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

Thank you for responding to my question. I haven’t been thinking about this problem recently but I had hoped to develop a better understanding of the issue (and have, so far, failed). This is the paradox: the ‘1/8 rule’ (as declared in the On-Site Guide, 7.2.2 on page 76), appears to contradict the very idea that every socket-outlet on a ring circuit can have one spur  (H2.4, page 189...’the number of non-fused spurs should not exceed the total number of socket-outlets’...) i.e. if the maximum length at the ‘furthest point’ is zero then you cannot have a spur at that point. I am convinced that my understanding is at fault and I should be happier were it resolved (but, clearly, I need to study more).


For example, if I wished to put a spur at the ‘furthest point’ on a radial circuit, I should, effectively, be simply extending the length of that radial (which would be fine, so long as I did not exceed the maximum length for the cable as identified in Table 7.1, pages 65 onwards). However, because a ring-final circuit operates differently, the electrical load is ‘distributed’ (i.e. the electrical power is drawn further one way or shorter the other way, depending upon which location on the ring circuit the socket-outlet is sited). This is why I doubt the conclusion being drawn from the interpretation of the 1/8 rule which suggests that you cannot place a spur at the furthest point.


Look at it this way, if you have a socket-outlet at the furthest point on a ring final circuit, the electrical power runs half way around the first half of the circuit and half way around the second half of the circuit (i.e. one full circuit); if you have a socket outlet at, or very close to, the consumer unit, the electrical power, effectively, runs all the way around the whole circuit. My conclusion being that, the electrical power on a ring-final circuit will, indeed, run the same distance around that circuit regardless of the location of the socket-outlet.


Therefore, if you had a longer ring-final circuit, there may be implications for the maximum length of any spur; that maximum length will have been determined by the 1/8 rule. Is this the seed of a valid conclusion? I can’t quite fully get my head around this puzzle at this very moment. What a shame that the On-Site Guide isn’t proving its worth as something comprehensively helpful to me as a novice domestic electrical installer.

Thank you for responding to my question. I haven’t been thinking about this problem recently but I had hoped to develop a better understanding of the issue (and have, so far, failed). This is the paradox: the ‘1/8 rule’ (as declared in the On-Site Guide, 7.2.2 on page 76), appears to contradict the very idea that every socket-outlet on a ring circuit can have one spur  (H2.4, page 189...’the number of non-fused spurs should not exceed the total number of socket-outlets’...) i.e. if the maximum length at the ‘furthest point’ is zero then you cannot have a spur at that point. I am convinced that my understanding is at fault and I should be happier were it resolved (but, clearly, I need to study more).


For example, if I wished to put a spur at the ‘furthest point’ on a radial circuit, I should, effectively, be simply extending the length of that radial (which would be fine, so long as I did not exceed the maximum length for the cable as identified in Table 7.1, pages 65 onwards). However, because a ring-final circuit operates differently, the electrical load is ‘distributed’ (i.e. the electrical power is drawn further one way or shorter the other way, depending upon which location on the ring circuit the socket-outlet is sited). This is why I doubt the conclusion being drawn from the interpretation of the 1/8 rule which suggests that you cannot place a spur at the furthest point.


Look at it this way, if you have a socket-outlet at the furthest point on a ring final circuit, the electrical power runs half way around the first half of the circuit and half way around the second half of the circuit (i.e. one full circuit); if you have a socket outlet at, or very close to, the consumer unit, the electrical power, effectively, runs all the way around the whole circuit. My conclusion being that, the electrical power on a ring-final circuit will, indeed, run the same distance around that circuit regardless of the location of the socket-outlet.


Therefore, if you had a longer ring-final circuit, there may be implications for the maximum length of any spur; that maximum length will have been determined by the 1/8 rule. Is this the seed of a valid conclusion? I can’t quite fully get my head around this puzzle at this very moment. What a shame that the On-Site Guide isn’t proving its worth as something comprehensively helpful to me as a novice domestic electrical installer.