I'm trying to do some under cabinet lighting. I found the 15 amp circuit that runs the kitchen and foyer lights. I added up all the wattage and it can handle more lights. I was thinking I could add another switch to the existing outlet to control the under cabinet lighting. I've been following this tutorial on The Family Handyman
But, my setup isn't exactly the same because I want to tap into an outlet that controls a kitchen light. This outlet also controls the garbage disposal (on a different circuit that also has the dishwasher). See my diagram.
Circuit A - holds the dishwasher and disposal (20 amp)
Circuit B - holds the kitchen light and is the circuit I want to tap into
Assume everything is grounded. I just didn't draw it.
So, my question is how would I add a new line into the box on circuit B? Just add a new line and connect the blacks before it goes into the switch?