Well there is no regulation saying they can't, and water pipes aren't designed to leak so no reason why they shouldn't. Just employ some common sense such as not having valves or joints directly over the DB, and if it's cold water, then insulate the pipe so there is no risk of condensation forming on them and dripping down onto the DB.
I bet that there are many domestic installations which have water pipes above the CU. The thing is that there is a ceiling between them, so it isn't at all obvious.
522.3.1 says ' Wiring system shall be selected and erected so that no damage is caused by condensation or ingress of water during installation, use and maintenace.' I think that this question would be caught in the meshes of this regulation. Am I wrong?
That reg also could be considered to include leaking roofs, condensation in kitchens etc flood risks, and any number of water sources. Pipes, except perhaps very badly jointed ones, do not normally leak and a more of a solid hot or cold obstruction., The other consideration not yet mentioned is access - if the location of the pipe run means you cannot take the covers off the electrical kit easily, or there is insulation that gets in the way of somewhere you need to rest a ladder, then that is a problem. Equally if the pipe is a drain pipe with a rodding eye or perhaps a heating pipe with a drain point that may need to be opened and could release water, then it needs more thought - well something needs re-positioning !
