"Keith Carlson" wrote in message
news:%zEyd.245829$V41.11293@attbi_s52...
I know I'm going to get the "DAGS" for this question, but I just came from
there, and I didn't see this specific information. Lots of debate about how
much current the neutral wire carries, though :-)


Agreed, this is a confusing issue. But there are lot of people with
experience on the rec.

With a 3-wire circuit, is it okay to wire both 120V and 240V outlets on
that same circuit? I know it's *possible* by using the two hots for 240
and either hot and neutral for 120, but is it recommended? Maybe this is
the main reason for this type of circuit, so I didn't see reference to it
on Google. Much of the discussion I read was on balancing the load, which
could imply using the 3-wire circuit with all 120V outlets.

Yes, mixing 120V and 240V is possible, and many circuits are designed this
way. That is why (almost) all 240V installations have a neutral wire. If
you used only 240V loads, there would be no need for that neutral wire.

If you want to mix 240V and 120V on the same circuit, at least 4 wires will
be needed. Call them L1, L2, N, and G (for Line1, Line2, Neutral, and
Ground). The "3-wire" terminology was confusing. Hence 3-wire with ground
is common, if a bit of a mouthful.

My tablesaw and jointer have 3 prongs on their plugs (L1, L2, and G) -- no
neutral is needed, since only the 240V motor is used. My bandsaw has 3
prongs (L1, N, and G) If, however, I wanted to add a 120V light bulb at a
240V tool, 4 prongs would be needed (L1 L2 N & G). I would need to add the
neutral wire to carry the 120V current. That's a hassle, which is why most
people would just run a separate plug.

I've got a dust collector coming soon, and the extra 6-7 continuous amps
is going to result in some frequent breaker trips when I start a saw or
planer. Looked at my electrical service, and found there's an unused
double-pole 30A breaker, feeding an unused dryer outlet (house had a gas
dryer when I moved in). That should give me the 2 hots for a 3-wire
circuit.

My thought on this type of circuit is to wire one outlet at 240V (re-wire
DC motor to 220V), and the rest wired at 120V. With that 30A breaker and
10 AWG wire, should have no problem handling the loads from a DC and air
cleaner running, and startup surge from another tool.

Even if the startup surge would cause the current to exceed the breaker
rating, the breaker probably would not open. Circuit breakers have what is
called a time-current curve, which tells you how long a circuit breaker can
carry excess current before it opens. Most breakers will carry 2x or 3x
their rated current (ex: 30 or 45A on a 15A breaker) for ten seconds or so
before they open; even short time delay models will do it for a second or
so, which covers the startup surge. Google on "circuit breaker time current
curve" to check it out for yourself.

Once you go above this level, the breaker will trip very quickly. For
example, when a hot wire shorts to the neutral or ground, a current of many
time the rated current occurs (10x? 20x?), and the breaker trips within
milliseconds.

The general idea is that larger overcurrents trip quickly, but that 2x or 3x
currents can run for a long time. This protects the wiring, which is one of
the reasons for circuit breakers; a "shorted curcuit" level of overcurrent
would damage wires quickly, but lower overcurrents would take a long time to
overheat the wiring.

An electrician would be a good idea, too. Hopefully I can find someone
willing to consult with me; I can do the wiring myself.
But I'm hoping to get some idea if this is the way I want to go so I can
cost materials. Won't get an electrician or the inspector on the phone
until next week.

Calling qualified help is never a bad idea, but this sounds like a simple
project. And this group will be helpful.

Matthew

TIA