A Guide To Installing Outdoor Electrical Junction Boxes 

An approved enclosure can be any standard outlet, switch box, or light fixture box. Still, where a wiring splice is required in other areas along the electrical circuit, the approved enclosure is typically a junction box.  

A junction box is simply an electrical box securely mounted to the house framing or another structure and contains the linkage (splice) of two or more circuit cables. To create an enclosure, secure the wires to the box with conduit connectors (if the circuit includes a conduit) or cable clamps if it doesn’t, and the box must possess a removable cover. Ensure the junction box covers are accessible; never conceal them behind drywall or other surface material. 

When a circuit branches off in two or even more directions from a location where an outlet or fixture is not practical, a junction box is most often used. It is also a popular approach when extending an electrical circuit. 

Factors To Consider When Selecting The Correct Junction Box 

Purchase the appropriate junction box for your installation. Ensure that your junction box fits the purpose for which you purchased it. Make sure that the box is big enough to house the number of wiring connections you’ll be making. Some boxes, for example, can comfortably house only two cables (four or five conducting wires), while larger boxes accommodate up to six cables (and up to 18 individual conducting wires). If you are installing the box yourself, choose the largest, most practical junction box for the job to ease installation. 

To better decide the box size (in cubic inches) you need, inspect the total wires entering the box. For example, if employing 14-gauge cables, multiply the total by 2; if using 12-gauge wires, multiply the sum by 2.25. Afterwards, take the biggest ground wire and add it to two if it’s a 14-gauge wire or 2.25 if it’s a 12-gauge wire. 

Choosing a box with conductor fill volume (CFV) is also an excellent option. The conductor fill volume of a junction box is the highest amount of conductors (wires) permitted in a junction box. It’s advisable to choose a box with a conductor fill volume greater than or equal to the number of cables entering the junction box (that is, after multiplying that number by two and including the ground wire). Filling a junction box with cables poses a fire hazard and is a dangerous risk. All cables should fit neatly inside the junction box, and when in doubt, go for the larger size. 

To prevent moisture intrusion in outdoor locations, a weatherproof outdoor electrical junction box with gaskets is necessary. 

Safety Precautions Before You Begin 

  • Wear safety equipment. Wear a pair of rubber gloves to reduce the possibility of an accident. Eye goggles can also help keep stray wire fragments out of your eyes. 
  • Switch off the central power source. Turn off the power supply to avoid accidents when working on a wire already connected to an electric circuit. Locate your home’s central electrical panel. Pull the lever on the main circuit breaker, or unscrew the fuse to turn off the power supply. 
  • Using a voltmeter, verify that the circuit has no current flowing. Check the wire to see if the current is still flowing to stay safe. To do this, employ a voltage tester by placing its probes on the wires. If the voltage reading does not remain at zero, then a current is still flowing through the wire, and it is dangerous to interfere with. Double-check the circuit breaker to completely turn off the power to ensure it’s properly switched off. 

What You Will Require 

  • Voltage tester with no contact 
  • Screwdriver 
  • Hammer (for a metal box) 
  • Tweezers (as needed for a metal box) 
  • Drill and screwdriver tips 
  • Strippers for wires (as needed) 
  • a code-compliant electrical box with a cover 
  • Clamps for cables (as needed for a metal box) 
  • Screws made of wood 
  • Connectors for wire 


Install the Box 

To create space for the new junction box, separate the circuit wires at the existing splice and loosen the cables. Secure the box to the framing or any other support structure available through screws driven through factory-made holes in the back or side of the box. 

Tip: If your box is metallic, remove the cutout on the box for each cable going into the box before doing anything else (even separating the circuit wires). Break out each knockout (metal disc) with a screwdriver or hammer, then twist off the metal knockout disk with pliers. 

Install Clamps for Each Cable 

Attach a cable clamp to each cable. Standard electrical boxes (plastic) lack knockouts and have internal cable clamps. Install a locknut-type clamp for each cable if your metal box lacks internal clamps. Insert the clamp’s threaded end through a knockout hole and secure it with the ring-shaped nut inside the box. Using pliers, tighten the nut. 

Secure The Wires Properly 

Feed the cables into the box through the clamps. Beyond the clamp, the cable’s outer jacket should extend a quarter to half an inch into the box, while the individual conducting cables should be extended by about six inches. 

Prune the wires as required and use wire strippers to remove a three-quarters inch of insulation from the ends of every wire. Secure the screws on the clamps to hold the cables in place, taking care not to overtighten and damage the wires. Plastic boxes typically have spring-tab clamps that do not need tightening. 

Tip: Individual wire ends should be clean, straight, and undamaged, with no more than 3/4 of exposed wire visible beyond the insulation. Using wire strippers, trim any damaged cables and strip insulation as needed. 

Connect the Dots 

Connect the wires using approved wire connectors, as directed by the manufacturer. Begin by connecting the bare copper (or green insulated) ground wires. If the box is made of metal, connect the ground wire connection with a pigtail—a 6-inch length of the same type of ground wire—then connect the loose end of the pigtail to the box’s ground screw. Special green wire nut connectors are typically used to connect the grounding wires. 

Connect the white (neutral) and black (hot) wires using a wire nut or other approved connector for each wire pair. If there are any red (hot) wires, connect them. By gently tugging on each wire, you can ensure that all wires are secure. 

Complete the Task 

Fold the wires into the box with care. Set the box cover in place and secure it with two screws. According to the code, the cover must be a solid “blank” with no holes. Switch on the circuit breaker to restore power to the circuit. 


Junction boxes protect electrical cables from damage and prevent shocks and sparks from igniting nearby flammable material. Make sure to follow safety precautions to ensure that your home is safely supplied with long-lasting electricity.