Thank you in advance!
I am wiring up a 110V outlet to go on the side of my table saw outfeed
extension table recently built.
I would like to tap this off of the 220V outlet for the table saw, directly
beneath the new bench.
I know I can get 110 from this by putting the black wire on one of the 220V
outlets hot post and the
white wire on the neutral ...now...should the green wire (in the cord going
to the new outlet) also
go to the same post as the white / neutral since both are at ground
First, it's a code requirement that neutral and ground
only be connected a *one* place, generally at the service
entrance. The practical reason is that connecting them
together in other places can cause ground loops and this
can put voltages onto grounded metal cases. Not good.
If your 220V outlet case is truly grounded, then yes
you would connect the ground lead to the metal
case. This might be the situation if you have metal
conduit leading to the 220V outlet. You could do
that and also use a GFCI, then if the case isn't
grounded you are still OK.
Note that line voltage in a residential setting varies between 110 and
120 plus or minus 5% or so. I'll use 240 and 120 in this article.
Generally tablesaws are not wired with a neutral at all. For what you
want to work safely, your 240 outlet must be wired with four (4)
wires - two current carrying conductors (typically red and black), one
grounded conductor (typically white) and one grounding conductor (typically
green or uninsulated). Note that when romex XX-2 with ground cable is
used for 240 three-wire outlets, the NEC allows the white conductor to be used
as a current carrying conductor (although it should have been permanently
marked with black tape or other black marking at both ends). Be very
careful not to assume that this white conductor is a grounded conductor,
you cannot make 120 from this configuration safely (simple check, use
an DVM set to AC 600V to measure the potential difference between
the two conductors - it should be either 120 or 240).
A 120 outlet will need three conductors. One current carrying
conductor (typically black), one grounded conductor (typically white)
and a grounding conductor (typically uninsulated). If your 240/220
volt outlet doesn't have a grounded conductor (which is what I expect
you'll find, if the outlet was run specifically for the tablesaw),
you _cannot_ safely tap a 120 volt circuit from it.
Note the NEC refers to the neutral conductor as a ground_ed_ conductor
and the ground wire as a ground_ing_ conductor.
The grounded conductor and grounding conductors may only be connected
together at a single location, and that single location _must_ be the
service entrance (this is to prevent inadvertent current flows in the
grounding conductor that can cause appliance metal frames to become
If your receptacle is installed in a metal box, the box must be bonded
to the grounding conductor.
I would recommend that you have an electrician evaluate your setup if you
are unclear on what to do.
Any type of wire fault, such as vibration, wirenut coming loose, part
pincing wire, ect.
Correct. The saw has two hot wires, and one ground. This ground is all of
the metal on the saw, and is intended to trip a breaker, if one of the hot
legs shorts to the frame. When you use that ground as neutral, whatever
electricity is being used by the 110, is going from the one hot wire,
through the 110 using device, and then back into the frame of the saw.
No. That is as it is supposed to be. The problem is using the ground wire
in the saw cord as a neutral. Ground wires are not intended to carry
current. It is not a neutral. It is a ground. There is no neutral in a 3
wire 220 system. One needs to be added.
PLEASE do not listen to ranck. What he proposes can be done, it will work,
but if anything else goes wrong, you have removed all safety margins.
A new 4 prong receptacle, a new cord and cord cap can all be done for under
$25 dollars. A life costs much more than that.
Wrong answer. That does not keep a fault between the neutral and ground
from lighting up the frame of the saw with 110v. You need to have a four
conductor plug and wire, and the white wire on the 110 plug should be
completely separate and isolated from the ground connections of the saw
frame. The ground of the 110 should still be connected to the ground of the
saw. If you want to use a GFCI, by all means, do.
To be completely right, the receptacle should be wired with 4 wires, all the
way to the box, with separate ground, neutral, and two hot feeds.
There are two types of outlets, three prong and four prong. Both have
a white wire connected to one of the prongs, but only one of them has
a neutral. On a three prong, the white is the other hot (black=hot1,
white=hot2, bare/green=ground). On a four prong, black and red are
hots, white is neutral, and bare/green is ground.
If you have a black/red/white four-prong, you can use either
black/white or red/white, plus ground, to safely power a 120v outlet.
Just because it is white does not mean that it is neutral. A lot of times
white is also used as a hot. Do you have a meter? Can you go to the other
end of the cable (panel box) and tell how it is wired?
Generally, you will not have a neutral wire in a 220v circuit as it has no
<snip> now...should the green wire (in the cord going to the new outlet)
No. The green wire (safety ground) is to provide a direct path to 'ground'
in_the_event_of_a_failure. If you don't care if your 110 outlet is
simply leave it unconnected. You will only have an ungrounded outlet.
(Also a GFCI outlet is no good without a ground!)
However, your 220 outlet on the table should have a green or bare ground
wire. If it does not then you don't have a grounded outlet for your saw.
(Don't know why that would be, since it's pretty standard practice
Incidently, your additional outlet idea sounds OK for a lamp or some small
appliance. If you plan on using it for a drill or shop vac with the saw
running, you maybe plagued with CB nuisence trips from too much power drain
on one of the 220 legs.
It might be better all around to just wire the outlet separatly.
Wha'd ya think?
This is incorrect. A GFCI operates by comparing the
current flow between the current carrying conductor
and the grounded conductor (aka neutral). If they don't
agree within some small percentage, the GFCI will
interrupt the current flow. While it is preferred that
it be grounded it is not strictly necessary to obtain
the benefit of a GFCI.
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