I am trying to install a new light fixture, but there are 9 wires--I wasn't able to see how it was connected and I'm pretty sure it was wrong to begin with because bulbs kept burning out right away.
Initially the whites were together, 3 of the black wires were together (including the hot wire), and one single black wire. There are 3 pairs, and one bunch with 2 blacks and one white. The casing is literally crumbling at the touch.
How can I properly wire this to a new fixture?
From the looks of those pictures you won't be able to properly wire this without some major rewiring. Those wires have insulation that is cracking/ crumbling all the way up up the connectors that hold them in the box. You would need to identify all the wires and would need to turn on the power to do that. That box is a fire waiting to happen and you do not want to be up on a ladder when it starts arcing. Those wires need to be replaced, up into the ceiling and maybe even further. If you have an attic and can get to that box, the job might be easier. If no attic, you're looking at making holes in the ceiling. I love DIYers but this is one of the very few times I'm going to say you need some professional help.
Professional Electrician here, I hate to say it, but those are fabric coated wires, they stopped being installed in new homes back in the 60's and very early 70's. Any surviving cloth wiring is way past its service life now. I know its going to be expensive, but if you dont want to risk burning down your house, this wire needs to be entirely replaced ASAP. Yes a total home rewire is going to be between a 2 and 10 thousand dollar job, but it needs to be done. and this is no DIY task, you need to call in the professional's for this one.
At first glance, I thought "Oh no. You pulled them all apart. Now we'll never know how they went."
This is actually quite terrible. Because now we're playing "Chinese Whispers" via your recall of how the last guy did it, and that work is not 100% reliable. Normally this is the point where I say "essential data has been lost, time to call a pro with tracing tools and experience".
In the future, simply cut the black and white small leads off the old fixture, and don't mess with the bundles of cable from the walls. Then splice the new leads to the old leads with a blue or gray wire nut.
But if you're sure about what you say about the configuration, that is plausible assuming the "solo" black wire is in the /3 cable. (that would be a modern post-2011 switch loop, actually.) I hope you know which one that is, though experimenting with that particular issue is probably not unsafe.
The disintegrating insulation on the wires may be from the heat of incandescent light bulbs previously used in the fixture. In particular, many people enjoy ignoring the "60 watt maximum" stickers inside fixtures, and that will cook the wire insulation and even start a fire.
If so, the insulation will be more sound as you get farther away from the fixture - the worst damage will be on the wires going to the fixture. If you find the insulation toward the end of the wire is solid, it'd be possible to save the day with some shrink-tube. You want the better shrink-tube that shrinks 3:1 so it can slip over the old insulation and still wrap the wire decently. A double layer (one over the bare wire and one over the insulation and first layer) would be better still.
Regardless, I would not continue this wire in service without installing an AFCI breaker at the panel, or AFCI receptacle or deadfront at the first opportunity past the panel. Wire arcing is at high risk for starting a fire, and an AFCI gives you the best chance to detect that.
The best workaround to both the "burning out" and the "thermal damage" issues is to fit a bulbless LED fixture. Those generally have a switching power supply which accepts a voltage range from 90V (Japan -10%) to 264V (UK +10%). As such they will not be bothered by weird voltages.
If you believe LEDs still suck like the early ones did, please revisit those assumptions because quality ones are readily available (not necessarily on the bottom shelf at the dollar store, if you get my drift). Regardless please DO NOT put any more incandescent bulbs in whatever fixture goes there, unless it's a chandelier that puts the bulbs far away from the junction box.
However that does nothing about the root problem, which is weird voltages should not be happening, and can burn out a whole lot more than light bulbs. The two most common voltage problems are
Both of these are supply wire problems with the service coming from the utility. 95% of the time, once you convince the utility that the problem is real, they fix it fast for free. Mine did it in an hour on a Sunday.
Regarding the wire quality and whether you should replace these wires, I personally can't tell from the picture. My experience with this style wire construction is that it always looks like that. But, you say it's crumbling off just from touching it. If that's exposing the copper, then some wire replacement is vital. Regarding the fixture, it looks pretty old and crusty. One of the wires looks stripped near the fixture. I'd get a new one. Or maybe that's the old one?
The only sensible reason there's multiple black wires in a fixture is splicing. The original installer needed the power to go to another location. That power could be either pre or post switch.
