I’m having an internal debate that maybe someone can help me settle…
I’m installing a fixture in a location where snaking a line is going to be a massive cramp. But, it’s inches from the switch that will control it, which has only a live conductor and a ground. Now, help me solidify in my brain why using the ground as my neutral is a bad thing. I have been chewing on this more as a mental problem than a practical one. I thought “well, if the circuit develops an open ground, then someone could get whacked.” But, wouldn’t the current flow simply be carried through the nearest ground junction and back to the panel?
Replies
The basic concept is that ALL "grounds" and anything that might have a charge on it be AT ONE AND THE SAME POTENTIAL.
Thus the equipment ground and also water pipe and gas pipe bonding and furnaces are connected back to the system ground.
And the only way that you can depend on them to be at the same potential is to never allow current to flow in the ground under normal circumstances, period.
"I thought "well, if the circuit develops an open ground, then someone could get whacked." But, wouldn't the current flow simply be carried through the nearest ground junction and back to the panel? "
Who know where it is ground?
Let me give you an example. A bedroom with all the receptacels and light wired on one circuit. Not the best way to wire, but certainly legal and commonly done.
So that cable goes from a receptacle to the light in that room And from the light to a switch leg where you want to boot leg off of using the ground for the neutral.
Now in that receptacle you have a computer plugged in (someting with a metal case and grounded).
Now the ground is broken at that receptacel or up stream from it.
Now the computer will work. The light will work, Nowever the light aht you boot leged won't because the return is now open.
But the light, when on, will connect through the bulb from the not to the wire that used to be a ground, though the used to be ground in the first light, through the used to be ground wire to the receptacel and to the case on the computer.
So the computer case is at 120 volts only limited by the resistance of the light bulb.
OUCH!
Hey Paul,
On a normal short circuit,say your drill motor shorts out,there is little resistance to current flow.Your shorted drill motor might draw hundreds of amps for a fraction of a second until the breaker opens.In the circuit that you're suggesting only a normal amount of current will flow in the circuit.Except it will be flowing on the ground back to the panel instead of the neutral.What happens if someone gets in parallel with that ground path,or worse yet opens that ground path to do service work?They're going to get rapped,maybe hard.We always bring up the example here of someone picking up a ground off of a plumbing pipe, and then fault current accidently being imposed on the pipe.Maybe it's only 2 amps for a light fixture,but when you cut into the pipe in the basement to make a change or fix something,then you're wet and in series with 2 amps,and that is something you may not survive.
Replacing or adding a piece of cable usually isn't as hard as people first think.
Barry
Thanks guys,
Thats essentially what I thought. (God, it's hard to believe I was an EE major, but that was long long ago in a galaxy far far away ;) ) More than anything I was chewing on the practical vs. the theoretical.
(BTW... I'll pull the new cable ;) )
> snaking a line is going to be a massive cramp.
That's why I love EMT. Easy to use the old wires to pull in new ones and do it right.
-- J.S.
Snaking wires in odd spots is something of a speciality of mine. Wencome to the nighmare world of 'old work'. Where nothing is easy and reinventing the wheel a daily event.
If you could post some description of what your facing I might be able to give you some hints that might make pulling a new run easier.
That's very kind of ya 4, I appreciate it.
The fixture is a sconce going on a stairwell wall, it needs to tie into an existing 3 way circuit. Plaster walls, and the only existing fixture on the circuit is directly overhead (ceiling mount). This was a late addition by the HO, so the subfloor's already down, I'm guessing I'm going to have to go up the stairwell wall, under the floor, up the adjacent wall and across the ceiling... heckuva lotta snaking for one stinking sconce :S
PaulB
Hmm. You have me confused. You say the switch your running it to has only two conductors, a hot and a ground. Is this wired in cable, a multi-conductor assembly, or is this a conduit system?
What color are the conductor insulations. How old is the system? Is it knob and tube?
As witch, to work needs a hot and a 'switched leg' or 'return'. Is it possible the box in being grounded by a conduit or armor. Is this running a ground. A lot of early homes had no grounds.
If one of those wires has white insulation and the other black and the system is run with cables the white, assuming it was wired correctly will be the hot and the black the 'switched leg', the leg controlled by the switch and feeding the power directly to the light.
Another poser: In the last post you say the the switch is a 3-way? This implies there has to be, outside the ground which may or may not be there, a third conductor. Typically a red if it is part of a cable.
