Thoughts on spurs (RFC)

[rjm2k]

I found 6 spurs in my lounge and it made me think about the whole spur thing, I believe according to accepted standards it's fine to add a double socket spur from a RFC socket using 2.5mm cable. Now that cable will probably be rated 23Amps, yet you can plug 2x13 amp devices into a double socket, giving a total of 26Amps.

At first I thought that's probably acceptable since the cable will have a degree of tolerence in rather than an absolute 23Amp cutoff so the extra 3 should be ok, but then there's adapters and extensions. There's nothing to stop someone plugging a double adapter into one of those sockets and plugging say a 2x13Amp and a 3 amp device say giving you a total of 29Amps on a cable rated only to 23Amps. A theoretical situation of course, but then a lot of the regs are about theoretical worst case scenarios. So would a 23Amps cable still be ok supplying 29Amps? Since the main circuit protection would be 32Amps, isn't there a chance that the cable will be damaged?

In theory there is no way to overload a ring in the same way since the circuit protection is lower than the combined cable can cope with anyway.

Will we see a requirement that all spur sockets must be fused sockets or maybe that they should always have a FCU? or maybe that you must use a cable which can cope with at least the maximum current that the circuit protection will allow you to draw? Interestingly, I looked at 4mm cable online and found it was only rated at 25Amps, so only 2 more than 2.5mm, so the next step up would be 6mm cable at 42Amps, difficult to work with? (OK, OK I know I'm ignoring things like voltage drop in this question).

I believe the regs used to allow 2x single spurs from a socket or 1 double, but they were changed to 1xsingle or 1xdouble because people kept upgrading the 2 singles to doubles, maybe they should have only allowed singles instead but that still allows you in theory to overload the spur cable to a point where the circuit protection wont help you.

What's wrong with more than 1 spur from a RFC socket anyway, the spur cables can carry the same load each (same issues as above) and the main protection would kick in at the same point. Is it just considered difficult to work with, or could be hiding a bridge in the ring?

 

[OwainDIYer]

The regs for a ring circuit assume that the load will be distributed fairly evenly around the ring.

Multiple spurs off the same point could cause an uneven distribution of current over both the legs feeding that point.

In domestic premises it's extremely unlikely the sustained load on the ring would be anything like 32A - with the exception of heavy load heating (most houses have central heating now; and when the ring was designed heating loads were less as people didn't keep their houses as warm as we do now) and kitchen appliances (washing machines etc were a lot less common in the 50s).

Therefore with the exception of the kitchen it is unlikely that the maximum loads will be exceeded.

 

[EFLImpudence]

The current carrying capacities you quote are wrong.

For twin and earth buried in the wall -
27A for 2.5mm²
37A for 4mm²
This is reduced for other installation methods.

Having said that your worries about what may be plugged in are correct but it is allowed.
There is probably more tolerance in the regulations than you think.

Only one spur per 'point' is allowed on a ring although it can be done on a radial.

Ring final circuits are a strange creation with separate regulations which do not relate directly to the characteristics of the single cable.

For example, you could separate the ring (at the centre) and each half could be a 20A or 25A radial but it is not acceptable to have a 40 or 50A ring (2.5mm² t&e).
It can only be and must be 32A (60% of the combined 54A (27+27)) and all cable and accessories must be capable of handling 20A.

 

[rjm2k]

Thanks, I was considering what the effect of plugging 2 wallpaper steamers into a socket that turned out to be a spur would be, seems that even a 2000W stripper would take 8.3Amps, so even 2 would still be under the 23Amps supported by the cable. Still, better to plug into different sockets and hope there's no ring fault I guess.

I prefer a damp sponge anyway

 

[PrenticeBoyofDerry]

As EFLI has mentioned your ratings of cables are incorrect, therefore the point you have made is not a valid point.

 

[rjm2k]

As EFLI has mentioned your ratings of cables are incorrect, therefore the point you have made is not a valid point.

Ok thanks, I just pulled the ratings off cable sold by B&Q, like like they quoted it wrong at 23, other products say 27

http://www.diy.com/nav/fix/electric...th-Cable-Grey-2-5mm-25m-9290436?skuId=9300135

 

[ericmark]

1) Cable with exception of flex does not have an amp rating what it can carry depends on how it is installed so a 2.5mm sq cable can be anywhere between 13.5 amp and 33 amp depending on the working temperature of the cable and the route it takes.

