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

wallywombat:

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.

Correcting myself: it's between 0.125r and 0.5r. Once you get near the CU, the Zs of the spur socket drops off rapidly: here the rule-of-thumb is badly underestimating the maximum safe length of spur. But from half-way to full-way to the furthest point the formula works well.

Thank you, I do appreciate all of your responses. During Lockdown I began to learn about ‘domestic electrical installation’ with the idea that I might enrol upon a short course to gain some competence. Perhaps the On-Site Guide and Wiring Regulations were not the best place to start, particularly without the support of experienced teachers. The old adage, ‘A little knowledge is a dangerous thing’, strikes me as pertinent.

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.

because the electricity is ‘evenly distributed’

I still say that's a misleading conclusion.


Try a few worked examples using some arbitrary cable resistances and Ohm's Law.


The other option is that one of the authors of the OSG has made something up that isn't of itself entirely logical (unlike many of the other "rules of thumb" for rings that are well known and go back generations, this 1/8th rule seems new has never appeared in the regs or any other guidance as far as I know).


   - Andy.

Well, I still don’t have a definitive answer but this is what I feel must be somewhere nearer to the truth than me just guessing...


On an 80m ring final circuit, the electricity is ‘evenly distributed’. So, if I plug a radio into a socket, it will draw electricity both ways around the ring. If the socket happens to be near to the supply it will draw the electricity a short way one way and a long way the other way. If the socket happens to be 1/4 of the way around the ring, the radio will draw electricity 1/4 one way and 3/4 the other way around the ring. Again, if the socket is half way around the ring it will draw the electricity 1/2 one way and 1/2 the other way. Effectively, in whichever location the socket happens to be the electricity ‘travels’ one whole circuit of the ring (...).


By induction, I suspect that the 1/8 rule must work on a similar principle. If the socket happens to be at the ‘furthest point‘, it is 0m one way but 80m measuring the other way. Dividing by two gives 40m. Applying the rule of thumb results in a maximum spur length of 5m. Somehow, because a ring final operates differently to a radial - because the electricity is ‘evenly distributed’ - the maximum length of any spur for a given ring is always the same (and the maximum length of the spur relates directly to the maximum length of the ring - up to the safe maximum for a ring)...


Hmm, well it just doesn’t make any useful sense interpreted the other way. The On-Site Guide can’t say that ‘you can have one unfused spur at every socket’ and then give you a rule of thumb that if interpreted literally, precludes the use of spurs anywhere approaching ‘the furthest point’.


Most unsatisfactory...

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.

Not quite. The current drawn though a socket will be divided between the two sides (legs) of the ring in proportion to the conductivity (1/resistance) of each leg. So a socket at the exact mid point drawing 13A will see 6.5A flowing through both sides. A socket near to the CU will see almost all its current flow through the shortest leg, and very little go the long way around.


You can verify that simply using Ohm's Law - noting that the voltage at both legs at the CU must be the same (as they're connected together) and likewise the voltage on both legs where they meet at a socket must also be the same (again as they're solidly connected together) - but the longer leg will have a higher resistance so Ohm's law say that it must therefore carry a proportionally lower current if V=IR is to hold true for both legs at the same time.


   - Andy.

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.

As Andy has pointed out, the 106 m max length (all the way around) of a standard T&E RFC is based upon voltage drop assuming that it is fully loaded (26 A) and that the load is distributed, but not all RFCs are going to be fully loaded, especially on the upper floors of a conventional house.

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?

Sort of. There'll be a number of different requirements, each of which can usually be reduced to a simple equation. Each calculation will have a slightly different basis - loop impedances will just depend on the cable size & length, so relatively straight-forward, but voltage drop also depends on the load current - which means making some different assumptions especially for circuits like socket circuits that have many different points where loads may or may not be connected. You get quite different results if you assume that the loads are concentrated at the far point, or spread evenly, or spread unevenly in some way. If calculating in full manually you'd often have to do several calculations and then pick the most limiting (shortest) result. The OSG used to note against some circuits what the limiting factor was - e.g. zs for earth fault loop impedance, sc for L-N loop impedance or ad for adiabatic (usually due to the smaller c.p.c. size), otherwise it was voltage drop.


In many cases you can treat as ring as being similar to a radial with half the cable of twice the c.s.a.. - which would suggest to me that a spur from the MCB could be up to a quarter of the overall length of the ring for the same result (I'm still puzzling where 1/8th has come from, unless someone thought that a 2x safety margin would be a good idea)

 

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.

I would have said that the furthest point of the ring was half way around (after that you start getting closer to the start again) - so even with the spur taken from the MCB you've only 1/8th of 53m rather than 1/8th of 106m to play with.


Don't discount the fact that these things are written by humans, and to err is human... Even IET texts aren't necessarily word perfect.


    - Andy.

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!