I have a kitchen cupboard that used to house an immersion heater....there is therefore a fairly heavy twin and earth terminating in what was once the switch for the immersion heater, on its own circuit with currently a 16A RCD at the consumer unit.

Along the line the switch has been replaced by a fused switch supplying a cupboard light and a spur off to a socket for the fridge.

Firstly, how legal or otherwise is that set up , secondly, if it's safe ( What ever happens I'm gong to replace the RCD at the CU with something of a much lower rating...6A? ) can I wire another socket out of that switch unit assuming the RCD is downrated accordingly?

  • I am on the other side of the pond but as long as the fuse was sized properly the larger conductors should not matter.
    – Ed Beal
    Mar 12, 2020 at 18:55
  • Yes, don't size down your breaker further than the smaller branch circuit wiring calls for. Leave as much current overhead as you can.
    – isherwood
    Mar 12, 2020 at 19:02
  • 1
    Yes, it's fairly axiomatic that there's no penalty for using wires larger than are required. However, my geolocating superpower tells me you're in Europe. Isn't "16A" a standard size for common receptacle circuits? Why not just proceed forward with sockets and extension wiring appropriate for 16A? Why nerf yourself down to 6A for no reason? Especially with an expensive RCD branch circuit breaker which - well, ours are $40 and you have to pay VAT too. Mar 12, 2020 at 22:21
  • Where are you on this planet, and what amperage are normal receptacles rated for where you're at? Mar 12, 2020 at 23:12
  • Blimey ...sorry ...never thought about the 'global' nature of the forum. Yes, I'm in the UK where, as far as I can work out, there is even legislation covering the colour smell and gender of plugs and sockets. I kind of assumed that lighting circuits had to be seperate....the lighting spur is fused from the switch box at 5A ( it's just one 5W flourescent ) Mar 13, 2020 at 9:56

1 Answer 1


16 amp MCB (not RCD) supplying a socket - OK fused connection unit with 3 amp fuse to supply light - OK

However, there are two important issues:

  1. The cable size. A standard radial socket circuit uses 2.5mm (minimum) cable on a 20A MCB. Sometimes (say pre 1980) immersion heaters were wired assuming a 2kW heater on 1.5mm cable. Before extending that circuit, you'd have to verify by calculation that 1.5mm cable is adequate for a 16A circuit. As a general purpose socket circuit the design current for the circuit would be the same as the protective device (16A)

  2. RCD protection. All new sockets must be RCD protected; all new cabling must be RCD protected (unless metal-sheathed cable or in metal conduit).

If the existing circuit is not RCD protected, you should add RCD protection. If you can't add an RCD to the existing consumer unit, there are ways round it (such as an RCD in a DIN rail enclosure next to the consumer unit, or an RCD fused spur, but if your consumer unit is that old it's probably due for replacement.

Don't reduce circuit protection down to 6A at the MCB - there is no need, and fridges/freezers can have high motor inrush currents.

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