The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 12 replies
  • Subscribers 31 subscribers
  • Views 1396 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Which is best, long ring final with 2 spurs or short ring final with more spurs?

Former Community Member
Former Community Member
Good afternoon, hope you are all well. I have been re-evaluating the number and position of sockets in my bungalow, and considering having a few more added for convenience. Currently I seem to have a relatively short ring with 2 long spurs. If I measure the new route for cables incorporating the existing spurs and new sockets, I anticipate a total cable run of about 100m. From what I gather on this forum this is the limit for a ring final per 100m square of floor. Question is, is this a good idea, or should the ring final be made to run a shorter route and add more spurs, not exceeding the number of sockets on the ring, in order to keep volt drop to within acceptable limits. Not being an electrician I have used online cable size calculators to see if my idea is anywhere close to being ok and they come back with a huge 25mm squared cable. I am clearly out of my depth here hence the question. As always, many thanks in advance. Kind regards, Simon   
  • 0
    The 100m. length limit is roughly the maximum to be within the electrical limits.

    It has nothing to do with the 100sq.m. area which, in my opinion, is an irrelevance.

    10x10 is a perimeter of 40m. and  33x3 is 72m.


    Some bungalows have the ring in the loft with all the sockets as spurs.


    It's up to you to design it so that it is within all the electrical limits.
  • 0
    One ring final circuit is one ring final circuit normally protected by a 32 Amp miniature circuit breaker. That limits the total loading to a maximum of about 7kW. What will you use at any  one time? Will the total circuit loading exceed 7kW? Perhaps more than one circuit is a wiser option, rather than just one ring final circuit. That way you are reducing the risk of overloading and the inconvenience caused by that if the M.C.B. trips off. A radial or two can be very useful, easy to install and self contained.


    Appendix 15 of B.S. 7671 suggests that appliances such as cookers, ovens and hobs of 2kW or above should be run on their own circuit, not from a ring final. 


    Fig. 15B in B.S. 7671 shows some radial options and requirements.


    Z.
  • 0
    I am not quite sure what you mean by a total 100 m cable run, please clarify. For example, it could be 25 m x 2 round the ring and then a 50 m spur, or 48 m x 2 + 4 m.
  • 0
    Are you adding significant new load, or just spreading about the loads you already have - if the latter there is no real need for an additional circuit. If possible I try and add sockets to a ring in a way that they are mid ring, rather than spur as this makes it easier to avoid joins and  jamming 3 cables in a box. 

    2.5mm is the "normal" ring cable,  unless very long  runs or well insulated and badly cooled routes, and it is for this configuration that the 100m round ring length rule of thumb, such as it is applies.

    More importantly, is the existing ring OK - for example is it RCD protected, are there any signs of trouble like nuisance tripping.
  • Former Community Member
    0 Former Community Member
    Hi Chris, to clarify, I meant 100m for the new ring with two 5m spurs. apologies for the confusion in the question. kind regards Simon 
  • 0
    It is generally accepted that a ring final should not serve an area greater than 100 square meters. This should be applied with common sense and is not an absolute requirement.

    I am not convinced that a corridor 2.1 meters wide and 50 meters long needs two ring finals !

    OTOH, I consider it good practice to install a dedicated ring final for a kitchen in all but the smallest flats, even if the floor area of the ground floor is under the 100 square meters.


    It is a useful rule of thumb, that the length of cable for a standard ring final should not exceed about 100 meters. This again is not an absolute rule, but more than 100 meters of cable requires careful consideration, and may need larger cable.
  • 0

    simon201:
    Hi Chris, to clarify, I meant 100m for the new ring with two 5m spurs. apologies for the confusion in the question. kind regards Simon 




    I'm still not quite clear. Is it a 100 m "ring", which could be squashed into avery tight oval, + 5 m spurs?


    As a rule of thumb, if you are installing a ring and the drum of cable runs out (been there, done that) you may need to think about a couple of things.

    (1) is the Zs within the acceptable limit? No reason why it should not be.

    (2) Is there a problem with voltage drop? If you are not going to plug in a 3 kW heater half way around the ring, there is nothing to worry about. And even if you did, it might run a few volts down and, therefore be 2 and a bit kW.


    I may be missing something, but these rules of thumb are not always helpful.

  • Former Community Member
    0 Former Community Member
    Hi Mike, The new run would be re-distributing existing loads apart from lamps. Sockets are approx equi-distant around ring with computer at far end and single and occasional 3kw heater plugged in around the place. Cables  air cooled under floorboards and around perimeter of roof void. no covering insulation. Regards Simon
  • Former Community Member
    0 Former Community Member
    Hi Chris, sorry for not making myself clear. looking at my plan i would say the closest description would be of some kind of antler shape, although I wouldn't want to be that animal. fanning out into separate rooms from main hallway. The 5m spurs would come from the tip of the points on the antler, if this makes any sense. Regards Simon
  • 0

    simon201:

    Hi Chris, sorry for not making myself clear. looking at my plan i would say the closest description would be of some kind of antler shape, although I wouldn't want to be that animal. fanning out into separate rooms from main hallway. The 5m spurs would come from the tip of the points on the antler, if this makes any sense. Regards Simon




    "Antler shape" suggests radial rather than ring. However, if the ring is passing up and down the main branches of the antlers, with spurs only at the tips, it should be in order.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines