I would like to install an external socket. Conveniently located on the other side of the wall where I'd like to put said socket is the combination boiler which has a dedicated supply from the consumer unit. The boiler itself heats water with gas, using electricity only to power the controls and ignition.

Is it a) safe and b) legal in the UK to turn this supply into a ring circuit with the boiler and socket, where the socket would be protected with an inline RCD? If not, is there a safe and legal way to do this?

  • ...the boiler which has a dedicated home-run from the service panel. Is it safe and legal in UK to convert this circuit into a general-use multiple outlet circuit, with a GFCI outlet protecting the outdoor outlet? ................... Translated that into NEC for you... Apr 15, 2016 at 20:28
  • So here's the operative question. Does the boiler make heat purely with electricity? (in that case, no, probably can't do it). Or is it a fuel-powered machine that uses electricity for minor incidental tasks like pump, blowers, controls and the like? Is the boiler's current draw in the normal range of things that plug into receptacles and share ring circuits? Is it common for this type of boiler to have a plug that goes into a common socket? Apr 15, 2016 at 20:33
  • @Harper thanks for the comments, have updated the question to reflect the fact it's a gas boiler.
    – SLD
    Apr 16, 2016 at 13:21

2 Answers 2


There is no rule in the UK preventing you from putting central heating and sockets on the same circuit (which may or may not be a ring). In my experiance a dedicated circuit for central heating is the exception not the rule.

I'm not sure why you would want to make it into a ring, making it into a ring would mean adding an extra cable which would seem to defeat the whole point of this excercise. So it sounds like what you really want to do is create a radial circuit.

The main question that needs to be answered is what size is the cable and how is it installed. Given that what is it's current rating (current rating of cables depends on installation method). This will determine whether you can design a workable combined circuit using the existing cable. Generally for a socket you want the breaker rating to be at least 16A. The breaker current rating should be less than or equal to the current rating of the cable. Volt drop and disconnect times may also be an issue on very long circuits but are unlikely to be a problem in a typical household situation.

If there isn't one already you should install a switched fused connection unit with a 3A fuse to provide overcurrent protection and isolation for the heating system from the shared circuit.

Any new concealed cabling either needs to be 30ma RCD protected or wired using methods that are considered acceptable for non-RCD cabling.

The socket outlets need to be 30ma RCD protected.

If the CU is very new the circuit may already be 30ma RCD protected but on an older install it likely wont be. There are several ways of providing RCD protection.

  1. You can replace the MCB with a RCBO which combines the functions of a MCB and a RCD.
  2. In a split-load configuration if there is space available in the CU you can move the MCB from the non-RCD side to the rcd side (remember to also move the neutral to the corresponding neutral bar).
  3. You can buy sockets with RCD protection built-in.
  4. You can buy fused connection units with RCD protection built in.
  5. You can fit a din-rail mount RCD in a suitable enclosure (normally you would only do this for higher-current circuits).

The RCDs i've seen sold as "inline" RCDs seem to be designed for portable use with flexible cables.

Obviously an RCD only protects stuff downstream of it (see comment above about concealed cabling).


It is ok to change it into a ring and add a spur or socket onto this ring. There is nothing in the regs forbidding it however if your outdoor circuit develops a fault your going to loose your heating. It's also advisable to put it onto an RCBO as opposed to RCD

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