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 potential?
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.
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.
So if I connect the white wire from the new outlet to the 220V outlet's centre pin (neutral), this will essentially put the white wire on the saw's frame?
If I connect the green wire from the new outlet to the metal box of the 220V outlet will this still cause a problem?
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 energized).
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.
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.
Yikes! Steve, 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 'grounded', 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 nowadays).
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.
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.
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.
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 ground.
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 GRN. 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 capped off.
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 suggested.
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 purpose
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