We have all seen lighting and power circuits doubled up but you ask if it is allowed!
First question is of course by whom? There are a whole host of organisations all which all impose their own rules so I will limit myself to BS 7671:2001 314.4 states quite simply “Where an installation comprises more than one final circuit, each final circuit shall be connected to a separate way in a distribution board.” Which seems to say you can’t do it but it continues “The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated.” And that leaves it all to interpretation if two circuits are combined are they then one circuit or two? You are allowed to add a spur at the consumer unit so is adding a second ring any different?
Personally I want an easy life and would not be prepared to argue it out.
This of course assumes everything else is OK as once you start to alter circuits at the consumer unit you become responsible for those circuits so a R1 + R2 for each circuit and inclusion of RCD protection may mean you have to change the consumer unit anyway.
My interpretation of regs 314; division of installation. I would assume that it would not be allowed, and arragements shown in the osg, have all final circuits on there own protective device and non are shared.
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