Need urgent help with electrical

I need urgent help with electrical in my basement. I have 6 recessed lights controlled by 3-way switch. Power comes to fixture #1, then to #2 then #3, then #4 and from #4 it goes to #5 and #6. Two days ago halogen 65W bulb on fixture #4 burnt out and I replaced it. As soon as I replaced it fixtures #4, #5 and #6 start to dim. I found that if in the group of fixtures #4, #5 and #6 I remove all but one bulb it works OK, if I put the second bulb in that group both lights dim, if I put all three they dim even more. Fixtures #1, #2 ans #3 work OK regardless of three others. I was advise that there may be a loose wire somewhere in circuit. I opened up fixture #4 and fixture #3. Voltage at fixture #3 is OK, 120V between hot and ground or neutral. However, at fixture #4 voltage is 60 V between hot and neutral, 60 V between neutral and ground and 120 V between hot and ground. What can be wrong with the wire connecting #3 and #4? The wire is BX armored cable AWG12. And what's the best and safest way to fix the problem? I would like not to remove that wire since to do so I have to rip sheetrock in half of the basement. However I can put another cable in parallel.
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Check your neutral and all electrical connections especially at the switch locations.
From your description there is more going on that what you describe. Me thinks.
If there is really only 3 wires, between the fixtures, hot, neutral and ground. Then connections problems come to mind. If there are more wires than just 3 then it is hard to say from here.
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Hey Sasha, The saga continues. The problem seems to be in the neutral between #3 and #4. Either leaving 3 or entering 4. Such that a load is drawing the neutral up from ground potential as I said before. Who made the fixtures? Are they retro fitted or original? You should be able to remove the trim rings and drop the cans down, as here for retrofits: http://doityourself.com/accent/installrecesslight.htm This is typical of Junos that I've worked on. You seem to be able to see the BX. Where are you seeing it? This seems to imply that you have access to the junction boxes for each of the recessed lights and can check the connections. You'll have to take the wire nuts completely off and inspect the connections. Richard
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I opened up the ceiling around fixture #3 and #4 meaning I cut sheetrock on celing to completely expose thefixture. All fixtures are Halo 6" new construction recessed lights. I verified that neutral, ground and hot wire nut connections at both #3 and #4 are OK. I unscrewed all wire nuts and examined all connections, I do not see any problems in them. I measured voltage at fixture #4 between floating hanging wires when two bulbs in problem group are dimmed. Again, 120V hot-ground, 60V hot - neutral, 60V neatral - ground. I can't understand what can cause such behavor? What I see is something is wrong with the BX wireconnecting #3 and #4. however I do not understand what exactly wrong. If wire in cable is broken there should be no voltage, if there is shortcut, the breaker would trip.
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Check all the nutted wire splices above fixture 4. I think one of the neutrals came open and somehow changed the parallel wiring to something such that two bulbs are powered in series. You should have gone from 4 to 5 to 6 instead of trying to parallel the last two connections. It should have worked anyway but it setup a situation in fixture 4 with more wires and more opportunities for miswires. Afterall it was fixture 4 you fooled with that started the problem. The bulb blowing was irrelevant. The problem happened when you moved stuff around changing the bulb. It worked once so it was correctly wired at one time. A short would blow a circuit breaker, an open is much more likely anyway.
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Sasha wrote:

I'd bet it's a crummy connection of stranded wire, wire-nutted together. This combination is used extensively, for reasons I'll never understand, because they are all problems waiting to happen.
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Yeah, there would be three solids and one stranded wire in the nuts over fixture 4. You may want to pigtail so that the stranded is nutted to only one solid wire.
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I've also had good luck with twisting only the 3 solids together, then wrapping the stranded (stripped long) along the twist grooves before wire nutting.
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I did this - disconnected #5 and #6 from #4. I also unscrewed wire nuts at #4 and just hag wires. So if wires are hung at #4 meaning there is no load voltage is 120V between hot and neutral. However if I screw just fixture #4 with #5 and #6 disconnected I have 100V hot-neutral and 20 V neutral - ground. I then thought may be something wrong with fixture #4 and unscrewed it but connected #5 at #4. Some effect, voltage seems to be divided between hot and neutral relative to ground depending on load, more load more voltage between neutral and ground and less voltage between hot and ground. note, that at any case and i measured it voltage at #3 is 120 V hot -neutral or ground and there is no voltage between neutral and ground as it is supposed to be. It seems I have eliminated all objects except that BX cable between #3 and #4. Of cause I can replace that cable but i still want to understand what can cause such weird behavior to avoid it in the future.
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Sasha, What you just did here was a load test of the circuit up to that point. This proves that the problem is ahead of #4. If you do a similar load test at #3 and it passes the test, the problem is between there and #4. I would completely take the wire nuts off and remake all connections. This is with the proper gage wire stripper and with new wire nuts. I use a pliers to twist the leads together (clean and shiney) as well as to twist on the wire nut. If after this it fails the load test at #4 and passes at #3 pull out that run of BX out and replace it! If you're curious why it failed you will have to perform an autopsy on that piece of BX. Unwrapping all the armor or pulling all the wire out of it and checking it all. You'll probably find a broken conductor somewhere in that run. Most likely there's a bad junction that you're not catching. usually a broken conductor leading into a junction that was partially nicked through then failed. Richard
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Since neutral is not at ground I would assume poor neutral connection at #3 or #4. Since your load is a 65W bulb the bad connection must be dissipating a fair amount of heat. Don't ignore it.

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And Sasha, this is a potentially dangerous problem. If you can't find an obvious problem you should get a licensed electrician to look at it.
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