Wiring regs says that fixed appliances should be hard-wired rather than use a socket. But in your own home, if you're sure that your ring mains are correct, I don't see a problem using an RCD plug with an appropriate fuse rating (3A?).
Domestic wiring doesn't come in less than 1mm2 AFAIK. Thinner wiring may not fit screw-down terminals correctly.
Yup. Although if the circuit already has RCD protection at 30mA then there would be no advantage from another RCD in the plug.
Assuming this is a mains cable and not an ELV one, it may just be allowing for a substantial de-rating due to being in a location with a high ambient temperature.
It's quite normal to plug in a shower pump. Indeed a plug and socket may be used to provide the mandatory isolation which is required, providing it's near enough to be under the direct control of anyone working on the pump.
Another tip for a shower pump - stand it in a plastic tray. That way, if you do ever need to disconnect the plumbing, the tray will catch any spillage. Also, if it starts leaking, the tray will make that obvious, rather than it invisibly vanishing into the floor, but the tray sides should not be high enough to have the electrical part of the pump submersed if it fills to the brim (can cut a slot down to the right level if they are). Standing the tray on a piece of soft carpet may reduce vibration transmitted to the floor.
see a problem using an RCD plug with an appropriate fuse rating (3A?).
fit screw-down terminals correctly.
and may not clamp correctly in whatever gland/cable clamp the pump has.
Definitely + 1. I have a shower pump and a single rotor pump for DHW in the loft above the shower. I use plastic storage boxes rather than trays but with fitted overflows which drain to a pipe coming through the shower ceiling, so that leaks are revealed.
Presumably 230V supply? Probably from the wiring regs, where a 1mm conductor was the minimum required for the 17th edition, but from January 2012 Amd.1, it should now be 1.5mm for power circuits, which I would class your shower as, though there is a caveat that .75mm can be used if the cable is non-sheafed, or sheafed flexible.
So if you are using T+E, you will need 1.5mm minimum, but flex can be .75mm.
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