Is 7 metres too far for a 2.5mm spur?

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Hi,
I am in the process of creating a spur from a kitchen socket to create a socket for a dishwasher.

The existing ring is 2.5mm, and while I don't know the total length of the ring, it is not substantial, serving just a medium sized kitchen and a bedroom above.

I had planned to run a 2.5mm cable up from the existing socket, a short distance above the ceiling, and then down to an isolator. I had then planned to run a further cable from the isolator to a single, unswitched socket for the dishwasher.

I could have the cable from the existing socket to the isolator as short as 3 metres, but had planned to use 5 metres of cable to allow reworking later. The route from the isolator to the new socket is 2 metres.

The spur would therefore be a total of 7 metres from the existing socket to the new socket (with the possiblity of getting it down to 5 metres if necessary).

Would that be too far?

Any thoughts would be much appreciated.
 
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No problem to run a spur that distance on 2.5mm² for that load.

why do you think you need an isolator?
 
Last edited:
If you want to comply with BS7671 then it does state a protective device can be up to 3 meters from the origin of the cable if no branches, as to if this applies to unfused spurs from a ring is debated, and there is not real answer, you would have to read the book and decide your self.

However as for cable length there are two reasons to restrict the length, one is volt drop, the other is the speed that the protective device can disconnect the supply.

So with the old fuse we wanted 0.4 seconds so with a 32A fuse the impedance (AC word for resistance) had to be 1.04Ω however if you exceeded that valve then it just made it a little slower, but with a B type MCB there are two devices in one, a thermal part which takes time to heat up, and a magnetic part that works in around 0.01 seconds, so unlike the fuse, if you go slightly over the impedance which was for years 1.44Ω for a 32A MCB then it jumps to around 5 seconds so it was changed to allow a 5% volt drop so now 1.37Ω.

Now with a RCD the earth loop impedance is not so important as the RCD will trip well within the time with quite a high figure, but the line to neutral impedance should still be low enough we for some reason never quite worked out why, measure the line - neutral as prospective short circuit current so simple ohms law 168 amp.

Volt drop uses the same impedance or prospective short circuit current to work it out, but there is a grey area, the design current for the circuit Ib is normally taken as 20 amp centre and 12 amp even distributed so we work on 26 amp, but there is no rule to say we should do this, so as the circuit designer you could take another figure. Also the correction factor Ct means we don't use 18 mA/A/M but around 16.5 mA/A/M so the sum becomes rather complex, but for a ring final with a incoming loop impedance at the consumer unit of 0.35Ω we would expect the centre point of the ring final to be 0.94Ω or less which would indicate the ring final has a 106 meters of cable used.

However the only modern item which is really affected by volt drop is the refrigeration plant used in non inverter controlled fridges, freezers and air conditioners, some old radios may produce a main hum with too much volt drop, and some old fluorescent lights may fail to start, but in the main the use of switch mode power supplies has resulted in very little units that are still affected by volt drop.

So with a RCD protected supply to a dish washer, the volt drop and earth loop impedance may mean the circuit does not comply, but the danger involved both to the dish washer and to personal is minimal even if exceeded.

However here in Wales a kitchen is a special location, and work needs registering under Part P building regulations, which means some one will need to test it and say if it is within the limits set by BS7671. So although the 3 meter limit 433.2.2 and the impedance table 41.3 may not in real terms be that important with RCD protection, it is down to the person doing the testing as to if he/she feels it complies or not.

So for an electrician he will measure the loop impedance and see if it is likely to be exceeded, and then decide if heavier cable, or different route, or use of a fused connection unit is required, unfortunately the cheap plug in testers like the EZ150 show all OK at 1.5Ω so you know if really bad, but not good enough to work out how much lee way you have.

But a 13A fuse can have an impedance of 2.42Ω so using a cheap plug in tester can say if within the limit if using a FCU, so for the DIY guy that is likely the easiest way around the problem, except when using a refrigeration device, as the fuse increases the volt drop, which is why all fridge/freezer instructions say do not use an extension lead, however I am using an extension lead with not problems.

So if in England where your not registering the work, likely you will have no problem, if in Wales where some one may such air through teeth and declare jobs worth, then you need to measure first.

