How to Hang a Ceiling Light Fixture (DIY) | Family Handyman
make sure the power is off
test the wires to make sure there is no power. move the tip of a non-contact voltage detector near each wire to make sure all the wires in the box are off (make sure the light switch is on). If the tester turns on, turn off the circuit breakers or loosen the fuses one at a time until the light on the tester goes out. disconnect the wires from the lamp. leave other wires connected and tucked into the electrical box.
It’s hard to believe, but many of the lamps now sold in home improvement centers and lighting showrooms cannot be installed safely in most homes wired before 1985. These lamps are clearly labeled with a warning that says “For supply connections, use cable rated for at least 90 degrees C.” The reason is simple: Fixtures with this label generate enough heat to damage the insulation of older cables and cause a fire hazard. Cables manufactured after 1985 are required to have jackets that can withstand the higher temperature.
if you know your wiring was installed before 1985, do not use fixtures that require 90 degree rated power cords. To confirm that you have a power cord rated for 90 degrees, look at the cable jacket or cable insulation. If you have a cable with a plastic jacket (often called Romex), look for the letters Nm-B or Uf-B printed on the plastic jacket. If your wiring is fed through conduit, look on the cable insulation for the letters thhn or thwn-2. If you’re still unsure, call an electrician or choose a fixture that isn’t labeled with a power cord temperature requirement.
check the capacity and strength of the electrical box
heavy fixtures require safes
If you choose a heavy fixture (the one we bought weighed 25 pounds), check your electrical box to make sure it will hold the weight. The National Electrical Code (NEC) allows you to hang up to 50 lbs. from any electrical box that is threaded to accept no. 8-32 machine screws to hold the crossbar (see “mounting with screws and acorn nuts” and “mounting with a threaded tube” in the additional information below). this includes almost all types of roof boxes.
For practical reasons, make sure your electrical box is securely fastened to a solid frame before hanging a new fixture. If your light fixture weighs more than 50 lbs., it must be supported independently of the electrical box. An easy solution is to install a fan support box (available at home and hardware stores) that is designed to be installed without having to drill additional holes in the ceiling. check the label to make sure the box is designed to hold more than 35 lbs.
Most ceiling boxes are big enough: the nec dictates how many wires and clamps you can safely fit in an electrical box. typical 1-1/2 to 2 in. octagonal or round-roofed deep boxes are quite large and crowding is rarely a problem. still, you need to do the math to be sure. see the section on “calculating box sizes”. but if you find a round box only 1/2 inch deep, replace it. Again, the easiest way to install a new electrical box on an existing ceiling is to use a special fan bracket and a box made for retrofitting.
calculate the size of the boxes
To calculate the minimum box size required by the National Electrical Code, add: 1 for each hot and neutral wire entering the box, 1 for all ground wires combined, 1 for all clamps combined, and 2 for each device (switch or receptacle, but usually not light fixtures) installed in the box. multiply this figure by 2 for 14-gauge wire and 2.25 for 12-gauge wire to get the minimum box volume in cubic inches. the plastic boxes have the volume stamped inside.