Ceiling fans can help lower heating and cooling costs, but the fans must be installed correctly using the correct junction box. The shape and material of the box must be specifically suitable to support a ceiling fan. If you use the wrong junction box, you may be doing so against the Underwriters Laboratories listed rating for the specific junction box.
Metal Junction Box
Ceiling fans are heavy and require a junction box that will support the weight of the fan. According to Ace Hardware, you cannot use a plastic junction box with a ceiling fan. You must use a metal junction box. The metal boxes are long lasting and durable enough to hold the weight of the fan and tolerate the movement and slight vibration from the fan.
Plastic Junction Box
The plastic junction box is easy to install and cheap, but the boxes should be used only when you are using cable that is nonmetallic. The plastic box is not as durable or as long lasting as a metal junction box. Ceiling fans have movement and some vibration, which a plastic junction box will not support.
Junction Box Type
Retailers sell different types of junction boxes. The one that is used for a ceiling fan is called a ceiling box. The junction box is round and can support heavier ceiling fans. The octagon junction boxes are for lightweight fixtures such as carbon or smoke detectors. The rectangular boxes are generally used for outlets or light switches.
Any junction box, even a metal one intended for use with a ceiling fan, must have an Underwriters Laboratories (UL) listing on the box that says that the junction box is rated for use with a ceiling fan. The box is rated for holding a ceiling fan 35 pounds or less unless otherwise stated on the junction box. If your ceiling fan is more than 35 pounds, you need a metal junction box that has a UL listing for the weight of your fan.