I have a switch box with two black wires only in the box. I wanted to add another switch box next to it for a new light. How do you connect thsi? Thanks!

 

You left out a lot of details. It's easy to add a switch. It's harder if you want that switch to actually control something. For simplicity, I'll make all of the following assumptions. Please give me the correct information:

• You want this new switch to control a new light.
• The existing switch controls an existing light.
• You live in a city where NM cable is allowed (perhaps not a good assumption given that you have only two black wires in your switch box).
• You plan on replacing the simplex switch with a duplex switch in the same box.
• There are no white wires in the back of the box.
• There are no grounding wires in the back of the box.

You need to find a source of power for your new light. The wires in the switch box are not such a source of power. One possibility is to get power from the light the existing switch controls, running a new cable from this existing light to the new light, and another new cable from the new light to the switch box. Another possibility is to replace the wiring from the existing light to the existing switch box. Another possibility is to find an independent source of power (e.g., an unswitched receptacle).

Post back with more information and we'll try to narrow in on the solution that is best for you.