2) Although you could get 26 amp from a double socket the sockets are not rated for 26 amp and to get two portable appliances over 2kW (fixed appliances need separate feed anyway over 2kW) which will run for an extended period is very unlikely.

3) Anything plugged into a 13A socket with the exception of some special medical items should always have a 13A fuse as a maximum. Yes I know some items break the rules but one can't consider rule breaking.

4) As far a kitchens go often they don't have a true ring but two conductors in parallel purely because it is hard to get two 4mm sq cables in a 20A grid switch. And normally the spurs will feed a single socket feeding in some cases fixed appliances, since not a true ring final circuit these are not covered by the same rules.

5) Because of the 3 meter rule (reg 433.2.2 and 434.2.1) within the rules it is difficult to install multi unfused spurs with out breaking the rules. No rule can caterer for rule breaking.

So as long as done within the rules laid down there are really no problems.

I would agree there is a problem with rings in that often the ring is broken. However this is in the main due to people working on the system not following the rules and one should not alter a rule because it is broken.

It would be like reducing the speed limit from 40 MPH to 30 MPH because there has been a series of accidents caused by people doing 60 MPH. Rather a pointless exercise.

The rule about connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit will in the main remove the problem. What I suppose should be asked is:-

Should fixed appliances over 2kW be supplied with a 13A plug? I would include tumble drier, washing machine, and dish washer to the list. Also any electric fires fixed into for example old fire places. Clearly immersion heaters, showers, and other heavy use items would also be included. If they are to be fitted with a plug then the un-fused 15A plug to BS 546 would be far better as often the plug and socket is hidden and there only for easy of maintenance so you don't want the heat generated by a fuse. A dedicated supply from a MCB/RCBO is far more appropriate.

 

[ericmark]

If you go to this site you will find what the rating for twin and earth cable is.

The rules say follow manufactures recommendations not DIY sheds. May be we should ban all sheds because some give misleading information?

 

[PrenticeBoyofDerry]

As EFLI has mentioned your ratings of cables are incorrect, therefore the point you have made is not a valid point.

Ok thanks, I just pulled the ratings off cable sold by B&Q, like like they quoted it wrong at 23, other products say 27

http://www.diy.com/nav/fix/electric...-Grey-2-5mm-25m-9290436?skuId=9300135[/QUOTE]


http://www.doncastercables.com/technical/datasheets/multi_core
http://www.prysmian.co.uk/export/sites/prysmian-enGB/attach/pdf/6241Y.pdf

and this at B&Q
6242YH

 

[JohnW2]

Only one spur per 'point' is allowed on a ring although it can be done on a radial.

We've been through this one a number of times before. Although what you say is certainly 'good practice' (both to reduce the theoretical risk of high point loading on the ring and, probably more importantly, because of the difficulties of terminating 4 cables), I think you'll struggle to find anything in the regs which says that it is 'not allowed'. I suppose it could become 'not allowed' if >3 x 2.5mm conductors exceeded the manufacturer's instructions regarding terminal capacity - but that would equally apply with a radial circuit.

Kind Regards, John

 

[stillp]

May be we should ban all sheds because some give misleading information?

That's given me a great idea for a username... ... ... oh. hang on a minute...

 

[ban-all-sheds]

this site

Think how much more useful that would be if they also showed what the various installation methods were.

 

[JohnW2]

If you go to this site you will find what the rating for twin and earth cable is. ... The rules say follow manufactures recommendations not DIY sheds.

I'm not even sure that the manufacturer's instructions are necessarily relevant in this respect. If, for whatever reason, a manufacturer claimed current carrying capacities which were higher than those given in the Appendices of BS7671, you wouldn't say that those "manufacturer's instructions" over-rode BSt671, would you?

In this context, I would have thought that BS7671 is what matters, regardless of what sheds or manufacturers may say (unless the manufacturers quote lower CCCs than BS7671, in which case I suppose one would have to adhere to those lower figures!).

Kind Regards, John