the purpose of Table 450.3(B) and calculating secondary current, what method should I take? Should I be using 120V or 240V in my calculations?

P=IV
3000=I x 120
I=25Amps

or
P=IV
180 = I x 120
I = 1.5 Amps

 

[charlie b]

Are you calculating the rated current capacity of the transformer? Then your first equation is close, but not exactly correct. For that one, you should use 240 volts, and you will get an answer of 12.5 amps.

Are you calculating the load that the branch circuit will actually draw from the transformer? Then your second equation is correct.

 

[NEC User]

Thanks Charlie