I'm an electrical apprentice and have been working on a job the past few weeks and was given the duty of trying to size the conductors for the secondary side of our 112.5Kva transformer. We would like to do it in parallel but its my first time sizing these conductors in any given job. I was wondering if there is a calculation or any which way to size them. Any help would be appreciated
Kevin
As others have mentioned, before you can size your secondary conductors you must know what size panelboard you will be installing.
The size of the panelboard is determined by one or more of several factors:
1. What is my need? [i.e. what is the calculated load going to be?]
2. What is my 'want' factor? [i.e. you may have a calculated load of 200A, but 'want' to have an extra 200A available for future use.]
3. What can my transformer deliver? [Obviously, if it is not big enough for 1 & 2 above you've got a problem].
Given a 112.5 kVa transformer, you must calculate how much this will provide. If I use a secondary voltage of 208 -
112,500 VA/ (208V x 1.73) = 312.6A
Table 450.3(B) indicates 2 things: 1) you can size your secondary OCP at 125%, and 2) if this ampacity does not correspond to a standard rating of an OCPD, you can go to the 'next higher' rated OCPD.
312.6A x 125% = 390.75A....next higher breaker = 400A
I calculate you can have a panelboard on the secondary rated as high as 400A
Forgive me if I made a mistake in information or calculations....but I feel confident fellow members will quickly chastise me for any errors I have commited.