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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!


  • 0
    The sentence in 7.2.2 makes no direct reference to the Wiring Regulations, and it is ambiguous.

    :

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

    Perhaps start off considering what bits of BS 7671 effectively limit the length of a circuit. Things like Zs, voltage drop (and occasionally L-N loop impedance) come to mind. There's no single place in the regs that define these requirement - its a combination of lots of separate requirements.


    Then consider the effect on circuit length of adding a spur to various points on a ring - both a ring that's already at its maximum circuit length and a ring that's somewhat shorter.


    Remember that this is all ultimately driven by the laws of physics - rather than what would be convenient to an electrician.


       -  Andy.
  • 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.
  • 0
    davezawadi:

    This statement is without foundation for practical rings, which are very rarely anywhere near maximum length. As RCD protection is mandatory the Zs question is fairly irrelevant, and voltage drop is just inconvenient and again unlikely to be a problem.

    The statement that RCD protection is mandatory only really applies in dwellings where cables are buried in walls without earthed metallic containment, where the ring final circuit serves socket-outlets (assuming that BS 7288 RCDs are not permitted, although they would have been considered acceptable to BS 7671 prior to BS 7671:2018). Hence, there is a need to consider


    It certainly doesn't cover every installation, and the OSG may be useful for older installations.


     
    Strangely what regulation does it reference? I suppose none.

    It says it's a rule of thumb - usually, calculation rules of thumb err on the side of caution - so you may have a point - but for many circuits in dwellings, a quick lookup is better than doing lengthy calculations (and making mistakes).

     
    I think that the OSG should probably be dumped, it has a number of "old wives tale" comments which are not supported, and any electrician should have a copy of the real regulations, which they should know fairly well (in my experience they don't), and I had a few people turn up for an 18th edition course with just an OSG under their arm! They were really surprised when given a copy of the book to use as to how much was not in the OSG.


    I really don't think providing lookup tables that err on the side of caution is responsible for many people being unaware of all of the issues that pertain to the 500 or so pages in BS 7671 (plus all of the attendant standards it references).


    I think that's a separate debate all together.


  • 0
    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 ?".
  • 0
    I think that if it is to have any real meaning it should be a fraction of the max permitted length of ring less the point it is to be actually  installed at. Or to put it more simply, to not put it any more onerous (Zs and Volt drop)  than any point on max permitted ring length would be.

    You`d need to invent a formula to do it
  • 0
    This statement is without foundation for practical rings, which are very rarely anywhere near maximum length. As RCD protection is mandatory the Zs question is fairly irrelevant, and voltage drop is just inconvenient and again unlikely to be a problem. Strangely what regulation does it reference? I suppose none. I think that the OSG should probably be dumped, it has a number of "old wives tale" comments which are not supported, and any electrician should have a copy of the real regulations, which they should know fairly well (in my experience they don't), and I had a few people turn up for an 18th edition course with just an OSG under their arm! They were really surprised when given a copy of the book to use as to how much was not in the OSG.
  • 0
    Ian2304:

    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 - !


    That's how I read it. Let's suppose that you have a ring which is 32 m long so the furthest point is at 16 m. You cannot put a spur there. If you put one at the CU, it can be 16/8 = 2 m long; and half way to the furthest point it would be only 1 m max.


    A ring serving a kitchen might be only 16 m long, which limits half your possible spurs to 0.5 m, which doesn't seem much use to anybody.


    It's only a rule of thumb in a guide. Just ignore it! ??


  • 0
    My guess would be your second option:

     
    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 - !

    And zero length at the furthest point does make sense in a way. A spur will always add some resistance - so increasing both voltage drop and Zs above that at the point where it's connected to the ring. If the ring is already as long as allowed - then yes the maximum spur you could add at the furthest point would indeed be zero - otherwise you'd be exceeding permitted loop impedances or voltage drop.


    I agree the wording you've quoted isn't that clear. Really the limit should be based on the maximum permitted ring length rather than the length of the ring you've actually got - and indeed they could be clearer about meaning the point on the ring where the spur is connected.


    (I'm still puzzling where they've got 1/8th from - I'll have to think about that).


       - Andy.


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