using 30A cable to supply 5A lighting circuit: good idea?

HI,
I decommissioned my electric shower so there is nothing attached to that 30A circuit. I want to add a load of lights to the master bedroom/ensuite which will (at full load) overload the exsting 5A lighting circuit. I can't add a new circuit to the consumer unit, plus the cables exiting the consumer unit are chased in to the plaster which I don't want to rip out.
Is there a good reason not to connect my new lights to the currently-unused 30A cable? Should I change the 30A fusewire for 5A if I do connect them? IE, have 5A fusewire connecting to some 30A cable which connects to 5A cable which supplies approx 1kw max load of lighting.
Also if in the future I upgrade my fusewire consumer unit to one with MCBs will any decent sparky (i'm wouldn't install the new CU ) refuse to connect 30A cable to a 6A tripswitch?
Thanks a lot.
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How much load are you thinking of taking to run lights ? There is nothing stopping you from doing what you suggest, but you'd need to replace the existing fuse with one marked at the new rating. To leave the fuse marked as 30 amp, but to rewire it with a lower rated fuse wire, is not good practice.

When you find out by how much you'll need to run the new lights, then go for the nearest rating to it. Remember though, that fuse wire only comes in certain ratings, so you may find that the next one up is not really safe enough to do the job properly, and you might need to place fused spurs in line with you new lighting circuits for your own protection.

A split load consumer unit is now the ideal way of distributing circuits around the house. Power to sockets is taken from an RCD protected supply, and lighting is taken from a similar set up as your existing box.
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I can't see a problem with what you want to do. The fuse must be the first thing to go - so a 5a fuse with 30a cable is ok but a 30a fuse with 5a cable definitely isn't. To avoid any future confusion, a prominent label on your CU, stating that this is a 5a lighting circuit, wouldn't do any harm.
I have a similar situation - where a 45a cable was installed for an electric shower, but in the event, we fitted a power shower (using stored hot water) so the electrics only had to power the pump. We thus downrated the breaker in the CU to 15a, but kept the heavy cable.
Roger
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As long as you de-rate the protection (ie. change the fuse to 5A) then you're ok. You could run the whole lighting circuit in 6mm cable if it floats your boat - and I'd happily connect it to a 6A breaker (and YES I'd consider myself a decent sparky - my plumbing on the other hand...) Regards, Richard
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