P:

I think I will break up the quoting a bit.

Pason wrote:
I would ultimately like to have 220V for a small welder that draws 20 amps.

You'd probably be well-advised to put in more than a 30A subpanel.
Welders
and air tools tend to go hand-in-hand, and a 3 hp compressor draws
about 10A, even disregarding the starting surge. 60A would certainly
give
you leeway, and I'd almost be inclined to recommend 100A instead, since
you'll have to dig up the concrete if you went to 60A anyway. On the
other
hand, as I said before, a 30A panel *now*, without the cost of digging
stuff
up, is better than a 100A panel somewhere in the undefined future, and
at
much greater cost, and certainly better than one 15A, 120V lighting
circuit.

That was what I was considering - individual conductors. 3-#10 for the
subpanel and 2-#14 for the existing circuit. Ground wires would
terminate and begin in the boxes at either end as you describe. The
idea behind leaving the existing 15a circuit would just be to maximize
the allowable current. I believe 3 #8 wires won't fit.

Actually you would be allowed 3 #8. Here, have a chart.
http://www.westernextralite.com/resources.asp?key=47

Leaving the existing lighting circuit seems...weird to me. I think
it's
allowed (once again, I don't have my handbook on me and I am just
Joe Homeowner, not Ed Electrician) but it just seems weird.
OTOH, you wouldn't have to be annoyed by blinking lights when
the air compressor kicked on.

My second question was can the sum of all the branch circuits off the
subpanel exceed the current capacity of the feeder (if I said that
right) if appropriate gauge with is used throughout. This would make
sense to me but I haven't seen it spelled out anywhere.

Yes. The panel should be sized to fit the expected load (include your
welder, other tools, and required lighting power, then apply
appropriate
demand factors. I suggest referring to the NEC residential manual
or some other non-watered-down book without the name Black and
Decker or Easy on the cover (-:, or to the electrical wiring FAQ )
but the ratings of the individual breakers, added together, can
certainly
exceed the main breaker's rating. What you don't want is to be
tripping
the main every time you try to run your welder and the air compressor
(sorry I keep mentioning that thing. You had better ask your wife
first)
kicks on. The demand factors &c will help with that.

As an example, my house's old Wadsworth panel has a 60A main
fuse, a 30A fuse to the dryer, four 15A fustats in the panel itself,
and six 20A fustats (five used) in a subpanel. Even allowing that the
main is 60A at 220V (13,200 volt-amps), and the branches are
(except the dryer) 110V, that's still a total of 26,400 VA on the
branches,
twice what the main could supply, but I never blow a main fuse.

For my fairly Luddite electrical needs, 60A actually (barely) satisfies

the service requirements of my house, but I am 'current'ly installing
a 200A service and breaker panel. That's another story.