I have a 2-gang box with a single pole switch and i took power from there to a dimmer next to it. Can i take power a second time from the original switch to install another switch?
Wiring – Taking power from a single pole switch
wiring
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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.
Can't be done, without installing more wires.
What you have is known as a "switch loop". An ungrounded (hot) conductor goes to the switch, and a switched ungrounded (hot) conductor comes back from the switch. There's no grounded (neutral), so no way to extend the circuit.
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Best Answer
You need to make sure the circuit is not overloaded. For residential there is no limit to the number of devices by the NEC so you can add, but your total load needs to be at 80% of the circuit to prevent overloading. The one limit you may run into is box fill where the number of wires devices is limited.