A lot of this depends on where the 'power feed', the end of the circuit closest to the panel, comes into the three-way circuit. It can enter at the box holding the light, the box holding the switch closest to your sconce or the switch farthest from your sconce.
If it is the switch location closest to the sconce your in luck. You will have a neutral to run the sconce. Typically this means there are two cables with five wires. Not counting ground conductors which typically are bare or green wires. Possibly a bit undersized. Maybe attached only to a screw on the metal box. Some are quite hard to spot. A strong flashlight helps.
If the power comes into the other switch location, count the cables or wires, your less lucky. If the power comes into the overhead light your also less lucky but it is good to know. Again count the cables or conductors.
Sometimes, if the house is balloon framed it is easier to run the cables down the wrong wall and back up to the switch or down to the sconce location. Even with situations involving platform framing going up and down is often far easier than side to side. A lot depends on the framing.
Given a choice between a long but easy pull and a short but hard one go for the long one. I have, not infrequently, run a cable up to an attic or down to a crawl space, sometimes thirty or forty feet, as opposed to trying to go across the studs.
Of course if you know a good drywall man it can be easier to either drop the drywall or cut out the drywall, a thin section, on top of each stud or joist and notch the wood to accept the cable. The wire is then only being fished a couple of feet at a time. A nail plate gets positioned over the cable at each stud or joist and the wall gets patched. A good mud man can make patches that are essentially impossible to find once the wall is painted.
If you could clarify the situation with how many, cables, conduits or wires are present at each box I might have a solution.
Another consideration if you have fairly large baseboard and can pull and replace it without doing too much damage: Make your drywall holes and holes thru the studs behind the baseboard, and do your horizontal run there.
-- J.S.
Thanks John, that's an approach I had considered but probably not hard enough. It's starting to look like a definate possibility. Thanks for taking the time to respond...
PaulB
Sorry 4, I guess I wasn't clear in my description. The circuit is a 3 way, so the switch (at least on first glance) has only 3 hot conductors and a ground. I believe it is fed from the existing overhead light. The layout of the studs is such that I'm going to have to cross quite a few no matter how I slice it, including finished plaster surfaces that I'd have to repair (again, this was a late add on...grrrrrr). While I'm no seasoned pro I've fished a good amount of wire in my time, and I can't seem to find a route that isn't going to be a dramatic PIA... but all the suggestions are appreciated.
PaulB
Don't forget you can cut holes after removing baseboards or cut holes and install a chair rail or crown mouldings over them with only rough patching needed. Another thing people forget is that the walls have two sides and that patching holes is less a PITA when it is in a closet, behind cabinets or other similar locations. More than once I have jumped floors by running the cables into the lower back corner of a closet and out through the top with conduit protecting the cable.Another thought is to check to see if there is a receptacle on the other side of any walls near your switch and sconce. You could take the light off of one circuit and run it off the one feeding the receptacle if it makes things any easier. Unfortunately you can't just pick up just the neutral to make it work. Seen it done that way. Real hack job. But if you can get a feed cable into the switch and a length of two conductor cable from the switch to the sconce it should be easy to rewire in a workmanlike manner.You would need to disconnect and cap off the existing power run going to the light. You could just cap it under the light, or wherever it comes into this three-way switch configuration and stuff the wires gently and neatly into the bottom of the box.
Just to polish off the thread (and pat myself on the back ;) )
I was sitting on the stairs today scratching my head when I realized I was locked into the typical German "ziss is ze only vay to run zee vire" mindset and realized if I turned the matter around (literally) I could fish the new wire no problemo. Took me an hour and a half start to finish. Thanks to everyone that convinced me not to take the easy way out ;)
PaulB
Soooo. Exactly how did you run the cable?Would it best be characterized as closer to Kosher, or improvisational.
4Lorn! I'm shocked you would even ask after all that.
As Kosher as a ...hmmm... dill pickle?
I realized if I came out of the top of the stairwell at an angle I could get to a joist bay, then up the wall opposite where I planned on, right between the stud bays. And that I didn't have to tie into the ceiling fixture since I already tied a sconce in the wallway to it, so from the stud bay to the hallway sconce. Total holes drilled: 2, no studs butchered up and virtually no patching. But if it wasn't for youse guys making me scratch my head I think I would've mounted a flashlight in the wall, and hoped the check cleared before the batteries ran out ;)
PaulB
Good to hear it worked out.