I currently have a 3-way circuit in the family room. I would like to install a 2 gang box @ one of the switch locations and add a single pole switch to control an exterior light. Can I wire the single pole off the 3-way switch, or do I need a separate power source?
It entirely depends on how the existing switch is wired. You need a constant hot and a neutral wire. It would be helpful if you can check what wires are in each location - what colors, and how many actual cables are coming in. You'll need to turn off the power at the breaker panel and physically remove the switches (don't disconnect them, just pull them out), as well as the light fixture. Compare to the diagrams below (or at the original source), or ask here again and we'll try and help you out (labelled pictures == very helpful).
There are several ways to wire 3-way switches, but what you'd be concerned with is the type where power comes to the switch first:
In the above diagram ,you can add another switch to the one on the left - you'd connect to the white and black wires coming from the "power source" line.
On the switch on the right, you have a neutral, but you don't have constant hot -- hot is switched, on either the red or black, depending on how the first switch is flipped.
Another common way the switches are wired is the power goes to the fixture first.
In this case, there is no neutral at the switches. (Also note, the black tape on the white wires indicates this. White is, by code, always neutral, and has to be marked if it's used for a switched circuit. That said -- keep in mind that not everyone follows code.)
If you're missing the hot/neutral, the only option you have is to run a totally new wire, or possibly (depending on how it's wired), you can convert the 3-way to a single switch, and then re-purpose the wires to supply constant hot and neutral to the new outside switch (effectively, one of your 3 way switches would get converted to control the outside light instead). This still may not be possible, and definitely isn't a beginner task - you need to understand electrical fairly well, and map out everything involved with this circuit.
I assume you are asking about adding it at the far end of the circuit. Not where your original power source is.
You can add a small 120v (coil) SPDT relay to create an always hot wire. It would draw a small amount of power when it's in the close position but save you the need to run another wire in the wall to the switch.
The con: it's going to make a click sound every time either of the 3 way switch is flicked.
Please check your local code before doing this.
In a circuit where there are two or more switches that operate lights, there is always two 3 way switches. Any additional switch(s) is a 4 way switch. One 3 way switch receives the power for the system of switches. The second 3 way switch is the last switch in the series of switches, this 3 way switch sends the power to the light or lights. If the single pole switch is installed next to the 3 way switch that receives the power from the electrical panel/circuit breaker, then installation of the single pole switch totally possible.
How to determine which 3 way switch is next to the single pole switch you wish to install? The three way switches have four screws, one for the ground (this is obvious as it is connected to the bare wire), two screws on one end and the forth sole screw on the opposite end. The sole screw is where the power is connected or sends the power to the lights. (If there are additional switches controlling the light(s), those additional switches are all 4 way switches with 5 screw, one for ground and four that transfer power.) Turn on the lights, then find the circuit breaker that turns off the lights. With the appropriate breaker off (lights do not illuminate), remove the two screws for one of the three way switches and pull the switch out of the electrical box to expose the wire screws being sure that the screws are not in contact with anything. Turn the circuit breaker back on. Be careful since the power is on and shock hazard now exists. Using an electric meter, place the red lead on the ground and the black on the "single" screw being careful that the lead does not touch anything else. With the lights on the meter will read 120V, with the lights off, if the meter still reads 120 volts, then this is the power source, if the meter reads zero volts when the lights are off it is is the power feed to the lights. Turn the breaker back off, verify the lights do not come on when the switches are operated.
If you have determined that the 3 way switch indeed receives the power, then (with the power off) remove the black wire from the power screw. Create two black wire pig tails and using a wire nut connect the two pig tails to a power wire. Reconnect one of the pig tails to the 3 way switch and the second pigtail to the single pole switch. White wires are connected together - all white wires in the box can be bundled/connected together.
Obviously the 3 way switch will need to be totally de-wired and removed to install the two position box. Remember that the wire that has a black, white and ground is the power wire. The wire that has the black, red, white and ground, connects to the two screws on the two screw end of the 3 way switch. Reconnect the red and black from the R/B/W/G wire to the two screws. It doesn't matter which screw the red and black wire are connected as long as they are connected to the two screws. I recommend watching online videos for wiring a 3 way switch and 4 way switch systems.
If you look at the new switch and new lamp as a unit, it needs to be fed by always-hot, and neutral.
With 3-way switching, there is usually one switch that is fed by the incoming supply power, and another switch which is a "remote". In some installations, both are "remotes".
If the switch in question is at the supply, this is a simple matter, because the supply includes always-hot and neutral.
If the switch in question is a "remote", simple answer: can't do it. Complicated answer: you'll need to redesign the 3-way circuit using smart switches.
In the smart-switch option, you seek out smart-switches that handle 3-ways with either radio, powerline signaling, or 1 single communication wire. Then you reallocate the 3 wires between the two 3-ways as follows:
- Black becomes always-hot
- White becomes actual neutral
- Red becomes reserved for the "signal line" if the smart switches need it.
Now you have always-hot and neutral at the remote location, and can add the new switch/lamp in the normal fashion.
The smart-switch requirement means the smart switch will require an entire switch space all to itself. If you were hoping to use a "double switch" that takes one space, that will not work and you'll need to enlarge the box.