Adding a spur socket: Total length of cable ?


Postby johnch » Sat Nov 14, 2009 3:01 pm

I need to add a new socket as a spur on my ring main - something I've done before, but this one needs to run for a longer length before the new socket. According to your article on doing this there's a limit to the total length of cable for the ring main + spur:

"Please also check the rules very carefully for ring mains and radial circuits. You are limited in the length of cable you are allowed to use in both circuits and long spurs could make you exceed the limit. If this is the case you are asking the circuit to use much more energy than the circuit is designed for. More energy = more heat and cables can catch fire."

Can you tell me what that total length is please so I can check I don't exceed it ?

Many thanks, John.
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Postby sparx » Sat Nov 14, 2009 8:33 pm

Hi,
I'm afraid there are some answers put on here which are less than factual!
The rules for a ring CIRCUIT with or without spurs are simple, but often misquoted.
The AREA covered by a ring must not exceed 100 square meters, eg;
10X10 or 20X5 or any mix to not exceed 100m2.
The limiting factors in practical terms are not overload but volt drop on long runs and too high an earth loop impedance reading which would exceed the max allowed for the protective device (fuse or mcb).
These readings/ calcs. can only be done on site by someone with test equipment and the knowledge to understand the results.
There have recently been comments about 3metre limits on spurs, NOT TRUE! also only one outlet per spur, again only partly true since if you fit a fused connection unit (Fused spur) there is no limit to how many outlets on a spur, also it is allowed to have a fused spur feeding a 1.5mm2 cable to a socket subject to above limits.
So afraid no straight answers except contact a registered 'competant person'
regards Sparx
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Postby ericmark » Sun Nov 15, 2009 12:06 am

As already said there is a limit to the amount of cable one can use on a ring main it was around the 80 meters and now under 17th Edition around the 106 meters can't remember exact figures but in real terms you can't measure cable length once fitted so you have to measure its AC resistance called impedance.

The meters used to measure this are not cheap around the £100 mark and the reading permitted vary according to protective devices used.

Often we are looking at volt drop which under 17th Edition is 5% of normal voltage so on a 32A supply that's 0.36 ohms but it does not work out quite that easy. To start with on a ring main only 20A is considered as being drawn at furthest point with 12A evenly distributed so we use 26A in calculations not 32A this raises the figure to 0.44 ohms but we haven't finished cable resistance varies with temperature so we have correction factors to add or subtract from that.

By this point I expect I am loosing you and I am not surprised some electricians have problems with the maths but in the main we are not working to the limits and so using the ELI (Earth Loop Impedance) we can have a reasonable good idea as to if we are sailing close to the wind or not. Many consider on a 32A ring main if the ELI is below 1.44 ohms it will be OK. Not really that simple but I suppose near enough.

Adding spurs from a fused connection unit the volt drop considerations don't change but because maximum current is now 13A the ELI can raise to 2.42 ohms so normally not a problem and once a fused connection unit is used you can add as many sockets as you like.

However the 17th Edition under regulation 433.2.2 explains how when a reduction in current carrying capacity is introduced into a circuit then you must use an over current device (Fuse) and this has to be within 3 meters of the change. So when installing a spur following the letter of the regs unless a FCU is used then there is a limit of 3 meters. However I would not worry too much about that. The problem with all the regs is if some third party is issuing the completion certificate i.e. LABC they can get pedantic so one would have a hard job proving you could exceed the 3 meters.

As Sparx says no straight answers.

As to catching fire as long as the rules as to one socket only on a unfused spur are observed and the ring is preserved then unlikely to be a problem but there are two reasons to limit the length of cable.
1) Volt drop and old TV's and other old audio equipment may hum is volts drop too much and motors may fail to start and should that motor drive the fan in a fan heater then it may cause a fire.
2) Is the ability to open the protective device in the prescribed time. i.e. Blow the fuse. With the introduction of RCD's the current needed to open the circuit is reduced and in today's' house this is not so much of a problem but old houses wired before June 2008 often don't have RCD's on all sockets although any new cable buried less than 50mm or any socket under 20A will need RCD protection.

As already stated without knowing the ELI or PSC it is impossible to know if you are under or over the limit. The prospective short circuit current (PSC) is really the figure one should use to work out volt drop.

But really with any meters costing around £200 and to get the three to do all measurements required costing £750 plus add cost of LABC and Part P should that be required and in most cases to do any electrical work to British Standard (BS 7671:2008) it is cheaper to employ an electrician.

However I know most don't believe that so that's why I have tried to say what is involved. Google "planningportal.gov.uk/england/professionals/buildingregs/technicalguidance/bcelectricalsafetypartp/bcapproveddocuments12" and look at Part P and it has copies of the documents you should fill in. If you think you can DIY look at them and decide if you could fill them in. Google "theiet.org/publishing/wiring-regulations/updates/" and down load BS 7671:2008, Corrigendum (July 2008) and that tells you the bones of what is allowed. The full BS 7671:2008 costs £60+ but that shows a little bit which should help.

Can't stop anyone DIY'ing but we can point out the dangers and the legislation that governs what can be done at a DIY level and where you should stop and call in a professional
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