How do I take a cut off the bathroom light source, to install an extractor fan?

Im wanting to installl an extractor fan with a timer, connected to my bathroom light, so every time I switch the bathroom light on, the extractor fan will switch on too. The trouble is I dont quite know how to take take the power from this switch. Do I take just before it reaches the light itself? Answers would be greatly appreciated. Thanks, Steven

Public Comments

1. But another problem is running the exhaust fan's output duct work to the outside of your house. Since the fan has a small motor, it may properly go on a non GFI (ground fault interrupter) circuit. Your bathroom AC plug outlets should be protected by a GFI circuit breaker at least for new wiring in the USA. Our bath fans usually are recessed into the ceiling of the bath with an exhaust duct running outside the house. But if the bath has one outside window and wall, you could mount it high on the wall with the duct going straight through to the outside.
2. this depends on where light actually gets its power from...most time the switch is just one leg of the power circuit...the power should come to where the light is mounted and then go to the switch and back to light...you will need to take light down and inspect how it is connected ...if two cables with two wires and grounds are in box then one is power wires the other is switch circuit.....white wire from power cable will hook directly to light ...black wire will connect to white wire to switch..then black wire from switch will tie to light...
3. go to your power controller board most of the time it is located in the basement or in a kitchen. then, read the name each switch assigned for and switch off the restroom light and remove the fun ( i will advice if you don't know electricity please call service man)