My source of power is originating from a light fixture and then goes to a switch.
I've pulled a cable from the switch box to a bathroom exhaust fan and wanted to find out how to go about wiring it up.
What would normally be a neutral wire (white wire) in this case ends up being the live wire (black wire) at the switch and the black wire ends up being the neutral.
Any help would be greatly appreciated.
Thank you.

I am guessing you only found two wires inside the switch box, a black and a white, and if the wiring is Romex, a bare wire. I get this from your statement:

My source of power is originating from a light fixture and then goes to a switch.

If this is the case, then you do not have a neutral in the switch box, only the feed to the switch, which is white, and the black switch leg back to the light.

You will not be able to make the fan work properly without a neutral at the switch box, and will need to add another feed cable to the switch box, or to the fan. If you bring a feed to the fan, then you can do the same as the light is done, use the two wire you ran and connect to the new switch.

Let me know more detail, I can make up a wiring sketch if needed.

Thanks for the info tkrussell.

Would this still be the same if I wanted the fan to be controlled simultaneously with the ligths.
In other words when the lights are on the fan goes on also.

I hope I was successful in attaching a diagram of what I currently have.

Sincerely,
Claude

The sketch seems fine, I am assuming the white and black on the bottom only splice through and do not connect to the light, and the white and black on the top only connect to the light.

I modified the sketch, not as neat as you did, to show what I mean.

Hi tkrussell.

Thanks for the info.
Because there is already a wire that has been fished from the exhaust fan into the light switch would you suspect that the following rendition be OK?

Please see attached.

Thanks again.

Nope, will not work at all, you now have the fan motor is series with the light, the light bulb will be about half brillance, and the fan motor will either hum or turn very slowly.Yoo have no choice but to bring the fan cable to the light.

Sorry. If you like give your method a try, see if I am right.

Thanks tkrussell. Your help was greatly appreciated.

Have a great X-Mas.

Sincerely,
Claude