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Since the wires for a Central Vac system are low voltage (less than 120 volts), any splices are not required to be in a junction box. However,
If you are splicing the wires, I would definitely keep the splices in accessible locations.
This is for future servicing purposes. If you ever have a splice come loose, you'll definitely want access to them, in order to make the necessary repair.
Sure. As long as you can remove the cabinet to get to them.
If you are open to it, you could make a removable bottom for the cabinet (just a portion of the bottom removable, to get to the splices).
You may never need to get to it. However, splices are known to be failure points for wires. I would try to avoid the frustration of trying to get to an inaccessible splice, if possible. In those cases, new wire is required, to bypass the failed splice.
No problem. It's not that the splices are required to be accessible. I'm just trying to prevent any possibility of future grief for you, if one of the splices should fail.