This post is similar (but different) to one I posted previously, and I got great answers at that time. This is a different bathroom:
I currently have an overhead bathroom light with a single pole switch at the door. The power for the light first comes into the ceiling box directly above the light via 14/2 cable. The hot for the light switch is tapped in right there by using the black wire of a 14/3 cable, said 14/3 then runs to the switch where the black wire supplies feed for the switch and when switch is turned on the red from same 14/3 runs back to the light as power (right before the light, another black wire is tapped in to feed vanity light). Now, here is something else; in the switch box on the wall, before the black feed from the 14/3 runs to the switch, another 14/2 cable taps into that hot feed to supply overhead lights to the bedroom on the other side of the bathroom wall (it runs straight to the switch that controls those lights from the bottom of my bathroom light switch box).
So, now I'm replacing my bathroom light with a light and fan combination. I will be installing a 2 function switch instead of the single pole so I can control the fan and the light separately. So, I'm thinking, why not just replace the 14/3 that runs to the switch now from the light junction box with 14/4 wire. I can do that in the attic. So, what kind of 14/4 should I buy, stranded or solid? Any special markings need to be on the cable? Also, it has to be able to be the feed for the bedroom lights I mention in the last sentence of paragraph #1 above. I'd tap into the 14/4 in the switch box to feed the bedroom lights. I also need to tie in to the 14/4 at the light box to feed the vanity light.
Best Answer
Any 14/4 you can find should do it. Ideally you want NM type, but UF will suffice.
I expect any 14/4 on the market will be solid wire.
If a conduit connection is feasible, you can then run individual THHN wires in conduit. This may make sense if thecost of 14/4 is prohibitive. THHN wires come in solid or stranded. I prefer stranded, but I am pretty good at getting stranded wire on screw terminals without frizzies. You must never use stranded wire in a backstab.