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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
  • As written it seems to preclude short rings and long spurs - which is a perfectly safe and economical method if for example you have a high ceilings and wish to drop the downstairs sockets as a spur each - the voltage drop is better than if you went up and down the wall each time.

    I'm incined to say it is indeed so much cobblers, and a simple total voltage drop requirement (no socket more than 50m from the origin by whichever route is shortest) would be a better approximate 'rule of thumb' for being sure of that.

    Clearly there is nothing in BS7671 itself about this.

    Thinking of other threads on here, I'm reminded this is just the sort of confusing and unscientific advice that causes false alarms and bitter wrangling when  EICRs are performed by folk who follow a recipe rather than design for a living I can almost see it  "what code do I give an over-long spur ?".
Reply
  • As written it seems to preclude short rings and long spurs - which is a perfectly safe and economical method if for example you have a high ceilings and wish to drop the downstairs sockets as a spur each - the voltage drop is better than if you went up and down the wall each time.

    I'm incined to say it is indeed so much cobblers, and a simple total voltage drop requirement (no socket more than 50m from the origin by whichever route is shortest) would be a better approximate 'rule of thumb' for being sure of that.

    Clearly there is nothing in BS7671 itself about this.

    Thinking of other threads on here, I'm reminded this is just the sort of confusing and unscientific advice that causes false alarms and bitter wrangling when  EICRs are performed by folk who follow a recipe rather than design for a living I can almost see it  "what code do I give an over-long spur ?".
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