Electric junction box

Yes, you currently HAVE to attach a ground wire with a green screw to the metal box, in case it becomes short. Plastic boxes do not become short.

In a crawl space or attic... Buy a small blue and a large (deeper) blue nail on boxes. A foot away from the tap attach the small, at the tap attch the large. You need a staple within 6 in. of each box on each cable. The small will create a splice point for an extension wire, the large will have room for the tap. (Boxes are sized by number of wires. small will have 5, large will have 7, code wise.) Grays cost a little more than blue, have a little piece of metal in screw hole. Get the blue.

Tims... I guess I'm not quite following you... Why do I need 2 boxes?? Thanks.

If you cut into an existing line, there is minimal slack. In order to get 6" slack in each box, cut the line, pull back 1 ft. apart, set 2 boxes, 1 ft apart, aha, 6 in. slack. Insert 2 ft. of wire, 1 ft apart, 6 in. slack to each. Believe me, I've done this hundreds of times.... Carry 2 boxes when you go under the house, and a cell phone, and a meter/wiggy of some sort.

The few times I have done this I have found enough slack, but two boxes would be necessary if the cable is tight between the two existing boxes.

You can argue the plastic/metal either way. Plastic insulates so it doesn't need to be grounded, which saves a couple minutes. However, steel can be grounded which could in some circumstances be safer. I use steel if I want a clamp right at the box; otherwise I use plastic. I used to use the blue plastic, but have switched to gray; for a couple cents more they just feel better. Any of the three is just fine most of the time.

TimS. So the "first box" is used to basically "extend" the original wire, giving it more slack as it goes into the second box where it is connected back up to the original wire and where the new wire is added to it... Right?

Exactly!!