This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

On-Site Guide BS 7671:2018 - 7.2.2 Socket-outlet circuits

Former Community Member
Former Community Member
Hello, I am studying the On-Site Guide and have reached Section 7 Final Circuits.


7.2.2 Socket-outlet circuits

...

As a rule of thumb for rings, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring.

...


I should like to ask, what does this mean?


The length of the cable used for the non-fused spur is the length of the cable from where it connects to ring final circuit at the terminals of a socket-outlet (or junction box...), to the terminals at the other end of the cable where it connects to a single or twin socket-outlet.


What is the length ‘from the spur to the furthest point of the ring’? Does ‘from the spur’ mean from the the point where the spur connects to the circuit? Does ‘to the furthest point of the ring’ mean to the furthest point away from where the spur connects to the circuit i.e. half the length of the ring final circuit? If so, the maximum lengths would be the same for all spurs on that circuit.


Or, does it mean from where the spur connects to the circuit to the point which is the furthest away from the protection device in the consumer unit i.e. a point half way around the circuit beginning from the consumer unit? If so, the maximum length of the spur cable would be longest nearer to the consumer unit and shorter as the furthest point away is reached... Surely this cannot be correct, because at the furthest point the maximum length would be zero - !


So, in conclusion: 

1/ establish the full length of the ring final circuit;

2/ find the furthest point of the ring by dividing the full length by two;

3/ divide that length by 8 to find the maximum length of a spur cable for that particular ring final circuit.


Please would someone confirm my understanding or explain what the sentence within 7.2.2 actually means?


Many thanks!


Parents
  • Former Community Member
    0 Former Community Member
    Thank you to one and all for your kind help.


    The sentence in 7.2.2 makes no direct reference to the Wiring Regulations, and it is ambiguous.


    To quote from the On-Site Guide, ‘Table 7.1(i) has been designed to enable a ... ring final circuit to be installed without calculation’.


    My belief is that when faced with any problem it is dangerous to assume anything but, presumably, the IET have included the sentence in 7.2.2 to be helpful to anyone learning about the installation of electrical wiring.


    In this instance, the IET appear to have failed to give ‘clear guidance’, and I am not qualified enough to do the electrical calculations myself.


    Perhaps my suspicion is wrong but, if I use my first interpretation of the sentence, taking the maximum cable length for a ring final circuit (32A, RCBO Type B, 2.5/1.5 mm2, TN-C-S), as 106m (Table 7.1 (i)); the maximum length of a spur would be 6.62m (106/2 => 53/8 => 6.25).


    Similar calculations for two ring final circuits with maximum lengths of 40m and 80m would result in maximum spur lengths of 2.5m and 5m. Both of these maximums would be of useful working lengths.


    Taking the meaning of 7.2.2 in the other way would mean that if a ring final circuit were to be extended (for example, a loft conversion), all the new socket-outlets would need to be on that ring final circuit, rather than on spurs taken off the ring final circuit. Which would not be very helpful or practicable.


    Presumably, the IET understand the reasoning for the sentence in 7.2.2 and are suggesting to novices and qualified electrical installers alike that, the larger the domestic dwelling (i.e. the longer any ring final circuit is likely to be), the longer the practical and safe working length of any spur taken from that ring final circuit. Hence my summation being, that the maximum spur length for any given ring final circuit (shorter in length than the maximum given in Table 7.1(i)), is 1/8 of 1/2 of its full length (effectively, 1/16).


    I am not assuming that I am correct, but I am trying to understand.


    Thank you once again to everyone who has given my question their considered time and thought.
Reply
  • Former Community Member
    0 Former Community Member
    Thank you to one and all for your kind help.


    The sentence in 7.2.2 makes no direct reference to the Wiring Regulations, and it is ambiguous.


    To quote from the On-Site Guide, ‘Table 7.1(i) has been designed to enable a ... ring final circuit to be installed without calculation’.


    My belief is that when faced with any problem it is dangerous to assume anything but, presumably, the IET have included the sentence in 7.2.2 to be helpful to anyone learning about the installation of electrical wiring.


    In this instance, the IET appear to have failed to give ‘clear guidance’, and I am not qualified enough to do the electrical calculations myself.


    Perhaps my suspicion is wrong but, if I use my first interpretation of the sentence, taking the maximum cable length for a ring final circuit (32A, RCBO Type B, 2.5/1.5 mm2, TN-C-S), as 106m (Table 7.1 (i)); the maximum length of a spur would be 6.62m (106/2 => 53/8 => 6.25).


    Similar calculations for two ring final circuits with maximum lengths of 40m and 80m would result in maximum spur lengths of 2.5m and 5m. Both of these maximums would be of useful working lengths.


    Taking the meaning of 7.2.2 in the other way would mean that if a ring final circuit were to be extended (for example, a loft conversion), all the new socket-outlets would need to be on that ring final circuit, rather than on spurs taken off the ring final circuit. Which would not be very helpful or practicable.


    Presumably, the IET understand the reasoning for the sentence in 7.2.2 and are suggesting to novices and qualified electrical installers alike that, the larger the domestic dwelling (i.e. the longer any ring final circuit is likely to be), the longer the practical and safe working length of any spur taken from that ring final circuit. Hence my summation being, that the maximum spur length for any given ring final circuit (shorter in length than the maximum given in Table 7.1(i)), is 1/8 of 1/2 of its full length (effectively, 1/16).


    I am not assuming that I am correct, but I am trying to understand.


    Thank you once again to everyone who has given my question their considered time and thought.
Children
No Data