Step 2 : Investigate the circuit
Tapping into an existing light circuit to make a new fan circuit depends on how the existing bathroom lighting is wired. There are two general configurations. The first has incoming power supplied directly to the light fixture. In this case more than one set of black and white wires are found in the electrical box for the overhead light and a single set of black, or possibly red and black wires, in the electrical box for the switch. The second wiring possibility is that power comes directly to the switch and then goes from the switch to the light. With this case you have more wires in the switch box and probably a single pair of black and white wires in the light box.
Determine which wiring scheme you have, and decide how to tap the circuit at the power source. Another alternative is to wire the new fan directly to the light without a switch and then the fan will go on with the light.
Regardless of what circuit you tap, bear in mind the following:
- If you make an electrical connection using an existing electrical ceiling box, a receptacle box or a box for a switch, don't overstuff it. If it seems the box is too crowded with wires, install a larger box.
- If you find it necessary to run cable into a new junction box and from there to the new light, the junction box must be left in an accessible place (in the attic or basement) and not covered with drywall or closed in any other manner.
- To run cable across ceiling joists, cut small holes in the drywall at each ceiling joist. Then cut notches in the bottom of the joist only as deep as the cable and cover the cable at each joist with a protective steel plate.
If you use a cable's white wire to carry current to the light, mark the cable's end with black electrical tape.
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