Ring Circuits - Fixed Electric Heating

If a dedicated "ring circuit" rated for a continuous 6kW load, suitably identified and fixed wiring i.e. no pluggable sockets; I see no objection, Suitably rated switches with neons should also be installed at the heaters for isolation.

Jaymack   

BS 7671 isn't particularly prescriptive about circuit layouts. Regulation 433.1.204 describes one common ring circuit arrangement (supplying BS 1363 accessories, which includes fused connection units as well as socket outlets) - but that is more of a  'do this and it is deemed to comply' to avoid the need for calculation in the common case. There's nothing to prohibit other arrangements at all, but a little more design effort is likely to be needed to show compliance with the basic requirements of BS 7671 (especially with regards to overload protection of the ring cables).

The arrangement you describe doesn't sound too far from the usual arranagement, but perhaps some care is needed to show that 'the circuit is unlikely for long periods to exceed the current carrying capacity of the cable'. The choice of 4mm² cable rather than the usual 2.5mm² may well provide reassurance on that score, but a lot depends on the installation method - it could have an ratiing of below 20A if installed within an insulating wall for example. Likewise the position of loads around the ring will affect how well the current divides between the two legs.

BTW 6kW feels like an awful lot of heat for a living room - many whole houses can be kept warm for a lot less than that.

     - Andy.

Is the heating all the ring will supply ?  25 amps of heaters could probably be a radial on 4mm though a ring will reduce the volt drop. It rather depends on the layout, run lengths  and what the cable has to route through and group with.

If  anything it may be a bit oversized, for example you cut it in half and had 2 radials each feeding 3kW of heaters, you'd probably think it was OK to use 2.5mm unless the routing/grouping was particularly onerous.

BS7671 has no issue with it, but you do need to design it , rather than wack it in without any thought. If there are other loads as well (you mention 13A sockets) then it is not so clever, as you have used up 24A of your 32A on the heaters alone, so not much left for ironing or the toaster..

Also, how big and draughty is this living room - 6kW is quite a lot for  one room - consider that a gas boiler for heating a whole semi-D house is only 24-36kW and does the hot taps as well.

M.

If the circuit is dedicated to the electric heaters and supplies no significant other loads then I would say that this is fine.

If the ring final circuit also supplies general purpose socket outlets but ONLY IN THE SAME ROOM then it is probably acceptable since significant other load is unlikely.

If the circuit also supplies other socket outlets in OTHER rooms, then this is not in my view acceptable, since very little capacity is available for heaters or other heavy loading appliances in these other areas.

Since there is only mention of heaters 4 x 1.5KW = 6KW and not additional sockets I would say this is OK and could be considered OK even with 2.5 T & E if design considerations allow.

It might not be our first choice but it could be one way of doing it

I am puzzled why you are asking.

If there is (only) 25A loading and a cable capable of 37A (assuming no derating) why is there a need for a ring?

There is no mention of the OPD rating but if it is not higher than the cable CCC then the special circumstances of 433.1.204 (which is still written for BS3036 fuses) have nothing to do with it.

Any conformant radial circuit could be extended back to the CU for no reason.

A standard 2.5 mm socket ring with both sockets and the heaters could be acceptable.

Why use a RFC?

what about the revolutionary idea of supplying 4 times 2.5mm² radial cables protected by 4 times B10A MCBs?

So far as I can see, we have not established what if anything else is connected to this ring final circuit. Nor do we know what size OCPD is used, though 32 amps is very likely in the absence of statement otherwise.

We therefore can not reliably comment on the suitability or otherwise of the arrangement, I therefore repeat my earlier comments that it depends.

If this is a 32 amp ring final dedicated to the heaters, without any other significant loads, then in my view it is fine. Circuit designed for 32 amps, actual load about 25 amps, no problem.

If this is a 32 amp ring final that also serves socket outlets in the same room, then it is probably acceptable since use of other heating appliances is unlikely without making the room uncomfortable warm and turning off some of the heaters. Brief or short term use of a kettle or a power tool, or a large vacuum cleaner  might produce a slight short term overload, but prolonged use would result in turning off some of the heaters.

If this is a 32 amp ring final that also supplies socket outlets in other rooms, then in my view the arrangement is not acceptable since only about 7 amps of capacity is available for the other rooms in which heaters or other high loading appliances might reasonably be used.

In the rather unlikely event that this is a 45 amp ring final that also supplies socket outlets in other rooms, then it is probably acceptable since about 20 amps remains remains for those other rooms, which is probably sufficient unless unusually heavy use is expected.

That covers most possibilities, but we do not know which of the above circumstances apply.

Legh Richardson said:

Why use a RFC?

what about the revolutionary idea of supplying 4 times 2.5mm² radial cables protected by 4 times B10A MCBs?

Add in a socket circuit and there’s around threes times the amount of cable actually required.