Shamefully, I have to admit that I did something very similar to this in my early years in the trade. My boss at the time was very patient with me and allowed me to take on side jobs for friends and family. He would even bail me out if I got in over my head. This time, I really screwed up.
I had some close family friends who owned a lighting supply store. They had expanded and needed to power up an addition of lighting displays. My solution was to install a sub panel for this new display area.
They had a 400 amp, three phase service, and it looked like a pretty straight-forward job. About a 75 foot run of SER cable, another disconnect and the panel was all I needed. This was an old place and the service consisted of a trough with four fused disconnects. I managed to muster up the guts to open the main (and unfused) trough, cut a hole and place another disconnect to feed the sub panel. My boss even came in and tied everything in for me live since I was afraid to do it.
Just as he was finishing, he asked me if I need any orange tape. Being 17 at the time, I asked him "why would I need orange tape, I have white, red and blue"?
I guess you know the rest of the story. It was a high-leg service and the C phase was of absolutely no use in the sub panel. This was my first attempt at working with three-phase services of any kind since all we did was residential. My suggestion was to just skip all C phase spaces in the sub panel, but he insisted that the only way he'd let me do that would be if we cut the third phase completely off at both ends so that it would be impossible to reconnect.
He taught me something very valuable that day and he didn't have to say a word about how I was doing this wrong. He let me make the mistake on my own and pay the price on my own without hardly saying a word. What a great teacher he was. I never made that assumption again and that was nearly thirty years ago.
[This message has been edited by EV607797 (edited 01-08-2007).]
