Just a thought on RFCs

Pete999 · May 30, 2017

After reading a post yesterday regarding a Timber House Install, got me wondering, someone mentioned, can't remember who it was, and asked has anyone ever wired a RFC in 4mm2 cable, now I have never gotten involved in all this fancy Timber all insulated structures, so I asked myself the question, "if after all calculations", you came to the conclusion that normal 2.5mm2 RFC was, or did not meet the requirements after applying all the derating factors, you needed to install 4mm2 instead of 2.5mm2, how would you note the distraction from the, norm, would you just add a note on the certificate? describing what and why you had installed the circuit in this way. Just asking out of interest.

D00DY_T00DY · May 30, 2017

Flanders said:
Used 4mm for ring circuits with long runs on commersial installs and 2.5 mm for lighting circuits , makes connecting up click roses difficult though when you use 2.5mm , we just fill out the certs with the relevant details

After 4 years of house bashing those numbers just seem wrong to me D:

davesparks · May 30, 2017

Pete999 said:
Not disputing that Westy just wondered how people document it that's all.

Just record the cable size In the appropriate box on the schedule of test results.
If providing design paperwork with the installation then it would be recorded in there too, but that's almost certainly not going to be required, or done, for a domestic installation.

Toneyz · May 30, 2017

davesparks said:
It's common on commercial specs to standardise lighting as 2.5mm minimum size and small power as 4mm minimum size. This is done to negate grouping factors and make calculations quicker and easier.

Used 4.0mm for RFC but not 2.5 for lighting yet guess you have to take consideration of terminal capacity /back box room

westward10 · May 30, 2017

anthonybragg said:
Used 4.0mm for RFC but not 2.5 for lighting yet guess you have to take consideration of terminal capacity /back box room

Large warehouses I have used up to 4.0 on the lighting terminated at 13A sockets. Fitting any more than one 2.5 at a Klix terminal is stupid as they do not terminate correctly.

d.l.rob · May 30, 2017

It's stated on p82 or p84 of bs7671 that every single leg of cable on a ring final circuit has to be able to manage 20A alone for the ref method it's Installed. P364 of bs7671 gives the current carrying capacities of twin and earth cable. A ring circuit wired with a ref method of 101 in 2.5mm won't achieve this regulation becaus it can't carry 20A so in that instance a 4mm twin and earth cable would be suitable. Hope that makes sense? This is a scenario when you would use a 4mm twin and earth to wire a ring circuit