I've seen doorbell transformers mounted on the outside of main service panels. I don't recall how the 120V was connected, but IIRC, it just goes via simply insulated wires (vs. cable) to the outboard transformer that is mechanically mounted via "feet" in a bracket. It that correct and allowable? It seems somewhat suspect since there is not cable, but individual conductors with very simple insulation (not much mechanical protection).

I see instructions for mounting HVAC control transformers in a special box cover that encloses the high side connections leaving the output, low voltage side free and visible. These instructions also include mooting like in the previous paragraph, apparently, but I find them somewhat ambiguous.

Can someone explain to me, or point me to an explanation about how this transformer mounting is supposed to be done? By way of explanation, this is not a home-wiring question. I want to put a low voltage transformer on or in an existing (purpose-built) junction box on a commercial appliance chassis, but the same safety principles should apply.