I have an overhead utility room light that is controlled by two 3-way switches. I have added a light over the washer and dryer that I'd like to control from a single pole switch. I could only add the single pole switch to the non directly powered 3 way switch box. I am okay with the light only coming on with the one 3 way switch being powered on. How do I do this? The only thing I have found is how to make it power up when the overhead light is turned off by the 3 way I want to feed it.
That's not so clever. The problem is if the washer light is ON, and the utility room light is OFF, then flipping the farther 3-way will turn the washer light OFF and the utility room light ON, necessitating a trip to the other 3-way to put it right.
You need a major reassignment of wire functions in the 3-way circuit.
- Ground is ground
- White is neutral, always
- Black is always-hot, always
- Red is a digital control line (or it can be a relay actuation signal line)
The 3-way box that feeds the lamp gets the "master" smart switch (or a 3-way and the relay). The "remote" smart switch goes in the other 3-way box. Some smart switches use a data-communication wire, that's what red is for. Others use radio or powerline-induction signaling, in that case red is unused.
Now there's always-hot and neutral in every box regardless of the topology of the 3-way system. Now you can add the second 1-way switch in the normal fashion for adding a 1-way.
I think this is what you want. Definitely weird and maybe not to code. Basically you want to splice the new supply to one of the travelers. In the diagram below the light will have power only when the supply side 3 way switch is in the up position.
As a side note, it would cool if someone made a 3 way switch that had a constant hot connection(for the load side switch). This should be possible using relays. It would probably be cost prohibitive but might be useful in certain circumstances.