Can a low voltage transformer for a doorbell sit loose (not screwed to anything) inside the main panel or does it have to be affixed somehow? It came with holes but I'd need to drill holes in the panel to attach it, unless I can just wire it and sit it on the bottom of the panel.
You won't want to install the transformer inside the panel. Instead you'll install it on the panel.
For safety's sake you should mount the bell transformer in a manner that the low voltage wiring does not come into the same enclosure as the mains wiring. There are available bell transformers that come already mounted on a electrical box cover. The transformer is on the outside with the low voltage leads exposed. The transformer primary leads project down through the plate so that they end up inside a box where then can be safely attached to the mains wiring.
So unless that transformer already has a threaded mount for an electrical box knockout then you should set that transformer aside and get one of these:
No. The NEC requires a distinct separation of low voltage wiring and high voltage wiring, so you cannot have any part of the low voltage wires or the transformer itself inside the electrical panel.
National Electrical Code 2014
725.136(A) General. Cables and conductors of Class 2 and Class 3 circuits shall not be placed in any cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, or similar fitting with conductors of electric light, power, Class 1, non-power-limited fire alarm circuits, and medium power network-powered broadband communications circuits unless permitted by 725.136(B) through (I).
If it's a proper doorbell transformer, then it will include safety features internally to protect it from overcurrent and overheat conditions, and it will include a fitting on the primary (household voltage) side that allows it to be properly installed into a knockout on an AC electrical box. It is permitted to use a knockout on the electrical panel itself for this purpose, as long as the transformer and all low voltage wiring are outside the electrical box or panel.
NEC says no. Going beyond this requires some common sense. Question: Your circuit breaker panel is flush mounted with the sheet rock then what? Answer: Do not bury the transformer behind sheetrock. Use a Junction box and ensure to mount it in a location which provides access , yes it will fail. Annotate on circuit breaker panel its location.
Electrician for 30+ years says some idiot will try to connect the low voltage 12/24 to the high 120/240
I think it shouldn't be inside the main panel at all. And it shouldn't be rattling around loose anywhere. At least bolt it to the outside of the panel.
See also this related question for advice on connecting it.
