transformer question

[billk554]

i came across a situation at work that i have not seen before. 2 single phase transformers fed from a 400 amp 3 phase 480v. buss plug. both xfmr's are 480v primary and 240 v secondary. one(xfmr a) is 167.5 kva and the other (xfmr b) is 100 kva. phase a and b go to xfmr a and phase c and one of the other legs(either phase a or b leaving xfmr a and feeding xfmr b). on the secondary side we have 240 v 3 phase 4 wire, again coming from 2 transformers, feeding a disconnect fused at 600 amps with paralell 350 mcm cables. i guess i am dumbfounded as to why someone with obvious electrical expierence would install 2 single phase xfmrs instead of one 3 phase xfmr? isnt there an issue with efficiency? would a 3 phase xfmr be a better choice as to the cost of running these 2 transformers?


question number 2 is this. we have panels there that are 125 amp 4 wire 3 phase panels. 240 volts. the top half of the panel has been reworked to provide more single phase circuits. that is the buss detail has been modified to accomadate single pole breakers for the top 10 spaces (high leg removed and the buss is modified to do this). so the top 10 spaces are a phase and c phase only.the bottom 10 spaces are 240 volt 3 phase with the high leg right in the middle where it is supposed to be. to my understanding once you modify any panel from its original state you loose the u.l. standing of that panel. i gues i am asking any inspector if he would pass this install?

 

[LarryFine]

1. That's called an Open Delta, and it's a legitimate way of providing "real" 3-phase with two transformers and their reduced associated losses.

2. If the panel is field-modifiable, then it shouldn't be an issue. If faced with the same loads and had the same panel, that's what I'd do, too.

In case you don't know, the A-N-C portion of a high-leg Delta is exactly the same as 120/240v 1ph.