I have one more question, would that consider the three conductor cable to be one conductor (which will required 53% filling instead 40% filling for the three single conductors)? thanks again
There are equally as many downsides to 3 conductor cable. Are you serving predominately 3 phase loads? If not, single conductor cable has many benefits. How large a wire size do you need? You will have many more limitations if you need 500 kcmil for example.
I have one more question, would that consider the three conductor cable to be one conductor (which will required 53% filling instead 40% filling for the three single conductors)? thanks again
See the beginning of Chapter 9 in the NEC (I am using 2005 edition). Note 9 to the table states in part:
"A multiconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area."
This surprised me when I looked it up. Intuitively I would have thought that you would still count a multiconductor cable as multiple conductors, since you still have 3 conductors.
Edit to add: Treating it as one conductor helps you in that it raises the maximum conduit fill allowed from 40% to 53%. You make it sound like a bad thing!
With concentric designs most common to HV & MV cables, I believe skin effect and perhaps XL is reduced more reliably. I see how surface-area ratios remove proximity / skin effects, but not sure how that dissipates heat or EMF more reliably than solid-core wires, with identical circular mills, or mm^2 conductor area. Trouble is, outside linemen are the only one's I've seen equipped to terminate concentric cables.