I am trying to install some sub cabinet (low voltage) lights in my kitchen. I want them on the same switch as a regular ceiling (120V) light. The switch for this light is conveniently close to the cabinet and I can fish a wire into there from above to feed my transformer.
The switch is in a double box with a duplex which is on a different circuit to the ceiling light. I have to switch off two basement breakers to make the whole double box dead. Two questions (1) is there any code reason why I cannot put the live feed to my transformer through the same switch (in other words just take it from the bottom of the switch); and (2) does the white from the transformer have to go back to the ceiling light for a return or can I just grab another white from the box (duplex side) even though that is on another circuit?
NEC 300.3 All conductors of the same circuit, including grounding and bonding conductors, shall be contained in the same raceway, cable, or trench.
In other words, you cannot use a hot conductor from one circuit and a neutral from another. If these two circuits are a MWBC (Multi Wire Branch Circuit) then it wouldn't be a problem. Check you panel and see if these two breakers run through the same cable as they exit the panel, one breaker should have a black wire connected to it and the other should have a red wire.