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

I must thank you all once again. All of your helpful ideas have nudged my thinking into a different direction. I am beginning to hope that I may now understand what that sentence in the On-Site Guide may mean. 


To begin again, ‘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 on the ring.’


My nascent understanding of electricity and electrical wiring includes the belief that a ring circuit operates somewhat differently to a radial circuit. In a domestic situation, an installed circuit is supplied with a nominal 230V of electricity which can be drawn off at socket-outlets and suchlike to provide power for electrical equipment. Would it be true to say that for a radial circuit one equation (more or less) can be used to determine the limits to how that circuit may be used? Presumably, that same equation could not be used alone to determine the limits to a ring circuit because it is supplied with power at both ends rather than just one. Hence, the IET have designed Table 7.1(i) to save electrical installers the time and effort of performing these various calculations for both radial circuits and ring final circuits. Please be patient, I am not pretending to know but to get the gist.


Looking at Table 7.1(i), if I install a ring final circuit with a 32A RCBO and 2.5/1.5 mm2 cable installed correctly (according to an allowed method), I could run the circuit to a maximum length of 106m. The beginning and end of the circuit will be at the consumer unit or distribution board. Applying the rule of thumb given in 7.2.2, I can install an unfused spur at the consumer unit, not longer than 1/8 the cable length from the spur to the furthest point of the ring, which just happens to be the cable length of the ring i.e. 106m. Therefore, the maximum length of any spur at the consumer unit will be 13.25m (106/8). Should I wish to install a spur half way around the ring circuit, at the far end of the dwelling say, the maximum length of the spur would be half i.e. 6.62m (53/8). However, if I were to install a spur a quarter of the way around the ring, the furthest point of the ring from the spur is three quarters of the full length i.e. 79.5m (106 x 0.75). Therefore, the maximum length of a spur at that point would be 9.94m (79.5/8).


Say, my ring final circuit is 80m long. The maximum length of a spur at the consumer unit is 10m; half way around the circuit it is at its shortest, 5m; a quarter of the way around the circuit it is 7.5m. All of these are useful lengths for installing unfused spurs in a practical situation. The difficulty being that unless you have installed the whole circuit yourself or have the electrical diagrams of the installation at hand, you would be left guessing the locations of the spurs. But that would be missing the point, because the idea of the rule of thumb is to provide you with a guesstimate. What 7.2.2 is saying to me is that, if I wanted to put a spur longer than 5m half way around my 80m ring circuit, I should consider extending the ring further (maximum 106m), rather than attempting to do any calculations myself.


By Jove, I think I’ve cracked it! Unless you know differently, of course.


Thank you, thank you, thank you!

I must thank you all once again. All of your helpful ideas have nudged my thinking into a different direction. I am beginning to hope that I may now understand what that sentence in the On-Site Guide may mean. 


To begin again, ‘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 on the ring.’


My nascent understanding of electricity and electrical wiring includes the belief that a ring circuit operates somewhat differently to a radial circuit. In a domestic situation, an installed circuit is supplied with a nominal 230V of electricity which can be drawn off at socket-outlets and suchlike to provide power for electrical equipment. Would it be true to say that for a radial circuit one equation (more or less) can be used to determine the limits to how that circuit may be used? Presumably, that same equation could not be used alone to determine the limits to a ring circuit because it is supplied with power at both ends rather than just one. Hence, the IET have designed Table 7.1(i) to save electrical installers the time and effort of performing these various calculations for both radial circuits and ring final circuits. Please be patient, I am not pretending to know but to get the gist.


Looking at Table 7.1(i), if I install a ring final circuit with a 32A RCBO and 2.5/1.5 mm2 cable installed correctly (according to an allowed method), I could run the circuit to a maximum length of 106m. The beginning and end of the circuit will be at the consumer unit or distribution board. Applying the rule of thumb given in 7.2.2, I can install an unfused spur at the consumer unit, not longer than 1/8 the cable length from the spur to the furthest point of the ring, which just happens to be the cable length of the ring i.e. 106m. Therefore, the maximum length of any spur at the consumer unit will be 13.25m (106/8). Should I wish to install a spur half way around the ring circuit, at the far end of the dwelling say, the maximum length of the spur would be half i.e. 6.62m (53/8). However, if I were to install a spur a quarter of the way around the ring, the furthest point of the ring from the spur is three quarters of the full length i.e. 79.5m (106 x 0.75). Therefore, the maximum length of a spur at that point would be 9.94m (79.5/8).


Say, my ring final circuit is 80m long. The maximum length of a spur at the consumer unit is 10m; half way around the circuit it is at its shortest, 5m; a quarter of the way around the circuit it is 7.5m. All of these are useful lengths for installing unfused spurs in a practical situation. The difficulty being that unless you have installed the whole circuit yourself or have the electrical diagrams of the installation at hand, you would be left guessing the locations of the spurs. But that would be missing the point, because the idea of the rule of thumb is to provide you with a guesstimate. What 7.2.2 is saying to me is that, if I wanted to put a spur longer than 5m half way around my 80m ring circuit, I should consider extending the ring further (maximum 106m), rather than attempting to do any calculations myself.


By Jove, I think I’ve cracked it! Unless you know differently, of course.


Thank you, thank you, thank you!