Correct cable size?

[roundthebend]

Further to this post: http://www.diynot.com/diy/threads/adding-a-plug-socket-round-a-corner.467371/

I still intend to DIY extend the ring circuit and add 2 double sockets side by side behind my TV. My plan is to run the cable horizontally from an existing socket. The length is about 50cm from the existing socket to the corner of the room (external wall), then 250cm from the corner to the new socket position (internal stud wall). The house is <5 years old and has RCD protection on the consumer unit. I expect both walls will have Celotex type insulation behind the plasterboard so I am prepared to remove an entire channel of plasterboard and Celotex, and remove notches in the studs to pass the cable through. As such, the cable will be just behind the plasterboard. I believe this is OK with RCD protection and observation of the safe zones, but I may add conduit if advice here is to do so.

I believe I'll be using Twin and Earth cable.

What I need to do next is work out the correct diameter of cable. For that length of run, that fixing technique and the environment that the cable will be in. And also, for the use of 2 double sockets (possibly with a 4 gang extension lead) running an LCD TV, PS4, Wireless Router, Soundbar, Humax Freeview box.

I'm not sure of the power consumption of all that, but a similar TV seems to be about 85W so must be no more than 1kW total. One website calculator suggested that 1mm sq cable would be sufficient, but I wanted to get more informed advice before continuing.

Thanks

 

[Iggifer]

Well, if as you suggest, you're 'extending the ring' then you have no choice to do it in 2.5mm² (assuming you actually have a ring to extend).

If you're not extending and just spurring, by adding an FCU then you could use 1mm², I would stick with 1.5mm² personally, but 1mm² would technically be sufficient. I'd try and get the cable in front of the celotex and touching the plasterboard, as this gives it better thermal displacement.

 

[JohnW2]

If you're not extending and just spurring, by adding an FCU then you could use 1mm², I would stick with 1.5mm² personally, but 1mm² would technically be sufficient.

Not as far as BS7671 is concerned, it wouldn't - unless you could find a way of defining it as a 'lighting circuit'!

Kind Regards, John

 

[Iggifer]

Not as far as BS7671 is concerned, it wouldn't - unless you could find a way of defining it as a 'lighting circuit'!

Kind Regards, John

True, it would be against that table. But in the real world, 1mm is good for 13A even 'in an insulated stud wall, touching the plasterboard'

One of the regs I have an issue with that one, I'm not saying it's wrong, per se. Just another one that could be worded much better.

 

[JohnW2]

True, it would be against that table.

Indeed - a.k.a "non-compliant with the regulations" - which I think is something that probably should be pointed out in a DIY forum

But in the real world, 1mm is good for 13A even 'in an insulated stud wall, touching the plasterboard'

Indeed, electrically speaking, that's true.

One of the regs I have an issue with that one, I'm not saying it's wrong, per se. Just another one that could be worded much better.

I think the issue actually goes beyond just the wording - it would be nice to know what is the rationale behind having a blanket 'minimum requirement' of 1.5mm² for "power circuits" (whatever they may be!).

Kind Regards, John

 

[roundthebend]

Well, if as you suggest, you're 'extending the ring' then you have no choice to do it in 2.5mm² (assuming you actually have a ring to extend).

Ah, it seems what you're saying is that when I am "extending a ring" then I should use the same diameter of cable as is already in use - which would probably be 2.5mm² for the majority of domestic socket circuits?

I wasn't aware of that, so I'm glad I checked. I don't have a particular problem with that, I suspect it's a bit harder to work with and possibly a touch more expensive.

Your reply also has me thinking about whether I do have a ring, or something else? I assumed a ring because I vaguely recall reading that it was the preferred installation for normal domestic socket circuits. But I'm willing to be corrected.

And on further reading, I'm wondering if "extending a ring" is something that can only be done from either end. What I'm suggesting sounds more like I'm "taking a spur". Are there different regs for me to consider in that case?

 

[EFLImpudence]

which would probably be 2.5mm² for the majority of domestic socket circuits?

It would. 2.5mm² is the minimum allowed for ring circuits.
If you use larger cable then configuring the circuit as a ring is not necessary.

I vaguely recall reading that it was the preferred installation for normal domestic socket circuits. But I'm willing to be corrected.

For historical reasons which no longer apply, the ring circuit was devised.

And on further reading, I'm wondering if "extending a ring" is something that can only be done from either end.

No, you can add to it anywhere - i.e. make the ring longer.

What I'm suggesting sounds more like I'm "taking a spur". Are there different regs for me to consider in that case?

A spur is a single cable branching off the ring.
There are different regulations in the sense that the ring regulation does not apply and others may apply.

