Electrical Box Basics
What are the main purposes of an electrical box?
To mount devices, protect people from shock, keep wires from combustibles, and act as a pull point.
What type of box is mounted to a joist and used for light fixtures?
Ceiling box.
What is a knockout?
A pre-cut or scored hole in a box designed to be removed for wire entry.
Do nonmetallic boxes require grounding?
No, they are nonconductive.
What is the main purpose of a box cover or faceplate?
To cover and protect exposed wiring and terminals.
To cover and protect exposed wiring and terminals.
What article of the NEC covers the installation and use of boxes and conduit bodies?
Article 314.
What is a wall box typically used for?
Switches, receptacles, and splices.
How do knockouts in nonmetallic boxes differ from those in metallic boxes?
Nonmetallic knockouts are made from thin material that must be scored to remove, while metallic knockouts are pre-cut circles held by a single weld that are punched out.
What is a ground screw?
A screw threaded into a metal box to attach and bond a grounding conductor.
Name the three common materials used for faceplates.
Plastic, nylon, and metal.
What are the four common shapes of electrical boxes?
Square, rectangular, octagon, and round.
Which box is used wherever conductor splices or pulls are needed and is covered with a solid plate?
Junction box.
What must you do with unused knockout openings according to the NEC?
They must be covered with a seal or plate—never left open.
What is a ground clip used for?
To hold a ground wire tightly against the metal box edge for electrical bonding.
What are the three common sizes of faceplates?
Standard, mid-size, and jumbo.
What does it mean when a box is “listed” or “labeled”?
It’s approved by UL or another testing agency for safety and compliance.
Electrical boxes are primarily made from which two types of materials?
Non metallic and Metallic
What is the purpose of an extension ring?
To add volume, bring a box flush to the wall surface, or allow for surface wiring
Which component must be grounded in a system that uses a metallic 4" square box, a plastic round Nail on, and NM Cable in between?
The metal box
What is a P-ring (plaster ring) used for?
To bring a recessed box flush with the finished wall surface.
What does “ganging” mean in electrical boxes?
Joining boxes together to form a larger box for multiple devices.
What is the difference between a “multigang” box and a “gangable” box?
A multigang box is pre-sized for multiple devices; a gangable box can be joined to others to create a custom-size box.
What do connectors and clamps do?
They secure cables/conduit to the box
Why is grounding not just a code requirement but a safety measure?
It ensures that any fault current travels safely to ground, preventing electric shock and fire.
What is the difference between a damp location and a wet location?
Damp = sometimes moist; Wet = often exposed to water or rain.