finding a short circuit in the wall

I am wondering if there are any tricks electricians use before I start ripping walls to find a short. (Sigh).
I have isolated the short between the 2nd and 3rd light fixture in a hallway. The 2nd fixture is a recessed lighting box. The 3rd fixture mounts on a simple round wall box. There is no visible short in the accessible parts of the wire at the two ends. All three wires (hot, neutral, ground) are shorted together.
This is relatively new construction, finished about 1.5 years ago. The wire is standard 15A or 20A romex.
I tried measuring the resistance of the short from the two ends to see if I could tell where it is. My voltmeters measures down to 0.1 ohm which is a lot higher than a short length of romex. So I rigged a circuit with a 100W lightbulb to push about 1A through the short, and tried to measure the voltage drop from each side. This is tricky because a lot of the resistance is in the contacts and it varies a lot depending on how good they are. I was using alligator clips. The approximate readings are 220mV from the 3rd fixture and 200mV from the 2nd fixture.
Any tips/suggestions before I start breaking the wall near the 2nd fixture?
Thanks! Luigi
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Luigi, The easy way is with a cable fault finder. There is currently one on eBay for about $100. Auction# 250019824403 I got mine for about $70. These send a pulse down the wire and calculate how long it takes to come back which is indicated on a digital read out as the distance to the short or open circuit. They are extremely simple to use and are also handy for measuring how much cable is on a reel. They only work with 2 or more conductors and will not work with a single wire.
John
snipped-for-privacy@gmail.com wrote:

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All three? Yikes.
Telephone servicemen (and presumably some computer network geeks) have special instrumentation that allows them to "echo" check shorts and figure out how far away they are. Electricians wouldn't. Unless you can know of a tame phone serviceman, you're probably SOL.

All three wires is a pretty catastrophic cable failure. Eg: getting "scissored" between two structural elements, really badly crushed under a box clamp, or punctured by a screw.
With the segment isolated:
1) loosen the box clamps and wiggle the cable. See if the resistance becomes intermittent.
2) Try to isolate the ground from the boxes. See if there's still continuity from ground to the box (clamp crush or insulation slice).
3) Using some sort of metal detector (eg: a compass or magnetic stud finder), find the fasteners in the ceiling (eg: drywall screws), and see if there's continuity from the wire end to the fastener (you'll need a very sharp probe). If you find any, you've found the short.
4) Commence chopping ceiling.
Good luck.
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Chris Lewis, Una confibula non set est
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run new cable between areas.
the short is likely at a box but might be a screw thru romex or other fun stuff.
best to just replace that cable run.
yeah there will be colateral damage and perhaps some holes.
tink of closets etc on other side of wall, if its drywall its easily patched
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Thanks to all for the quick replies.
The cable fault finder is a good idea. I know of at least another place where I can use it (a short in the wire that turns on the central vacuum when the hose is inserted in a plug). The TS100 accuracy could be better (2 to 5 feet, depending on cable length) but it's still useful I guess.
I wasn't able to unclamp the wire at either end but I will try harder. Not much room to work with in the recessed box.
Thanks! Luigi
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snipped-for-privacy@gmail.com wrote:

There are tools to find it, but why? It would be far better to just run a new wire. You are going to need two holes or one big one for the splices.
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There are tools to find it, but why? It would be far better to just run

yeah and the box must be permanetely accessible with removable cover, worse theres probably not enough cable buried in the wall to do it right.
some people go to so much extra work and gain so little
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pierced/crushed it may keep the wire from being pulled. Another thing to keep in mind, it might not be just one place. My mothers storm door became electrified after having a door lock installed. When I pulled off the paneling to get at the wiring found several of the nails holding the paneling had also nicked the wires . Each job is going to be different. Good luck to the OP.
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wire may well be stapled to insude of wall framing, in which case you cant pull it anyway.....
fixing stuff right sadly sometimes involves colaterral damage.........
fix the problem then fix the wall.........
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T Shadow wrote:

You don't need to follow the path of the old wire. Just cut off the power to the existing wires and pretend they don't exist any more. Run a new wire and it may well require running it in a totally different area.
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Joseph Meehan

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Only 1.5 years old, shouldn't there be some warranty? How did you know there was a short and how did you isolated it?
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