Can I use a 14-3 Romex for two different electrical circuits on a 15A QT/tandem breaker? Or should I use one 14-2 for each?
My mistake, apologize.
Post edited: the QT breaker is a space saver. My panel is 30 space 40 circuits. So I intend to have two different circuits on one space. I will use a 1 pole Siemens Q1515 with no clip. Here is the link Tandem breaker
So turning off individual circuit.
With two adjacent regular breakers, you can share the neutral, a multiwire branch circuit. Adjacent breakers are on different legs, so the neutral only carries the difference in current between the two hots due to a cancellation effect.
With a tandem breaker, both breakers are on the same leg / line, there is no cancellation effect on the neutral, so you can't share the neutral - you'd be doubling the load on the neutral conductor!
So yes you must use two 14/2 cables rather than a single 14/3.
No No No!
First, that trick with the 14/3 wire and shared neutrals, is actually one circuit. It is called a multi-wire branch circuit or MWBC.
MWBCs have a number of special rules. Top of the hit parade is you can't use a Tandem or double-stuff breaker!
The two hots must have 240V voltage between them (be on opposite poles). If so, the neutral wire will carry the difference of currents. (if one is 11A and the other is 13A, the neutral carries 2A). A double-stuff/duplex has both circuits on the same pole, and the neutral current would stack (24A) instead of cancel, overloading the neutral.
Common maintenance shutoff -- the handles must be tied; shutting off one subcircuit must also shut the other one off. Either by using a 2-pole 240V breaker (same as you would for a dryer) or a factory approved handle tie between two single breakers. They don't make handle ties for double-stuffs, so if you respect this rule, the above rule takes care of itself.
Must pigtail neutrals -- you can't have it where removing, say, a receptacle interrupts neutral for the other half-circuit.
These types of circuits have fallen out of favor because they make life very complicated when you bring GFCI or AFCI into the picture.
