I installed a ceiling fan recently and it required me to use black (hot), white (neutral) and ground wire. The rough-in had an additional hot wire (red) which was left unused because the fan had remote functionality. My question is that can I use this unused hot wire to connect to another wire and connect that wire to ceiling lights?
A:
To really figure this out, we need a picture showing all cables/wires in the switch box. That will tell us, definitively, whether this is one switch switching two wires, one switch switching one wire (with the other wire always hot) or two switches each switching one wire.
Black, red, white and ground in the ceiling box can actually fit 3 different scenarios. From this point forward, the bare ground wires will be ignored as they are not relevant to this situation – they should be in every cable, connected to metal boxes, connected to every device.
Switch loop
Always hot (typically black) included as well as switched hot (typically red)
Two switched hots (black and red)
A switch loop would, typically, send hot down to the switch on black and neutral on white and bring switched hot back up on red. However, it also requires an additional cable (black/white) for incoming power. Since OP did not mention a second cable, I will assume that this is not a switch loop.
So that leaves two very different possibilities:
Always hot included as well as switched hot
This seems a little odd to me. Why would a typical penny-pinching builder include an extra wire (always hot) when it is not required? But it is possible. If this is the case then the fan black wire should actually be connected to the red wire, not the black wire. Doing so will allow the wall switch to actually turn the fan/light on and off. However, depending on the design of the fan control (fan/light remote? fan speed controller remote? pull string switches?) that wall switch may or may not function as expected.
Two switched hots
This is the traditional configuration for a fan/light – one switched hot to the fan, one switched hot to the light. In this case, the second switched hot can be connected to a separate light (i.e., it doesn’t have to be part of the fan) as OP wants, with the neutral (white) extended as well. Don’t worry about the colors – you can use black for the fan and add a black/white cable to extend red/white to the light fixture.
However, it is extremely common for a fan to include a remote control module. The remote control module gets always hot (which often means “wall switch with a piece of tape over it so nobody turns it off”) and the remote is placed on the wall next to it to use for turning on/off the light (and often dimming it as well) and turning on/off the fan (and often speed control as well).