Cable length for a ring main spur ?

[Hans Freikit]

Hi folks,

Just discovered this handy forum, and have a question.

I'm planning to install a junction box and spur from my ring main.

Question is, how long can the spur cable safely be ?

Thanks in advance,

HF

 

[dietcoke]

better off making it into a ring (then you have no worries)

 

[plugwash]

its depends on exactly where you are on the ring and how long the ring is

how long were you thinking of? normally its not a problem

 

[Malc]

AFAIR there is only an area limit for rings. And that is just so there aren't too many appliances going at the same time and thus tripping the fuse. AFAIR it's one double socket at the end of a spur and preferably 1 spur per ring junction (limit 2 I think).

If you think about it all this makes sense. The cable has an amp limit which is more than 2 sockets and more than 1/2 a ring fuse. Of course if you are trying to go a very long distance then voltage drop will be a concern. The cable will be ok but at the far end the mains voltage will be low which something might not like (bulbs will last longer though ).

If you want to check why not get the IEE wiring reg from your local library. Mine seems to carry this.

 

[Hans Freikit]

Thanks for the advice so far: to clarify, it seems as though my cable run would need to be around 5 metres in length. The spur will run from a 30A junction box, and end at a RCD fused connection unit.

Does that seem reasonable ?

HF

 

[Lectrician]

Fine and dandy!

 

[ban-all-sheds]

AFAIR there is only an area limit for rings.

There is also a length limit, which depends on the type and rating of the protective device, the method of installation of the cable, and the value of Ze.

If you want to check why not get the IEE wiring reg from your local library. Yours seems to carry this...

 

[Malc]

Well I did say AFAIR. I seem to remember this now you mention it. I usually get the regs out when I do work since things change anyway.

I would imagine the length has more to do with the earth function and insuring a fault trip happens quick enough so presumably CB have a longer length than fuse wire. I think I worked this out and measured it when installing a ring (and was nowhere near ).