Connection a junction box to a 10mm ring

[jcdotcom]

Plugwash wrote

there is nothing wrong with mixing cable sizes in a cuircuit (and with very long runs you often have little choice because of volt drop and terminal size issues)

using cables with ratings below the current rating of the cuircuit is a different matter. Its generally considered best not to but provided the short cuircuit protection upstream is adequte and there is suitable overcurrent protection downstream it is ok.

look at your spurs from rings (protection from the plug fuse for the spur)

So, at the risk of flogging this one to death, there's nothing wrong in taking a 2.5mm2 line to a fcu from a 10mm2 cooker radial, because the 13A fuse in the fcu will pop, protecting both the line before the fcu and the line and appliance after the fcu?

 

[dw3101]

I personally wouldnt use 2.5mm but thats up to you..

10mm is the best idea, but its going to be hard without a 45mm patress.. the bigger the cable the better imho.

David

 

[plugwash]

So, at the risk of flogging this one to death, there's nothing wrong in taking a 2.5mm2 line to a fcu from a 10mm2 cooker radial, because the 13A fuse in the fcu will pop, protecting both the line before the fcu and the line and appliance after the fcu?

and with a short run the smaller cable shouldn't cause any problem with short cuircuit disconnection.

 

[notb665]

So how much current can 2.5mm2 cable take?

 

[Careful_Bodger]

My understanding is that Table 4D5A of BS7671: 2001 Amendment 1 allows for 20 Amps when 'enclosed in conduit in an insulated wall'. This increases to 21 Amps for 'installed directly in an insulated wall' and 27Amps when 'clipped direct'.

The IEE's Wiring Matters publication, dated Spring 2002, goes on to say that this allows a 2.5mm2 Ring Circuit to be protected by a 32 Amp breaker. (Thus relaxing Reg 433-02-04, which describes the, now apparently defunct, protective device 0.67 derating factor for Rings). But this (a ring main) is not the main point of the original qaustion.

These values (for a cable run) are restated in Table 4.7A of the Guide to the Regs (Ed 7), but all these are slightly higher than the prevously values given in Table 4.7 of the same (i.e. 18.5 Amps for 20Amps).

If anyone thinks my interpretation is a bit amiss then I like to continue this thread

 

[andy]

So, at the risk of flogging this one to death, there's nothing wrong in taking a 2.5mm2 line to a fcu from a 10mm2 cooker radial, because the 13A fuse in the fcu will pop, protecting both the line before the fcu and the line and appliance after the fcu?

yes, there is a problem with that. you should the same size cable as the main circuit to the FCU, because the FCU doesnt protect the cable before it, only after. if you have a length of 2.5 from a 40A breaker to the FCU, what happens if there is a fault just before the FCU. nothin there to stop the cable melting and possibly causing a fire, since the breaker is rated at 40A. if the FCU is located immediatly to the side of where its being fed from, there is very little chance of anything happening and the cable burning out, but the FCU could be some distance away


havin said all that, back to the ring/radial spur. you wire a 32A radial with 4mm, so you have to spur using 4mm. this is adequatly protected by the 32A breaker. if you spur from a ring using 2.5 on a 32A breaker, you use 2.5mm cable. the breaker aint gonna protect that to a certain degree, and the cable can overheat (altho there should not be more that a 27A load on a spur, but there could be a fault with the cable which is 32A. or some idiot with no knowlegde takes another spur from it)

 

[plugwash]

thing to remember is that damaged cables don't tend to start drawing 30 amps (and if they do you have a far more serious problem than the fact the cable isn't rated to deliver 30 amps to the fault)

most cable damage will (if it results in any current flow at all) result in a short cuircuit. So provided the earth fault loop impedence is kept low enough to fast trip the mcb the cable isn't going to have time to get hot.

any cable fault that did draw 30A would be producing enough heat to start a fire even if the cable supplying it could handle that current. Such faults are basically impossible to protect against lukilly they are also extremely rare.

 

[notb665]

That's what I thought. But only being a DIYer, I didn't like to say.

 

[kai]

Think about your mains service cable to your house, a massive fuse at the substation protects the street main, divided into thirds, ie the red yellow and blue lines, It can be rated at say 1200 amps etc, however your service cable branched off the street main is only 25.sq.mm. rated at 100 amp, what is there to protect the service cable if a fault occured just outside your house? - bugger all....

 

[jcdotcom]

The situation I have is that I've inherited a cooker radial, using 6mm2 cable, (I originally thought it was 10mm2, but it isn't!) and protected by a 30A cartridge fuse. (Cable run is under floorboards, and down hollow plasterboard stud partition, total length approx 4m) This has had a large cooker (10.7kW) running on it for over 5yrs (afaict) without incident (ie the fuse has never gone) This will be replaced by a separate hob/oven, rated at 5.8 and 2.4kW respectively, making a total of 8.2kW. (Diversity calcs suggest the old cooker should draw ca 21A, the new setup totalling ca 18A - no socket on the ccu in either arrangement)

Although the oven fcu will be right next to the hob disconnection switch, I'll try to use 6mm2 to connect into the live side of the switch if it'll all fit in the terminals.

Thanks for all the help guys.

jc