tranny calc

[clayton]

ok i need a little help with figuring out what the primary ocp should be for 2, 3phase transformers,
480v to 240v trans. 25kva apiece,
going to come from a 480v power dist.panel to a disconnect, (one disconect per tranny,) here is what im thinking

25kva * 2 = 50kva
50,000va/(480*1.732) =180.41 amps

does that seem right, i would need to provide 200amps of power, to provide power enough for both?
(dont know the pf. trannys are not here this is just all the info i have to go on project is still weeks away just want to have an idea about how to do the math thanks)

clay

 

[kiloamp7]

Re: tranny calc

For starters, your calc is wrong. Pri. rated FLA for each 25KVA 3-PH xfmr would be appx. 30A.
For most common applications, you would need to protect the pri. of each xfmr.
NEC art. 450 allows a lot of different approaches, but for the most common applications usually 40A time lag fuses or bkr. would suffice.

Remember that the conductors leaving the secondary would have to meet O/C protection of NEC art 240.

You say that sec. is 240V. - not 208Y/120V. & not 120/240V. hi leg delta. Right? Not that that has anything to do with pri. protection

 

[clayton]

Re: tranny calc

yup, i made a mistake.
ok
one transformer.
25kva
25,000/830.4= 30.10 amps

then according to article 450.*** 600v or less
need 125% protection,

125%* 30.1= 37.63 soooo
what im seeing is i need 40amp fuses

does this seem more reasonable??

Transformers will directly feed a machine machine has built in ocp,(disconnect)

thanks again,
clay