Ring final circuit grouping

[Tuttle]

Please could someone enlighten me on how grouping factors apply to ring final circuits run together, e.g. sharing holes through joists and walls, in a domestic situation?

For one RFC run in 2.5mm T&E under first floor boards (ref method B) the cable rating is 23A.

If you group two RFCs together, i.e. four 2.5mm T&E cables, then I think this counts as two circuits giving a grouping factor of 0.80 and therefore a cable rating of 18.4A, which is less than the allowed minimum of 20A for each leg of the ring.

This seems to suggest to me that you can't group a 32A 2.5mm T&E RFC with any other circuit?

What I really want to get to the bottom of is: is it ok to run two 32A 2.5mm T&E RFCs together, though the same joist holes, and if so how can this be justified?

Thanks

 

[Engineer54]

Another area where you need to use a bit of commonsense. Look at your question again, this time considering that it will be highly unlikely that either of these two ring circuits will be carrying it's full load capability (or anywhere near it).

 

[Amp David]

Domestic and grouping factors, in the real world it doesn't exist does it?

 

[malcolmsanford]

Tuttle I've been doing this for more years than I care to mention and have to agree with E54 that in these type of situations we are always in danger of trying to "over engineer" things.

Yes your correct technically in some ways, but I have yet to see any problems caused by bunching cables through a joist in any domestic situation and you running 2 RFCs together, or any circuits together in a domestic situation is unlikely to cause any problems at all

 

[Engineer54]

Hate to think what the de-rating factors going to be where all the installations cables come together to drop down to the CU then!!! lol!!

As Malcolm has stated, don't over think these situations, the real loadings on a domestic installation is generally far lower than most think, or even calculate it will be....

 

[telectrix]

as long as you don't need lubricant to pull cables through joists, i wouldn't worry about grouping factors. agree with previous posts by the other members of the old farts club.

 

[malcolmsanford]

as long as you don't need lubricant to pull cables through joists, i wouldn't worry about grouping factors. agree with previous posts by the other members of the old farts club.

Oi you less of the old

 

[Octopus]

Most CU's have all of their cables exiting together - does that mean that all the circuits need derating? I don't think so.

 

[Engineer54]

Theoretically and correctly, ...Yes!! But the grouping de-rating tables in BS7671 and OSG's are based on worst case scenario's, ...(eg, all circuits operating at it's full load capacity) which 9 times out of 10 or more, will not be the case!!

 

[drew35]

What happens when they add underfloor heating pipes and get the void nice and toasty. Your grouping factors fine but you could fry an egg on your cables!

 

[Engineer54]

That's not really your concern is it, you can't design and install an installation based on what might or might not happen in the future. (especially second guessing a plumbers installation) So long as You ideally keep all cable runs away from existing central heating pipework as far as is possible, then all should be fine...

 

[Tuttle]

Thanks guys, I thought that was the case, I just wanted to make sure

 

[fylthespider]

I seem to remember reading somewhere that the classic ring final (2.5 on 30A ) had all the factors taken into account (insulation, grouping, thermal,etc).
it kind of makes sense cos effectivly you got 5mm cable. 2x2.5mm equals 2x27Amp=54amp. so your only loading 3/5 of cable capacity.
lots of ISH-ness to be taken into account of coarse.
only ever seen overheating in old pub with about 20 circuits pulled tight in joist holes.... it did catch fire.
bit unlikely domestic.

 

[ackbarthestar]

Hate to think what the de-rating factors going to be where all the installations cables come together to drop down to the CU then!!! lol!!

As Malcolm has stated, don't over think these situations, the real loadings on a domestic installation is generally far lower than most think, or even calculate it will be....

I have found grouped cables running up from CU into a ceiling void are usually quite drafty and thus have a 'cooling' effect. With this in mind I tend to drop the Cg at this point and only ever consider grouping where cables are bunched together with cable ties.
However, each installation has its own problems and needs to be looked at on an individual basis

 

[lauriehurman]

OSG 7.2.1
"if the following rules are followed, derating for grouping is not necessary."