I’ve decided to replace my decades-old bedroom ceiling light. According to the instructions that came with my new light, there should be three wires coming out of the ceiling: black (live), white (neutral) and green (ground). What I actually have are two thick mottled-black-and-white wires that were attached to the old fixture, plus at least four smaller wires that were not attached to anything.
Can anyone make sense of this?
Sounds like old wiring, perhaps cloth-over-rubber. There should be SOME difference in them to identify the hot lead, but I wouldn’t count on it being right. If you have a voltmeter, you can (with the power ON, so be careful–no potato necessary) use it to measure the voltage between each wire and the metal fixture box (which SHOULD be grounded–again, a crap shoot on old installations). The one with about 120 VAC between it and the box is the hot. That will go to the black wire on the new fixture.
With only 2 wires, it really doesn’t matter much; the light will work however you connect them.
There is a minor safety advantage when they are connected properly; only the little button at the bottom of the light bulb socket is hot. So if you are working with live wires, and are touching a grounded water pipe while sticking your thumb into the light bulb socket, you have to stick your thumb all the way to the bottom to get shocked. (Personally, I think people doing that many silly things at once deserve a shock!)
And the extra 4 smaller wires not connected to anything – they are probably abandoned leftovers from some old fixture. Maybe some kind of a ceiling fan control wiries. Just leave them alone. (Do make sure there are no bare ends that might touch anything in the box, though.)
The idea is to keep you safe while you change a bulb–when you begin unscrewing it, the threaded base is exposed, but still electrically connected to the socket. If that part was hot, rather than the button in the bottom, you could very easily get a shock.
So these two wires are actually interchangeable?
Electrically, yes. For purposes of safety, no. As stated, the lamp will work just fine wired either way, but if the collar is hot, rather than neutral as it’s supposed to be, you could get a shock, which probably won’t be fatal if you aren’t well-grounded; however, if you’re up on a ladder or chair changing the bulb when it happens, the shock could startle you into losing your balance. I’d try to wire it correctly if at all possible–an ounce of prevention and all.
Only if you hold the bulb by the metal threads at the bottom, instead of the glass bulb part. And only if, at the same time, you are touching a grounded conductor like a metal water pipe or similar.
I really don’t think doing both of those at once is “very easy”. (‘Very stupid’ I would accept.) There are thousands of old houses out there with reversed hots on light fixtures, but you very seldom read about people getting shocks while changing light bulbs. It’s just not that big a problem.
Still worthwhile for panache45 to try to wire it properly, though, checking the voltages like you suggested.
I agree to a point, but as I just pointed out there are other ways to be injured by electric shock than having a dangerous current flow through you. I’d rather someone did the job right the first time, rather than simply wishing he’d done it right while he’s laying in bed recovering from an injury. 