While building codes usually allow junction boxes in attics, the codes impose installation standards to prevent electrical fires and accidental shock. Junction boxes are basically standard electrical boxes that act as waypoints and connection centers for one or more circuits. Since they're not something you actively use, it's easy to forget where they are, which might lead to accidents and property damage during home maintenance. If you learn about installing junction boxes in attics, you can plan a new project or inspect your existing work for problems.
About Junction Boxes
The term junction box doesn't describe a particular type of electrical box, it describes a particular application of electrical boxes. Junction boxes hold wire connections, such as a point where a circuit diverts to a switch or a receptacle along its path. Additionally, a junction box might serve as the jumping off point for a cable that supplies several electrical fixtures. Electricians regularly use switch, receptacle and light box fixtures to wire junctions; however, this is only acceptable if the box is large enough to safely house all the wires.
The primary requirement for junction boxes is that they remain permanently accessible. According to most building codes, the phrase "permanently accessible" means that the junction box is clearly visible and serviceable without removing any walls, ceilings or building materials. Concealed junction boxes are easily forgotten and therefore pose a great danger to the home's occupants. However, the accessibility requirements do not prohibit you from installing a junction box in the attic. A common solution to accessibility problem is to have the box open to the underlying room's ceiling. To soften the box's appearance, you may install a low-profile face plate and paint the cover to match your ceiling.
Junction Boxes in Insulated Attics
Some areas might allow for junction boxes that don't penetrate ceiling drywall. In this case, the boxes will open into the attic; occasionally they're uncovered. Since insulation is combustible, you must install face plates on all attic junction boxes to prevent contact between bare wires and insulation materials.
Junction Box Mounting Locations
There are two options for mounting boxes that are wholly contained within the attic: you can attach them to the joist's side surface, called the face, or to the joist's top surface, called the edge. Although code inspectors might accept either arrangement, many builders prefer to mount junction boxes on the joist's top surface. A top-mounted box remains clearly visible, even from a distance. Clear visibility prevents homeowners from accidentally placing insulation, decking or storage items on top of the box.