Adjustable mounting devices allow you to install boxes between framing members to support light fixtures and ceiling fans.
The type of mounting bracket, bar, or tab you use depends on whether you’re mounting a box to finish surfaces or structural members, plus you have to be sure that the box is rated for the weight of the light fixture or ceiling fan you plan to attach to it. When you’re attaching a box to an exposed stud or joist, you’re engaged in “new work,” even if the house is old. New-work boxes are usually side-nailed or face-nailed through a bracket; nail-on boxes have integral nail holders.
The mounting bracket for adjustable boxes is particularly ingenious. Once attached to framing, the box depth can be screw-adjusted until it’s flush to the finish surface.
Adjustable bar hangers enable you to mount boxes between joists and studs; typically, hangers adjust from 14 in. to 22 in. Boxes mount to hangers via threaded posts or, more simply, by being screwed to the hangers. Bar hangers vary in thickness and strength, with heavier strap types (rated for ceiling fans) required to support ceiling fans and heavier fixtures.
Excerpted from Wiring Complete, 3rd Edition (The Taunton Press, 2017) by Michael Litchfield and Michael McAlister