Shared Knowledge

"Electrical Wiring - Mechanically Wiring a A Junction Box"

5 February 2011




Junction box? Why, where? A junction box is very handy when you are adding a new outlet or switch and need to tie on to existing wiring or you are modifying wiring and simply need to extend an existing wire run.

Shown below is where I used a junction box to connect 4 outlets to power. By using a junction box, I could route one outlet line at a time.


Hints and tips:

Note: you can buy steel or plastic junction boxes. I do not use steel. If you nick a wire when working with a metal box, it is possible that nicked wire will short against the grounded metal case, if not immediately, then at some future time. A nicked wire in a metal box can be a bear to find. Also, sometimes, I have to work inside a junction box with the power "on" and with a metal box, there is always the possibility a wire will come in contact with the metal side and short to ground. If you must, for some reason, use a metal box, make sure to route all grounds in the box to the ground tab on the metal box.

Always use a very sharp knife or other tool to strip insulation off wires. No way quicker to cut yourself than using a dull knife.


Wiring a junction box:

(1) Prepare junction box. Shown below is a blue plastic box, which needs the cable tabs lifted, raised before inserting a cable or it can be quite difficult to get the cable to start into the junction box.


(2) Insert the cables into the box and leave plenty of cable inside the box. One of the most common mistakes made in doing electrical wiring is to leave too little wire inside the box. Note here the length of all cables run into the junction box is about the same. Also note that I like to staple down all cables going into the junction box. Finally, I use a protocol when I insert cables into the junction box: power always comes in the bottom and if power goes out of the box to else where, it goes out the bottom of the box.


(3) Strip sheath off of all cables inside the junction box. Note that the sheath does not provide anything of value inside the box, so it can be striped off all the way to where the cable comes into the box. When ever stripping cable sheath, be careful to not nick any cable wires. An unseen cable nick can cause a nasty short circuit and is dangerous to you or someone who follows you into the junction box. Finally, note how much cable is left inside the box after the sheath has been removed.



(4) Separate wires inside the junction box and get ground wires untangled and get them together. Once ground wires are together, user your hands to begin to twist them together. Again, a protocol I use but twist wires clockwise using pliers.


(5) With grounds twisted together good and tight, use wire cutters to trim end so it is even.


(6) Tuck ground wire bundle back into box out of the way and proceed to untangle the white wires and again, clockwise twist them together by hand. Note the length of the white wires in the box.


(7) Cut all white wires to same length. Again, do not cut much length from white wire bundle.


(8) Slightly separate white wires in bundle and strip about 1 inch of insulation off each wire. Once stripped, twist end of wires together by hand and then tightly using pliers. With all white wires twisted together, place wire cap on the end and twist clockwise. Again, another common error made in wiring is to use too small of a wiring cap. A wiring cap should screw on tightly and not come off with a reasonable amount of pulling and it should completely cover bare wires (If a large cap does not cover all bare wires of the white wire bundle, trim the end of the wire bundle). I find that large caps with thumb twists built into the cap serves me well in most all wiring situations. With the white wire bundle properly capped, push it back into the box keeping it away from the the ground wire bundle.


(9) Twist all black wires, minus the hot wire, (clockwise) together by hand and trim all wires to same length. In image blow, the hot wire is on the bottom left and power out black wire is on the bottom right. Although it can not be seen in this image, protocol has me make a small label (Hot) and wrap it around the incoming hot wire. Untwist the black wires at the end of the bundle and strip about 1 inch of insulation off each black wire and then twist them all together tightly and trim such that the end is even.


(10) Take black "hot" wire and align it with black wire bundle such that hot wire length is just about the same length as the bundle. Strip the insulation off the hot wire and then align it with the black wire bundle and wire cap all black wires together by twisting clockwise. Note: I do not twist hot wire with other wires so later, if needed, I can easily separate the hot wire.


(11) Once all done, place a plastic cover over the junction box. If you a mind to, add label(s) to the cover that define the purpose of the box.




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Ron - Shared Knowledge Home