Splitting a wire from the light switch to another switch in your bathroom requires basic electrical wiring knowledge. You can add a new switch using your original switch as the power supply to any room in your home, provided you do not exceed the electrical wattage supplied by the breaker. A 15-amp breaker installed on a 120-volt circuit provides you with 1,800 watts (amps times volts equals watts). Add up all the watts on your circuit to confirm you can add a switch and fixture to your power supply.
Turn the breaker off at the main service panel to disconnect the electricity to the original light switch.
Remove the switch cover plate to reach the wires inside the electrical switch box. Turn a multimeter dial to the 250 alternating current (AC) setting. Touch one prong from the meter to a black wire attached to the terminal screw on the right side of the switch and the other prong to any metal on the switch or electrical switch box. The multimeter needle should remain at zero with proper disconnection of electrical power. Repeat this electrical test for the remaining black wire and terminal screw on the right side of the switch.
Remove the screws holding the switch into the electrical switch box and gently pull it from the box. Loosen the screws holding the black wires to the switch. Pull the wires from beneath the two screws.
Pull a length of 12/2 nonmetallic (NM) electrical cable from your additional switch in the bathroom to the original light switch location. Use a dual NM wire cutter/stripper to remove the exterior insulation from the electrical cable, exposing the interior black, white and bare copper wires. Remove 3/4-inch of insulation off the interior electrical cable wires.
Cut a piece of 12-gauge solid black thermoplastic high heat nylon (THHN) electrical wire to create a 6-inch pigtail. Strip at least 3/4-inch of insulation from each end of the pigtail wire.
Connect the pigtail wire, the black power supply wire and the new black wire you pulled in Step 4 with a red wire connector. Twist the wire connector onto the wires, connecting all three together. The power supply wire connects directly back to a breaker in the main service panel.
Wrap the 6-inch pigtail wire around one of the terminal screws on the right side of the original switch. Wrap the original black wire leading to the light fixture around the remaining terminal screw on the right side of the switch.
Reattach the original switch into the electrical switch box. Replace the cover onto the switch.
Turn the breaker on at the main service panel to connect the electricity to the original light switch and the additional switch in the bathroom.