Feed a length of 14 3 type nm cable or 12 3 if youre connecting to 12 gauge wire between the two boxes. Enjoy your new light fixture dave.

How To Install A Three Way Switch To A Existing One Way Switch
How to add a 3 way switch to existing light. To control lights from 2 locations you need two 3 way switches. The 14 3 cable has three insulated conductors. Connect the wires to the new 3 way switches with ground screws using one of the two wiring diagrams fig. I have scabbed the neutral white from a near by outlet because there isnt a neutral white wire in the switch box. I have added an additional light sconce to a three way switch. The location for the lights and power source may vary but regardless of this the connections to the switches will always be the same.
Connect the hot wire from the source to the common terminal on one 3 way and the hot wire from the lights to the common on the other 3 way. If i can do it so can you. This is about a 15 min project plus the time it takes to chase the wire took me about 30 minutes to chase the wire. Dominick amorosso decemberj2012 110 volt electrical 11 comments 24039 views in this video i explain just how easy it is to add a three way switch to an existing single pole one switch lighting circuit. White black and red plus a bare ground wire. If you wish to add a light fixture it is best to extend the wire from the existing light fixture or connect the new fixture wiring at the switch box where the existing fixture is fed from.
Yet here i am adding a switch to an existing light. How to wire a three way switch to a existing single pole light switch circuit. Light fixtures and 3 way switch diagrams electrical question. This way you will be sure not to interfere with the wiring that is dedicated for the 3 way switches at each end of the stairs.