You need a multimeter to identify what wires actually have current.
I'd start at the switch. Turn on your breaker (or make sure the fuse is good if your stuff is that old) and see if either wire attached to the switch has current. If yes, we should expect that all the black wires in the fixture do not have current when the switch is off. If any do, call a professional because the setup is irregular. We should also expect that the switch controls multiple fixtures, which you already know I'm sure, but simply haven't said so. If this all checks out, tie all the black wires together and all the white wires together.
If neither wire in the switch is hot, then the current is coming from the fixture. Very likely only one black wire is hot. If you find more than 1 hot black wire, I'd consider a professional for an evaluation of your entire electrical system. Again, this would be irregular.
If more than one white successfully grounds the hot, that's fine.
After determining which wire is hot, determine which white/black pair goes to the switch. If the last guy did his job, he should have permanently colored the white switch wire black. If not, you'll have to troubleshoot. With the switch on, tie the hot to the other black wires one at a time, and use the multimeter to test if the return white is hot. When you find that, that's your switch white/black pair. Color the white wire black.
If you've made it this far, it means the remaining black and white wires run to other fixtures or plugs to provide power and neutrals. Evaluate your nearby plugs and fixtures to see if they work. If not, those are likely where they run to. Decide if these are supposed to have power all the time or only when the switches on, and then tie them together accordingly.
You have to find out with certainty what each of those wires are. One may be the supply of power, one may go on to other outlets, one or more may feed switched power to other lights in the same room, one or two may be switch loops for a light and fan. Some of them may be disconnected at the other end, some may even terminate in walls. Hopefully in junction boxes with wire nuts but in many cases you'll find disused wires terminated poorly with just tape. With that many wires, hopefully you can figure out where each one goes but if not, you have good reason here to worry about poorly terminated wires inside the walls.
Some of those are badly damaged, as noted in another answer. Sometimes they can be repaired but you have so many wires in such a small box it will be impossible to repair the brittle old ones properly.
I'm afraid I agree with other answers here: you should crack open at least a bit of the ceiling, and probably more than a bit, to replace damaged wires, discover and remove disused ones and understand what they all do.
I agree with all the other posters that you need to repair/replace these wires.
However, I can't help myself from trying to solve the problem.
I am trying to install a new light fixture, but there are 9 wires--I wasn't able to see how it was connected and I'm pretty sure it was wrong to begin with because bulbs kept burning out right away. Initially the whites were together, 3 of the black wires were together (including the hot wire), and one single black wire. There are 3 pairs, and one bunch with 2 blacks and one white. The casing is literally crumbling at the touch.
One very important clue is that all the white wires were tied together. This suggests one circuit coming into the box, and from there it feeds a switch and some plugs in the room. In older houses the wires typically run across the ceiling in the attic and straight down the walls to the plugs and switches to minimize the number of holes that had to be drilled by hand. Hopefully the attic is above this box and you can get to it.
First, with everything apart like you have, check to see if there is one hot wire. This will be the hot wire from the breaker. All the white wires are the neutrals. Mark the hot wire (piece of colored electrical tape).
Second, the single wire probably goes to the switch, along with one of the hot wires. Check for continuity (with the power off) and get in the attic and trace it out. The single wire would be connected to the lamp.
The other two hot wires (and corresponding neutrals) probably go to plugs in the room, or plugs in adjoining rooms. Are there any plugs that are not working with everything apart? Again, get in the attic and try to trace the wires out. There may be multiple plugs tied together down low. There should be at least two that are not working.
Finally, now that you have everything traced, you should replace all the wires. If you are lucky, all the wires will run along the attic and go straight down into the plugs and switches. If so, the job won't be too bad.
I made several assumptions. If any are not true, the job becomes more complicated.
Once you replace all these wires, you need to think about replacing wires in the other parts of the house. If you are lucky, each room is wired similarly and you can do one room at a time.
Good luck!
One final comment, you said that you thought it was wired incorrectly because the bulbs kept burning out. I can't think of a reason where incorrect wiring would cause the bulbs to burn out quicker. They would either work or not work, but not burn out faster. My guess would be the bulbs are getting too hot in the enclosure, or the bulbs are just cheap. It seems like all the new bulbs I buy anymore burn out fairly quickly.
