Electrical Wiring on a Roof Ventilation Fan

Before Wiring

• Attic fans are stocked by most home improvement warehouses. It is recommended that an attic fan be used, rather than a regular extractor, because most have an integral thermostat prewired. The thermostat switches the fan on and off according to the temperature in the attic, saving on wasted electricity and obviating the need to remember to manually switch the extractor on and off.

  Follow the manufacturer’s instructions for fitting the fan; this usually involves constructing a wooden frame to the fan casing’s dimensions, which supports it against a grilled hole cut in a shaded exterior wall. An alternative design allows for fitting in the roof field itself -- the large, flat, pitched part of the roof -- using an integral flashing sheet to prevent moisture from entering the attic.

Establishing a Supply

• Attics almost always have pre-installed electrical supplies in abundance. Circuit boxes are fastened to the joists in the centers of most rooms, from which the ceiling lights below receive their power. Junction boxes are fastened to joists, usually above doorways, that feed down to and back from light switches. Some buildings also have unshared supplies to heavy-demand 120-volt appliances such as instantaneous water heaters and air movers, and supplies to wall outlets. Choose a supply that is not switched, so that the attic extractor will not be controlled along with another appliance, and ensure that the added wattage of the fan will not exceed the rating of the fuse or circuit breaker that protects the supply.

Preparation

• It is not best practice to have loose wires strung across roof voids. Use an electric drill and a 5/8-inch wood-boring bit to make holes through the joists between the supply and the location of the fan, so you can feed the new supply wire through the holes.

Making the Connection

• Isolate power to the supply. If it is not possible to be absolutely certain which fuse or breaker protects the supply, disconnect power to the entire building at the main switch. Follow the ventilation fan manufacturer’s instructions for fitting. Typically this will involve connecting a 14-gauge wire from the supply to a connector block inside the fan’s casing. Fans with integral thermostats seldom require twin connections; a single supply powers both the thermostat and the extractor fan. Some manufacturers may require another junction box be installed close to the fan and that a pre-installed wire from the fan then be joined to the supply inside the junction box. Connect the two white wires together and the two black wires together; if it has bare or green wires, these should also be connected. Fasten the safety covers back onto the junction box(es) before re-establishing the power supply and testing the fan.