I'd like to add a 240V-only receptacle to my basement workshop to run
two induction motor loads. I have experience running 120V household
circuits, and I just have a couple questions on 240V circuits:
1) My understanding is that for NEMA 6-15 (240V 2-pole Grounded 15
amp) plugs and receptacles, the two hot prongs are undistinguished,
i.e. there is no polarization as on 120V plugs and receptacles. Is
that correct? Is there any convention?
2) I believe the NEC does not require GFCI protection on 240V
circuits. Are economic 240V GFCI receptacles available?
OK, just to be redundant, that means that in a 240V circuit, you don't
have to keep track of black versus red and you can connect black to red,
red to black willy-nilly?
On a related note, if I rewire a 120/240V induction motor to 240V and
change the 120V plug on its power cord, then I should mark the white
conductor at the new plug and at the motor junction box?
Hmm, a SquareD QO220 breaker costs $15, while a QO220GFI breaker costs
$90. I have two induction motor loads, a jointer and a dust
collector. I wonder if the greater expense of GFCI protection would
be a good reason not to rewire these motors to 220V.
You use your jointer right after stepping out of the shower? Why do you
want a GFCI?
Bear in mind that to get 240v you have to contact both hots. That is highly
unlikely, and wouldn't trip a GFCI anyhow. Assuming your tools are grounded
properly, any kind of shock (except from directly touching one of the wires)
is unlikely, but would be exactly the same as a 120v shock anyhow.
(I shouldn't admit this, but in the interest of full disclosure... while
drywalling I forgot an outlet was hot and pulled on it, contacting the hot
with one hand and the neutral with the other. Fortunately it was just a big
surprise, but I might not have been so lucky with 240v! Stupid things like
that are possible. A GFCI would not have helped because there was no
To answer your other question, the hots are interchangable; there is no
reason to distinquish between them. Accordingly, you have to mark the white
wire with black tape on both ends so no one mistakenly thinks it is a
neutral. You might want to think about using 4wire so your hots will be
black and red. You will have the white available as a neutral if you ever
need it. I didn't do that when I ran 240v to my shop and I regret it.
240v is no more difficult to wire than 120v, but you might want to read a
wiring book first, because you don't seem to understand what 240v is. It is
no more difficult when you know what it is; when you don't get it, I suppose
a dumb mistake could easily happen.
Most zaps from a 240V circuit will be 120V from hot-to-ground. Even if
you hit hot-to-hot, you're likely to have _some_ additional leakage
to ground too. As such, a GFCI _would_ work.
However, the electrical code tends not to require GFCI's for
large machinery that has solid grounds. Especially if they're
single outlets. Think fridges or freezers for example, even
in places that need GFCIs for normal outlets.
I wouldn't put GFCIs on large 240V stationary power tools, but
I'd take considerable care to ensure that their grounds are
there, solid and work.
If the ground is working properly, nothing short of taking
the machine apart and sticking your fingers in the electricals
will give you a shock.
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
In theory, true. But for my own house, I don't like it. On all the
240V circuits, I have checked that black and red are not mixed up.
This is really easy to do when wiring the house, and makes everything
Here's another thing that worked out good. When we built the house,
the local electrical distributor had a sale on strange-colored wire.
So I used quite a bit of blue, purple, red, and such. This makes a
complicated box with multiple switches easier to understand. For
example: Black is the hot feed going in; red is the output to the
light, and blue and purple are the switch legs between three-way
Absolutely. All of my 240V outlet circuits are run with 4-wire cable
(but not the dedicated 240V circuits for pump motors, where a neutral
will never be needed). Sadly, all but two of the neutrals remain
unused. The two used ones are: The dryer (which uses the currently
code-legal 4-pin connector), and the drill press (where rewiring the
motor to 240V will require a neutral conductor so the work light on it
can stay at 120V).
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