I had a new 100 amp panel installed in my house 10 years ago. There is one plug in the kitchen wired (14/2) to a 120V, tandem 2 x 15 amp breaker (using only one pole). What type of wiring do I use to wire more plugs and use this breaker properly? Most topics are concerning tandem 240V breakers using 12/3 wire. Thank you!
With 14 2 with ground you are limited to a 15 amp breakers as 3phase has stated in earlier post this has been code for many years in the U.S. that's why most outlets in kitchens deal with 12 AWG. If you have 14 awg 15 amp breakers are correct and by putting in a double stuff or tandem you can run a new line to add more outlets or split the existing run to have multiple breakers with the existing runs so 2 appliances or devices won't trip a 15 amp when the next outlet over if fed by the second breaker. Never combine the 2 tandem breakers on the same circuit as this would be a code violation and risk a fire hazard.
Rather than worrying about what to do about a 15A tandem breaker. Why not just purchase 2 single pole breakers, one being a 15A to protect the the circuit you have and the other can be a 15 or 20 amp you can use for expansion? Now you are connected up in a more normal procedural manner and there are plenty of topics out there on circuits of that nature.