I need assistance with adding a switch to an existing light in my garage. The best way to explain what I have is:
The 12/2 line comes in from my left and connects to a light. But I want to add a switch 10 feet down from the light to my right. So I am thinking that I can keep the existing 12/2 line but add a junction with a new line leading to the new switch and then back to the existing light.

I am having trouble visualizing what wire to run and how to connect.
If I leave the existing black wire connected to the light and then disconnect the white, run a wire from that white to the new switch and then back to the light, I believe that would do it..............thoughts?
If so, is there a single wire available that is covered (protected) like a 12/1 ?

 

What you need is a switch loop. Using 12-2, you will repurpose the white wire on the loop to black, so you will technically have two black wires going to your switch. At the switch, connect the black and remarked white wire to the two screws, one each, of course. In the light fixture, you have a black and white from the fixture and your live black and white. Connect the whites together. Connect the remarked white to the hot wire in your 12-2. Then connect the black from the switch loop to the black to your fixture.

 

Grounds not shown. Connect per code.

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Currently adopted code in most jurisdictions requires a grounded conductor (neutral wire) in all switch boxes. Having that there allows some energy-saving devices such as motion detectors and timers to work when they otherwise wouldn't.

To do that, run 12-3/G cable between the light and the switch box. Protect the cable inside a conduit sleeve up to 8' above the floor if you're running Type NM-B, aka Romex.

That cable will have a black, red and white set of conductors plus a bare ground wire. At the light, connect the incoming black wire to the black wire going to the switch. Connect the red wire from the switch to the "hot" lead for the light. Add the white wire going to the switch to the incoming white wire. Use a short pigtail if you need to to connect the white wires to the neutral feed for the light. Ground everything.

At the switch, ground everything. Put a cap on the white wire if you don't need it. Connect the red and black wires to the terminal on the switch.

 

Grounds not shown. Connect per code.
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I was hoping you'd do that, Ray!

 

He is good with pictures, isn't he?? Worth a thousand words.

 

10-4 on that, good buddy.