Click image to enlarge.
Q I’m adding a new receptacle in my family room. Can I tie in to nearby switch box wires?
A Sometimes. First, check to see if the box contains a circuit you can tie into. Turn off the power to the switch box, remove the switch plate and unscrew the switch so you can examine the wires. If there’s a pair of neutral (white) wires that aren’t connected to the switch, you’re in luck (see photo). You can then add the new black wire to the black, the white to the white, and the ground to the ground wires. However, keep in mind that the switch box might contain other wires or have improper connections. If you’re not sure, look elsewhere or call a licensed electrician to advise you.
Second, do a wire count to make sure the box is big enough for the three new wires (see chart). The box size (in cubic inches) is often stamped on the inside of the back. If it’s not, measure the width, height and depth of the box and multiply to calculate its volume. If the box isn’t large enough, replace it with a larger one or look for another place to tie in.
How to calculate box size:
Count the wires coming into the box shown in the photo.
| 1 red wire |
1 |
| 2 black wires |
2 |
| 2 white wires |
2 |
| All ground wires (count as 1) |
1 |
| 1 switch (counts as 2) |
2 |
| Total |
8 |
Add cable for the new receptacle:
| 1 new black wire |
1 |
| 1 new white wire |
1 |
| 1 new ground wire (combined with other ground wires) |
0 |
| Total |
2 |
8+2=10 Total wire count with new cable
For 14-gauge wires, multiply by 2 cu. in. (For 12-gauge, multiply by 2.25.)
Minimum box size needed: 10 x 2 = 20 cu. in.
