You’ve likely noticed a square or round box on your wall with electrical cables entering it. That’s a junction box, an essential part of your home’s electrical layout and safety. Here’s all you need to know about how they work.
What Is a Junction Box?
A junction box is an enclosed unit found on walls and outside buildings. These units, which are usually box-shaped but can also be round, contain the ends of electrical wire where they connect and split.
A junction box, also known as an electric box, is an invaluable part of your home’s electrical layout. This system helps organize wires safely. Fireproof connections and boxes confine sparks to prevent a fire from starting or spreading.
Purpose of a Junction Box
Junction boxes contain wiring and prevent accidental electrocution. They should meet specific building codes to ensure your home is safe for occupation.
You can also use an electrical junction box to protect wires from rising dampness and dust, which can create short circuits. Rats and cockroaches often chew on wires. The junction box prevents them from reaching the wired connections, where rodent damage can cause the most harm.
Junction boxes help ensure the building’s power supply can flow without interruption. They secure wires and prevent undue tension on the cables, which could cause a wire to pull from a connection.
Some junction boxes are waterproof, which helps reduce electric shock if there’s flooding or rain, as with outside boxes.
A series of safe junction boxes makes the building’s wiring plan much more accessible for changes and repairs. They are also ideal places to transition and split wires for several connections, like wall plugs.
Parts of a Junction Box
Several junction box types are on the market, but they aren’t all the same. The components differ according to the specific type’s use.
Most common square or round junction boxes are empty inside, with a sealing gland or steel clamp where each wire enters. The entry point is a cut-out circle called a knockout.
When wiring the box, electricians knock it out with a steel punch or flathead screwdriver. The size of this opening will determine the size of the gland to use in securing the electrical cables.
The space inside helps isolate the area where the joined wires will sit. Terminal blocks and joining plugs cover them, but using insulation tape isn’t advised. The junction box lid or cover closes the wires into the space with the junction box lid and several safety screws or bolts.
Metal junction boxes have an earthing grounding screw. This screw lets the electrician determine a safe path for electricity to take if there’s a short circuit.
Types of Junction Boxes
Junction boxes are made for indoor or outdoor use. An outdoor box is suitable for indoor use, but the opposite is not true because they aren’t weatherproof.
Plastic and steel are the popular materials for junction boxes. You can build steel versions into walls, but plastic ones should remain attached to the surface. They don’t conduct electricity, providing safety from electrocution and short circuits.
Steel boxes best suit heavy-duty situations in factories or homes with higher-voltage connections. Their benefit lies in their heat resistance, making them less likely to melt when exposed to high temperatures, such as during a short circuit.
Junction boxes have a fire protection rating. Some are best for electrical cables running under the floor, usually at factories.
Different sizes and shapes of boxes fit specific uses. Round models are ideal for connecting lighting cables. In contrast, ground-level applications and power connection points in the home or office often favor square units.
Boxes have several connection options and can have two, three or four connection points per box. More than four increases the risk of wires shorting out. Wiring in more than one box inline prevents running too many wires through a single unit.
How to Install a Junction Box
A junction box provides an insulated space where the wires enter the box from opposing sides. The wire sometimes passes through sealing glands first. These rubber seals have edges that “seat” them on the circular opening in the junction box frame for the wires.
Planning and deciding where junction boxes should go is best done before cutting wires and cables to size. As the adage goes, “measure twice, cut once.” First, plan the junction box placements, then measure and cut to avoid costly wastage.
After deciding where to place the box, you should attach the box frame to the wall or floor with the provided screws. Large boxes may need anchor bolts to ensure heavy cables don’t pull them from the wall.
Use a wire stripper to safely and easily remove electrical insulation from wires. Twist the exposed ends together and seal them in a wire cap, connector block or insulation wire — depending on the wiring code in your state.
Press the junction box cover into place to keep the unit safe and sealed. Some have safety screws or bolts that further prevent tampering. Others have snap fasteners, making the boxes convenient for inspection.
Choosing the Right Size Junction Box
If you’re wondering how to choose a junction box, here are a few guidelines to help you:
- Consult your electrical dealer: The dealer where you purchase your electrical components is usually up to date on the latest electrical code for your state.
- Understand box placement: Use a weatherproof box outside with the correct ingress protection rating. Boxes mounted inside walls need a steel frame, while those on walls or floors can be plastic.
- Know the number of cables entering the box: This will determine how many access points it needs. No more than one electrical cable should enter the box per access point to avoid overheating.
- Consider the size of the cables: Cables of the same size should enter and leave a junction box. The joined sections inside should fit without touching each other or the sides of the unit.
Junction Box Regulations
Most state regulations determine that junction boxes should be in easy-to-access areas of the home. If located in your attic, you should ensure enough vertical space for an average person to stand up.
While junction boxes work well for electrical connections, they are not suited to electronic equipment like network switches or breakers. It’s best to use a NEMA-rated box, which rates the degree of protection against corrosion, gasket aging and construction practices for outdoor use. NEMA-coded boxes have complete ingress protection against water, wind and sleet. They are best for electronic switches and the accompanying wiring.
Electrical contractors should know the National Electrical Code (NEC) for their state and municipality. The code determines where you can place junction boxes and what may be nearby. The NEC is updated every three years, so staying up to date with the policies and correct specifics of the legislation is vital. This ensures your home remains safe and covered by insurance if you need to file a claim for electrical damage or fire.
Junction boxes should be visible and placed in accessible areas that are easily accessible in an emergency. You can’t install junction boxes near gas lines, switches or appliances.
While copper wire has excellent conductivity, it can produce significant heat, especially in the case of an overload. For this reason, use the correct graded junction box to prevent cables from melting the box.
The NEC also regulates using the correct thickness wire for your purposes. A wire that’s too thin will soften, often pulling from electrical connections and causing fires. The thickness has to match the electrical load it carries.
Junction Box Safety and Inspection
Once the initial wiring is in place, check for loose connections, heat or smoke. Secure all connections and junction boxes before reconnecting the main power. Ensure boxes are well-ventilated to avoid overheating. Avoid placing loose fabric, such as drapes or curtains, over them.
Note any discoloration of visible junction boxes, indicating electrical short circuits. Never work on them when the main power is still connected.
The Last Spark
Junction boxes are vital to home security and safety. They insulate electrical connections to prevent short circuits. Ensure your wire type matches the correct junction box to avoid overheating or contact faults.
When installed properly, junction boxes make electrical use much safer. They make it easier to implement upgrades to the electrical wiring plan.