That's so much for the info. This will probably be a spring project,
and I might get professional help, but I'm trying to learn what's
possible and just increase my electrical knowledge in general. I'd
really prefer to get away without breaking up the concrete. I would
ultimately like to have 220V for a small welder that draws 20 amps.
Also, I know codes vary from area to area, but some general info is
appreciated.
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Hmm. From the OP I can't tell if that's EMT with individual conductors
or
UF with EMT sleeved over it for protection. If it's the former, and it
is bonded
properly to the main ground, then I don't see any reason it couldn't be
used as
the ground. For instance, if the wiring changes from cable to piped
conductors
at a box in the basement, and the cable ground is bonded to the box,
and
the same situation occurs at the garage, that's a continuous ground,
no?
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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.

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.

Thanks again,
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What is the 15A circuit going to? If it's the garage, why not eliminate
that circuit
and feed it from the new subpanel? Probably I'd prefer at least a 60A
panel in
the garage over a 30A, for that matter, but I do know that sometimes
you need
to safely do something less than ideal. To have a 60A panel, you'd
need at
least 3/4" EMT for four #6 THHN/THWN conductors, since 1/2" is only
allowed 2.

Now for the conduit fill for your 1/2" EMT:

#10 THHN area = about .021 sq in
40% of 1/2" EMT area = .122 sq in allowed

You can legally pull 5 #10, THHN insulated conductors into your 1/2"
EMT.

If you must keep the 15A circuit in there (perhaps serves outdoor
lights?):

#14 THHN area = .010 sq in
#10 THHN area = .021 sq in
40% of 1/2" EMT area = .122 sq in allowed

4 #10 THHN = .084 sq in
3 #14 THHN = .030 sq in
Total .114

You can also legally have 4 #10s and 3 #14s. It will not be a picnic,
but
you were not looking for a picnic; you were trying to wire. I would
pull
the #14s out first, then pull all the wires at once, and once again be
sure
to use stranded wire and lube, and of course invest in a fish tape if
you
don't already have one. In fact, buy two, and green is prettier than
red.

One thing I don't know off hand without referring to the NEC: will he
need to
use THWN inside the conduit if it's (as it sounds to be) buried in
concrete?

Please feel free to poke holes in my figuring. I don't have my
handbook
in front of me just now (on break at work)

Cordially yours:
G P.