Q: I’ve seen electricians cram an amazing number of wires and switches into wall boxes—especially for gangs of three or four switches. What are the guidelines for determining box size, and how can you prevent overcrowding in a box?

A: David Herres, a licensed electrician in Clarksville, N.H., responds: When a box is overfilled to a point where excessive force is needed to push the devices (switches or receptacles) into place, you’re asking for trouble. Wires can loosen or spring free of wire nuts, causing the circuit to fail, or the insulation on the wires may become damaged. In the latter case, if you’re lucky, the breaker trips; if not, a partial arc fault can occur, creating a fire hazard. Also, an overcrowded box may be unable to properly dissipate heat, which will shorten the lives of the devices.

Most electricians know by experience what size box to choose for each application, but with larger devices with lots of wiring or in borderline cases, it may be necessary to perform box-fill calculations. These are covered in 2014 National Electrical Code (NEC) sections 314.16(A) and 314.16(B). It is a two-stage process in which the volume of the box is determined first, followed by the volume of the contents, or the fill calculation. (These sections also address required volume for conduit.) The code states that in no case is the volume of the box to be less than the fill calculation. The box volume is often stamped on the box; if not, it can be determined by measuring the inside of the box. The process for determining the required volume involves adding up the conductors and devices in the box and consulting the box-size table.

Here are several ways to avoid overcrowding electrical boxes. Use deep wall boxes where appropriate. Use grounding crimp connector sleeves instead of wire nuts—which occupy more space—for grounding splices. Choose the right size wire nut in critical situations—for example, a yellow or a red wire nut can accommodate two 12-AWG conductors or three 14-AWG conductors, but a yellow one takes up less space. Ground-fault circuit interrupters are bulkier than standard receptacles, so mount them in deep wall boxes if possible. Instead of octagonal junction boxes, use deep 4x4 square boxes, which have more space. “Pack” the box wisely. For example, if there are two or more wire nuts in the same box along with a wiring device, such as a switch, carefully lay the wire nuts side by side in the box—if stacked, they can interfere with placing the device.