Safe to re-use storage heater cable?

[ralphius]

Evenin' all,

So my house used to have storage heaters, they've been taken out and scrapped and the separate mini consumer unit for them taken out as well. This has left a switch that does nothing on the wall and a piece of cable (stranded twin+earth, 7 strands on the live, 11.5mm across width of outside insulation) that has been routed across the house under the floor and I can get at both ends of it.

I would like to convert this obsolete switch into a socket as it's in a convenient place and there's already a metal backbox in the wall. The 32A ring main for this floor has 6 double sockets on it and is wired with solid core twin+earth, 10mm across width of outside insulation. I would like to add this new socket into it between two other sockets so that I don't have to touch wires in the consumer unit.

As far as I can tell from the wiki (https://www.diynot.com/wiki/Electrics:cable_types:flatpvccables ) the storage heater wire is 4mm² so has a higher current capacity than the 2.5mm² but am I allowed to reuse it as part of the ring main? Would save me a lot of time taking up floorboards if I could reuse it in situ...

 

[oldbutnotdead]

Short answer-no. You have an RFC serving 6 sockets (so 2 x 2.5mm serving it from the CU) and a random bit of 4mm starting near the CU and ending on a wall somewhere. Can't see how you can integrate your 4mm into the ring without some work at the CU and some hacking of plaster at the other end

Medium answer- yes but you'll have to do things in the CU. You could in theory stuff one end of your 4mm in the same fuse/MCB that feeds your ring final and put a socket on the other end of it- that'll effectively be a spur from the RFC. In practice, using different sized cores in a common terminal is unwise and probably outside the type approval for your CU. To ensure compliance you'd need RCD protection on that circuit as well

You could (if you have the space in the CU) add a 20A fuse/MCB to your CU, stick your 4mm in that and stick a 13A socket on the other end BUT that'll be (a) notifiable and (b) again require RCD protection for the cable.

 

[ban-all-sheds]

So my house used to have storage heaters...

As a result of, or because of, what, did it used to have them?


Sounds like it would be a spur, rather than part of the ring.

How old is the cable? It would be odd for 4mm² to be used for one heater - might it be 7/.029?

How long is the cable? Is the existing socket circuit RCD protected?

 

[ban-all-sheds]

...some work at the CU ... you'll have to do things in the CU. You could in theory stuff one end of your 4mm in the same fuse/MCB that feeds your ring final and put a socket on the other end of it ... You could (if you have the space in the CU) add a 20A fuse/MCB to your CU ...

I would like to add this new socket into it between two other sockets so that I don't have to touch wires in the consumer unit.

 

[oldbutnotdead]

Yeah well I can't see any way (with the info available) of reusing that bit of cable without doing things in the CU. The OP doesn't want to mess in the CU- fine, abandon the plan or get someone in

 

[ban-all-sheds]

I assumed (yeah, I know, I know ) that if he thought he could do it without fiddling in the CU then the "supply" end of the cable must be near a socket, or the socket circuit cable...

 

[ralphius]

Sounds like I can't do it then, shame. For those wondering how I can avoid re-terminating things in the CU, a quick text diagram. (DS=Double Socket, CU=Consumer Unit)

   +--+         +---+
   |DS+---------+DS2|
   +--+         +---+
    |             |
    |             |
    |             |
    |             |
    |             |       +-+
    |             |       |S|
    |             |       +-+
    |             |        |
    |             |        |
   ++             |        |
   |              |        |
   |              |        |
 +--+           +---+      |
 |DS|           |DS1|      |
 +--+           +---+      |
   |              |        |
   +------------+ |  ------+
                | |
               +-----+
               |  CU |
               +-----+

The socket marked DS1 immediately above the consumer unit on my diagram is in my large understairs cupboard with the CU. My plan had been to take the DS1 end of the wire from DS2 to DS1 and stick it into S, and then re-use the wire from S and connect it into DS1 completing the ring circuit again like so:

  +--+         +--+
  |DS+---------+DS|
  +--+         +--+
   |             |
   |             |
   |             |
   |             |
   |             |       +-+
   |             +-------+S|
   |                     +-+
   |                      |
   |                      |
  ++                      |
  |                       |
  |                       |
+--+           +--+       |
|DS|           |DS+-------+
+--+           +--+
  |              |
  +------------+ |
               | |
              +-----+
              | CU  |
              +-----+

And in answer to the question from ban-all-sheds, yes it might be an old imperial cable (it has black and red inner sheath) I'll go and measure it more accurately tonight.

Also my remaining CU (the off-peak mini-CU was removed with the storage heaters) is split with 3 x 32A ring circuits on MCB's protected by a 30mA RCD.

Sounds like my best bet is to replace the old storage heater wire with a piece of new t+e 2.5mm² from 'S' to 'DS1' and take more floorboards up...

 

[EFLImpudence]

So my house used to have storage heaters, they've been taken out and scrapped and the separate mini consumer unit for them taken out as well.

That was silly.

 

[JohnW2]

Sounds like I can't do it then, shame. For those wondering how I can avoid re-terminating things in the CU, a quick text diagram. ....

Maybe I'm missing something but, provided that the cable is OK and appropriate, I don't see why you couldn't do what your diagrams suggest. Even simpler, you could have your new socket (on the end of the old heater cable) just as a spur off from 'DS1'.

The only practical issue I can think of has already been mentioned, and would apply to any re-use of the cable - namely that of terminating a solid-core and stranded core conductor ion the same terminal - but that is often done.

When you write "RCBO", do I take it that you mean RCD?

Kind Regards, John

 

[ralphius]

Sorry yes I did mean RCD, my mistake.

That was silly.

Why was it silly to have the 2nd CU removed? As far as I'm aware I couldn't have re-used the 5 radial circuits from the off-peak CU or left them live and connected?

 

[EFLImpudence]

Assuming everything is satisfactory, you could have fitted sockets instead of the heaters and connected the CU to the standard supply. Simple.

 

[ralphius]

Damn, yes that would have been simpler. Bit late now as a couple of the heater sockets have had the cable removed completely and I've reused the backbox for a cat5e network socket.

 

[John D v2.0]

Simple answer if I understand would be to get an RCD Fcu or standalone rcd, spur that onto the socket by the cu, connect your cable to that, then socket at the other end. Or even a plug in rcd with a flex into a fixed box with a connection block and strain relief on the flex if you want to go low budget.
My concern would be the condition of the wire, an electrician would likely IR test it first and may not use it on account of the age.

 

[ban-all-sheds]

Simple answer if I understand would be to get an RCD Fcu or standalone rcd...

Ralph - take no notice of this - you've already got an RCD for the socket circuit, you don't need to introduce another one to create a spur.

 

[Adam_151]

Evenin' all,
stranded twin+earth, 7 strands on the live, 11.5mm across

How many strands to the earth? I would suggest that 4mm² would be unusual, but 7/029 with a 3/036 earth would be possible