To sign the minor works or installation certificate you say you have the skill required and have done all the tests, in which case you would not be asking the question, so we all know 95% of DIY work will not comply, but it is still not dangerous, this is one reason the IET says should do an electrical installation condition report every 10 years or change of occupant which ever sooner, to catch the odd error, plus any degrade of materials.

So if you make that isolator a FCU then unlikely to be any problem. And when I tried to get copies of the compliance and completion certificates from council when selling last house, they could not provide them, so unless you workmanship is that bad you kill some one, in which case the council would be forced to find them, it is unlikely anyone will every worry that it is not registered with LABC.
 
If you want to comply with BS7671 then it does state a protective device can be up to 3 meters from the origin of the cable if no branches, as to if this applies to unfused spurs from a ring is debated, and there is not real answer, you would have to read the book and decide your self.
The other side of the debate being: that 3m rule is talking about fault protection and unless you're backing on to the substation the 1.5mm CPC in the cable would not overheat before the circuit breaker tripped.
 
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if the bedroom sockets are on the same ring, it maybe easier to run a cable from one of them? (rather than chasing wall).
 
But a 13A fuse can have an impedance of 2.42Ω so using a cheap plug in tester can say if within the limit if using a FCU, so for the DIY guy that is likely the easiest way around the problem, except when using a refrigeration device, as the fuse increases the volt drop, which is why all fridge/freezer instructions say do not use an extension lead, however I am using an extension lead with not problems.

So if you make that isolator a FCU then unlikely to be any problem.

Thanks. So I think you are saying that I could replace the isolator with something like this, and keep the single unswitched socket as it. Is that right?

https://www.screwfix.com/p/lap-13a-switched-fused-connection-unit-white/29377

if the bedroom sockets are on the same ring, it maybe easier to run a cable from one of them? (rather than chasing wall).

That is another option. In fact there are a couple of sockets together in the room above, but the placement means that spurring from the kitchen is actually a bit easier. I have put battens with plasterboard in the kitchen, so dropping any cables down behind that. The circuit is under RCD, so I think that there will be no need to bury the cable for compliance.

No problem to run a spur that distance on 2.5mm² for that load.

why do you think you need an isolator?

Two reason really, neither of them very strong. The first is that I just like the idea of having a separate isolator above the worktop (probably just because I have had them in other houses). The second is that it allows me to run the cable down from above, behind the plasterboard, have it exit from behind the plasterboard while still running vertically and then just run one cupboard over to the socket (cable clipped direct to the wall). I thought that would be a good idea, because the switch would mark the vertical zone and then there is just a small section of concealed cable behind the cupboards. So really to mark the zone in that area.
 
But a 13A fuse can have an impedance of 2.42Ω ....
With respect, that's surely total nonsense, isn't it? If a 13A fuse had a resistance of 2.42 Ω, then over 400W would be dissipated in it when 13A was flowing!!!

Did you perhaps mean 2.42 ? Even the 0.4W at 13A that would imply is higher than I would have guessed, but I suppose it's possible.

... except when using a refrigeration device, as the fuse increases the volt drop ...
As above, I don't think anything more than 2.42 mΩ would be credible for a 13A fuse, and with a full 13A flowing that would only result in a voltage drop of about 0.03 V. Could it seriously be suggested that such a tiny VD could make one iota of difference, particularly when we remember that a refrigeration device sold in the UK has to be able to work satisfactorily at any voltage between 216.2V to 253V?

Kind Regards, John
 
while I not too fussed either way on fitting a 13A FCU or 20A switch, I notice 13 FCU's tend to be cheaper, so can understand why they are used across a kitchen.

Also fitted appliances get wired in (don't understand why), so 13A fcu's cover this eventuality well. (and when you find the socket and plug is fitted in the wrong place and the appliance won't push all the way back!!!
 
while I not too fussed either way on fitting a 13A FCU or 20A switch, I notice 13 FCU's tend to be cheaper, so can understand why they are used across a kitchen.

Also fitted appliances get wired in (don't understand why), so 13A fcu's cover this eventuality well. (and when you find the socket and plug is fitted in the wrong place and the appliance won't push all the way back!!!
 

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