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

First, to avoid terminological confusion: the "furthest point" is the point furthest from the CU - if you have an 80m ring, then it's the point 40m from the CU.


It's best to think of the rule as a rough guide to not increasing the maximum resistance (Zs) of the ring beyond what it already is. This is important for two reasons: first to avoid excessive voltage drop across the cables (wastes power and things plugged in might not work properly) and to ensure that a fault will cause enough current to flow to trip the breaker. Given an existing ring, its reasonable to assume that the person who designed it did in such a way that at the socket at the furthest point from the CU will have a Zs which is within spec. If you follow the 1/8 guideline, this guarantees that the new socket will have a Zs not more than the highest Zs of any existing socket in the ring.


If you want the maths, consider a ring with total resistance 2r: so at the furthest point, each arm has resistance r, and the resistance from a socket at that point to the CU is r/2 (rule for resistances in parallel). If you consider a spur tap point in the ring of proportion k between the furthest point and the CU (so k=1 implies at the furthest point, k=0 implies at the CU), then the OSG rule says you can add a spur at that point whose extra resistance is kr/8. If you do the maths, you get that the resistance at the socket at the end of the spur is then


(1-k²)r/2 + kr/8


If you evaluate that for various values of k, you get values between about 0.3r and 0.5r. The latter is the value at the furthest point, so your spur socket is no worse than any existing ring socket.


Since its a rule of thumb, its not precise; in particular, when k = 0.75, you get 0.3r. In this case you could in fact put in a longer spur without exceeding 0.5r.


And remember that this is just a rule of thumb to not make things worse than they already are. If you really need the extra length, you can always measure Zdb and calculate exactly how long the spur can be.

First, to avoid terminological confusion: the "furthest point" is the point furthest from the CU - if you have an 80m ring, then it's the point 40m from the CU.


It's best to think of the rule as a rough guide to not increasing the maximum resistance (Zs) of the ring beyond what it already is. This is important for two reasons: first to avoid excessive voltage drop across the cables (wastes power and things plugged in might not work properly) and to ensure that a fault will cause enough current to flow to trip the breaker. Given an existing ring, its reasonable to assume that the person who designed it did in such a way that at the socket at the furthest point from the CU will have a Zs which is within spec. If you follow the 1/8 guideline, this guarantees that the new socket will have a Zs not more than the highest Zs of any existing socket in the ring.


If you want the maths, consider a ring with total resistance 2r: so at the furthest point, each arm has resistance r, and the resistance from a socket at that point to the CU is r/2 (rule for resistances in parallel). If you consider a spur tap point in the ring of proportion k between the furthest point and the CU (so k=1 implies at the furthest point, k=0 implies at the CU), then the OSG rule says you can add a spur at that point whose extra resistance is kr/8. If you do the maths, you get that the resistance at the socket at the end of the spur is then


(1-k²)r/2 + kr/8


If you evaluate that for various values of k, you get values between about 0.3r and 0.5r. The latter is the value at the furthest point, so your spur socket is no worse than any existing ring socket.


Since its a rule of thumb, its not precise; in particular, when k = 0.75, you get 0.3r. In this case you could in fact put in a longer spur without exceeding 0.5r.


And remember that this is just a rule of thumb to not make things worse than they already are. If you really need the extra length, you can always measure Zdb and calculate exactly how long the spur can be.