How does the "Test" feature on a GFI outlet work without a ground? My
understanding was that it was just a high-resistance shunt from the hot
leg to ground. When you push the button, the hot leg draws a little
current, the comparitor sees a current differential and pops the
internal circuit breaker.
On the other hand, just because the "Test" feature doesn't work, doesn't
mean the rest of it doesn't work. On the third hand, it seems pretty
dumb to put a GFI outlet someplace and not hook the ground up. On the
fourth hand, it seems pretty dumb to ever install an outlet without a
Thank you for all of the advice, although some of it appears conflicting.
I have examined and traced the wiring from the in-slab 240V metal table saw
outlet back to the panel. Here is what I see:
There are 3 wires going into the saw outlet from the panel end: BLK, WHT and
The BLK and WHT are connected to the two hot prongs while the GRN is
connected to the center pin as well as to the metal box.
These wires then go into the concrete slab through a pipe. I can't tell if
it is metal or plastic.
The wires re-emerge about 10' away in a junction box in the wall, From here
they connect with another cable that returns to the breaker panel.
This cable has a RED, BLK, WHT and BARE wire. It is connected to the saw
wire as such:
BLK-BLK, RED-WHT, BARE-GREEN. The WHT wire (from the breaker panel) is
What are the new implications of the WHT wire not being employed? Is it safe
in this configuration?
Sounds like it should be re-wired with a 4 wire system as so many have
Am GFI senses how much power comes out of the hot wire, and goes back into
the neutral. It does not care if the power is going through the tool, or up
one arm and down the other, and it will still think everything is OK. With
a true separate ground, it would have another path for the juice to go and
trip the GFCI. This still does not address the concern that the ground of
the table saw is carrying current, while all it is supposed to do is trip a
breaker. Juice in the saw frame is bad.
Do it the way I described. The safe way, the right way, the code way.
Hmm, Morgans makes a very good point here. While GFCIs are a code
approved way to add a 3 prong outlet to an existing 120V ungrounded
circuit, in this case there is a potential exposure because it is
a 240V circuit with a real 240V appliance attached. Not the same
Morgans is right.
Tug on the wires going into the pipe while watching the other end. If you can
pull them out attach a stout cord to the far end and pull the wire out. Add a
red wire to the pack and pull them back. Then hook them back up matching the
colors and using the red/black for the hots. Look in the panel and verify the
white is connected to the neutral bar. Then you will be OK for your 120v
Simply that it is a 240V *ONLY* sub-run. That _connected_ WHT wire (going
towards the saw outlet) _should_ be tagged with some black tape, at *both*
ends, to indicate that it _is_ a 'hot' lead, and not a 'neutral'.
Assuming everything else is done correctly, _yes_.
It's perfectly OK, as is, for a _240V_only_ device.
However, if you _really_ want that 120V outlet as well, then yes, you
_must_ modify the wiring from the saw outlet to the point where it
ties to that other cable. Note: you'll also have to investigate the wiring
between the outlet on the wall, and the saw itself. You may have to replace
_that_ wire, as well, to get a real 'neutral' all the way to the saw.
Getting a 'neutral' to the saw outlet will involve pulling another wire
through that pipe -- I'd recommend RED, using it for the 2nd hot, and
're-converting' the WHITE back to the 'neutral'. Be sure to remove the
black 'tagging' from both ends of the white lead.
There are some ways to do the 120V outlet, without pulling the additional
wire, *BUT*, they are contrary to building-code virtually *EVERYWHERE*,
and, regardless, are _extremely_ dangerous -- there are 'failure modes'
that can *kill* you, even if you don't touch anything connected to the
YOU are in LUCK!
You didn't say you had conduit to work with! Great!
First, verify one thing. The whit in the junction box that is capped off is
most likely the neutral you are looking for. Get a voltmeter and check some
things. 1.) The capped off white to the bare wire, with it set on AC
volts. Should read no volts. 2.) check red wire to ground, on AC volts.
Should be 118 or there abouts. Red wire to ground, AC, also 118.
If it checks that way, the white is the neutral. Turn off all of the
breakers to that area. You need to pull a red wire from the junction to the
outlet. Buy or rent or borrow an electrical fish tap, and take care to tape
up the hook so nothing sharp is exposed.
You then will go black-black, white to white, red to red, and green to bare
in the junction. At the outlet, (a new 4 prong receptacle) you will go
black to x, red to y, and white to z, and green (or bare) to g. Do the cord
to the saw the same way, with the black and red to the saw motor (switch),
and one of them to the 110 outlet. The white goes to the silver screw on
the 110 outlet, and is not connected to the saw or anything else. The green
goes to the ground screw on the outlet, and the frame of the saw, and to the
motor, if it has a separate wire.
I think I got all the details you need. Write more, if you need more help.
There are no conflicting replies by those who know what they are talking
about. They all say install a 4 wire system. I have it on one of my table
saws, and wired through a starter to turn it on and off with the saw. No
Got it! I will not jury rig anything. I will either abandon the bench outlet
or try to snake a 4th wire through and do it the safe way. I cannot put a
new dedicated 120v circuit through the slab. Thanks to all (except for one
goof) for the advice...maybe a life saved here today!
it will be a lot easier to pull that Romex out and add the wire instead of
trying to shove it in with the romex in place. Technically you should be
pullibng discrete conductors, labelled properly but the cops won't knock your
door down of you just strip the Romex jacket off and use the ones you have. You
won't believe how much easier it is to pull without that jacket.
Before we all sign off on this, I didn't notice anyone discussing the
breaker amperage vs. the wire gauge you plan to use to wire up the 120v
outlet. If, for example, the breaker is a duplex 30 amp and the wire you use
to connect the outlet is 12 or 14 gauge, you might have a fire hazard in the
Thanks Guy...I have come up with a new solution so I will not be doing the
re-wiring. I will just coil a short extension on the outlet box that I can
plug into the wall outlet about 5' away, without having to get out an
extension. I really don't anticipate using the outlet much. Good point re
I jumped in on this pretty late, but how many conductors are in the cable
you are using to power up your saw? I would be surprised if you actually
had a neutral going there, but if you did, you could pull 120 into a
receptacle. I wouldnt want it to be used at the same time the saw was
You are correct...only 3 cables...no neutral, so 120 out of the question.
And no, I had not planned on running them at the same time. My fixed
extension is working just fine and a lot less hassle!
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