Secondary conductors

[shawn73]

From an earlier post:

Heres my new project. I need to install a square D transformer (225 kva, 480 delta primary, 270 Primary side Amps, 208Y/120 Secondary, 625 Amp secondary) to power 3- 208/120, 200 Amp panels.


From what I understand from my other post, I may be able to run my cunductors directly from the secondary to the 200A panels with wire rated at 200A. Is this correct? 240.21 C(6) States that the secondary conductors shall have an ampacity that is not less than the value of the primary to secondary voltage ratio multipied by 1/3 of the rating of the OCPD protecting the primary of the transformer.
I don't understand the ratio section. I have a 480 primary and a 110V secondary.
So, that's 1:4 or 25%.
1/3 of the primary OCPD would be 350/3 or 117 A. 117 amps (25%) is 29 amps. This doesn't seem correct to me. Am I doing something wrong?
Connecting #10 wire to a transformer secondary with 626 Amps potential doesn't seem reasonabe!!

Thanks for your input.

 

[steve66]

Re: Secondary conductors

You have it backwards, multiply the actual ampacity of the secondary conductors by 1/4, and that must still be at least 1/3 of the primary circuit breaker rating.

If you had a 300A primary breaker, then 300x4= 1200 amps available on the secondary. You would need wire rated for 1200A/3 = 400 Amps. So you could run sets of 600KCM to each panel if the distance doesn't excede 25 feet.

Once again, you will want to make sure you can land that on the 200A breakers. Also note that for the transformer, this is primary only protection (see table 450.3(B))

P.S. Please reply to the origonal topic instead of starting a new thread. It makes it easier for others to follow.

Steve

[ March 15, 2005, 09:25 AM: Message edited by: steve66 ]