Thanks. Good to know.
Yep, some big differences at times.
I’m way past that point… 
Might want to consider that the red wire might be from a 3way switch or a switched outlet . Where half is powered all the time and other is switch power for table lamp or such .
None of us are too old to learn or can assume that there is ‘nothing for’ us to learn.
I’m sure there’s a book that will bring you back up to date (you read ALL the time)
And if nothing else, it’ll stand you if good stead for any domestic work you need to do (personal/friends/family, you know how it works : - “you are a ‘practical’ man, can you just… ?” 
)
My distain for ‘domestic’ is huge : - crap cabling, crap fittings, coz its all down to minimise cost. I’m sure that you on a “bad day” is better than most
I didn’t say I was going to stop learning.
I just said I wasn’t interested in going back to school for something like that again. 
Touché
10 char min
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I think the red wire might be for 3 way switch, open up other switch boxes in the same room and if you do see it turn ure breaker off and do a continuity test with a volt master, hopefully you know how to do that is very simple, Or it goes to an existing fan light combo, usually the red would be for the lights on the ceiling fan, black wire motor of fan, red wire would be for the lights of the fan…Hope this helps.
Happy holidays.
Ok, switch box right. Goes to light I am assuming since you don’t put a plug on a dimmer. Ok pull the light since it is piped in check and see what is in light box. If wire is capped there it is abandoned. If it continues on into another pipe my best guess it’s an abandoned switch that had an old style 3 way system called a Carter 3 way or aka dead man 3 way. I have 32 years in trade. If nothing else is wrong leave capped and ignore. If you must know start tracing the piping. It could be a buried junction just capped. Best I can do without seeing all involved and whole room. Hope this helped.
Ok, switch box right. Goes to light I am assuming since you don’t put a plug on a dimmer. Ok pull the light since it is piped in check and see what is in light box. If wire is capped there it is abandoned. If it continues on into another pipe my best guess it’s an abandoned switch that had an old style 3 way system called a Carter 3 way or aka dead man 3 way. I have 32 years in trade. If nothing else is wrong leave capped and ignore. If you must know start tracing the piping. It could be a buried junction just capped. Best I can do without seeing all involved and whole room. Hope this helped.
This has turned into quite the contest with people trying to guess at the “mystery wire” with the fewest clues possible. 
But it looks like the OP might have abandoned us so we may never know who won.
Asking the internet information about where a wire goes behind your wall is big mistake. A smart person will trace that wire to see where it runs back to, not make any guesses or assumptions. Its very possible its attached to another switch somewhere, so while it may be dead right now it might be a switched load, in which case you can cause some major damage.
You can also buy devices to trace circuits in homes. A qualified electrician will have this but it is available at most hardware stores.
I’m assuming you connected that wire considering we haven’t heard back… are you still alive?
You don’t need a neutral. You didn’t state in your post if it was a three-way dimmer. The red is capped off because the device that is currently installed is not a three way switch.
Or he may have encountered a live wire and thought twice about using an electrician 
He has gone to the crystal ball forum.
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That’s what you think. I’m still here, just with the holidays haven’t had a chance to pull the light. I’ll need a neutral anyway as putting in a smart switch there so need one. I’ll check shortly who is the winner. 
There’s good potential (ha ha ha, see what I did there?) that this could be a Neutral. I know you say there isn’t one, but can you be certain?
So the verdict is…
I guess it’s a wire for a potential fan? Took down the ceiling light and there was another caped unused red wire in there.
I know this is probably wrong, but even with conduit I couldn’t run a wire thru the conduit being small conduit so just reused the red wire as a neutral and wired it up to the light. Labeled the other end as a neutral. Not sure if I’m just not pulling hard enough or the 12AWG cable doesn’t pull thru easily as it’s ridged so have to just pull harder on them if I want to use an existing wire to pull thru other wires.
make sure you do that on both ends.
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See you are strugling with this problem, trying to give you a useful answer here, but first things first. Electricity IS DANGEROUS, you should trace / test / whatever anything only with the fuse pulled / switched off.
I live in europe so the standards are not the same, but never trust standards! You should always check for yourself!
The very first thing you should check at your main fuse box is the presence of a electrical protective relay (this may not be the correct english name for it. at my home we call if fi relay) It ha a little TEST button on it, and if you press it the electricity should go down EVERYWHERE in your house.
This is how it looks at us.
This should protect humans from electric shock.
If you do not have anything like that, then please do switch off all electricity before doing anything!
But here is the helping part.
I have attached the basic light switching diagrams that we use. the common in all is that NONE of these need a neutral in the wall socket. The ‘L’ is the Hot wire, the ‘N’ is the nuetral. They do not have a neutral, because if you switch the light off the wall switch should break the hot wire to prevent shock when you change the lightbulb.
1-pole ‘normal’ light:
Alternative switch (Can be switched off and on at any side):
Chandellier switch (More bulbs in th chandellier, one switch one bulb, all switches all bulbs):
the dimmer switch by the way should be no more special then a 1-pole normal light switch.
cheers
tom
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Usually the multimeter has a continuity test mode, which should be used to test wires if they are broken. Put the two ends of the multimeter on a strap of wire and if it beeps, then the wire is ok. If no beep then it is not ok, obviously.
With the electricity switched off at the fuse box, you can test any of your wires like that. You can check which goes to the ligth bulb by taking the bulb out and touching one end of multimeter to one of the bulbs connections. If it does not beep try the other leg of the bulb, if still no beep then choose another wire.
It is a bit cumbersome, I know, but you only need to do it once. After finding the right ends. label the wire, and thats all.