 

[roundthebend]

It would. 2.5mm² is the minimum allowed for ring circuits.
If you use larger cable then configuring the circuit as a ring is not necessary.


For historical reasons which no longer apply, the ring circuit was devised.


No, you can add to it anywhere - i.e. make the ring longer.


A spur is a single cable branching off the ring.
There are different regulations in the sense that the ring regulation does not apply and others may apply.

Thanks for the extra info.
I'm making some sense of this but I'm pretty sure what I was thinking of doing was actually a spur - that is, run a single cable from an existing socket to the new socket(s).
In order to extend the ring I would need to take the feed to the existing socket and instead extend it to the new socket(s). Then run a cable side-by-side with that back to the original socket.

So, if my circuit currently looks like this:

-----[]-------[]-------[]-------[]-------[]-------

Then I need to do this:

-----[]--| |--[]-------[]-------[]-------[]-------
         | |
         | |
         | |
         []-

Dotted lines (horizontal & vertical) are a length of T+E. So the new part of the circuit has 2 lengths, one to feed the new socket(s) and one to feed into the existing socket.

 

[EFLImpudence]

Thanks for the extra info.
I'm making some sense of this but I'm pretty sure what I was thinking of doing was actually a spur - that is, run a single cable from an existing socket to the new socket(s).

Yes, that's a spur.

In order to extend the ring I would need to take the feed to the existing socket and instead extend it to the new socket(s). Then run a cable side-by-side with that back to the original socket.

You would - pointless for one socket.
However, if you were to have more than one socket, then both cables would be necessary for the possible extra current.

Dotted lines (horizontal & vertical) are a length of T+E. So the new part of the circuit has 2 lengths, one to feed the new socket(s) and one to feed into the existing socket.

If you mean both cables connected to the socket then that is not extending the ring -
one cable would be connected to the socket with ONE of the existing cables and the other new piece of cable ONLY connected to the other existing cable.

However, as I said, for one extra socket - pointless.

 

[roundthebend]

However, as I said, for one extra socket - pointless.

I was hoping to install 2 double sockets - because it's to supply a TV unit which will have TV, Sky box, PS4, WiFi Router, Soundbar, Xbox and maybe a couple of other things. I was hoping to minimise the use of multigang extensions leads.

So, to do a spur the single new cable would be twisted into the existing socket? Creating a parallel circuit, rather than series.
Am I going to run into problems with current with the spur approach? I believe a spur is limited to 13A?

 

[roundthebend]

To extend the ring in my room, I don't believe I'll be able to do it without using junction boxes of some sort. This is because the wall I'm extending onto doesn't currently have any sockets so I'd have to run cable back to the new socket position, and then all the way past the existing socket to the next one in the ring. That will be a huge effort for a DIYer, especially since it will then need to be routed around a french door so I guess up to the ceiling and back down again.

There's a slim chance that the cable going to the existing socket (first or last on the ring) already runs where I need to position my socket. In which case I could just break into it and not even need to run new cable. I'm rarely that lucky.

 

[EFLImpudence]

I was hoping to install 2 double sockets - because it's to supply a TV unit which will have TV, Sky box, PS4, WiFi Router, Soundbar, Xbox and maybe a couple of other things. I was hoping to minimise the use of multigang extensions leads.

Am I going to run into problems with current with the spur approach? I believe a spur is limited to 13A?

Then use 4mm² cable for a single spur.

This will be adequate to be protected by the 32A MCB.

Others may disagree with this approach but it is perfectly acceptable.

 

[roundthebend]

Then use 4mm² cable for a single spur.

This will be adequate to be protected by the 32A MCB.

Others may disagree with this approach but it is perfectly acceptable.

I'm eternally grateful for your help. That approach will satisfy my needs, and abilities. I'm just a little concerned that it might be a contentious approach.

 

[securespark]

Contentious as in you should not put more than one socket on a spur off a ring final?

If your design current is 1kW, this is approx one third of 13A. Why not take a spur off the ring final with a single 2.5 cable, put a 13A FCU in and run 2 sockets off that?

 

[JohnW2]

Contentious as in you should not put more than one socket on a spur off a ring final?

Indeed - since some people seem worried by the fact that the guidance in Appendix 15, not being 'exhaustive', does not include a 4mm² unfused spur!

The same people would probably happily run two or more separate spurs from points very close on the ring, so it is not 'point loading' that they are concerned about. I think it's more a matter of their being happier to be spoon-fed with 'guidance' than to have to think (with the actual regulations in mind) for themselves!

Kind Regards, John