Maximum Length of Ring Final Circuit

[MicroMan]

Can anyone help to clarify what the maximum cable length for a ring final circuit is (conditions below).

The maximum surface area that a ring circuit may provide for is well documented being 100m^2, this of course can have various cable lengths from 10m*10m=40 meters to an unlikely 100m*1m = 202 meters length. In addition it is often quoted that the maximum cable length is 50 meters is permissible in a ring final circuit wired in 2.5mm^2 with a cpc of 1.5 mm^2, however the OSG quoted figures don’t seem to agree with this…. Information from table 7.1, pg 42, OSG: For a device rating of 32A and cable size phase/cpc of 2.5/1.5 mm^2, with a Type B CB, M6 cable installation in PVC (without de-rating) and a Ze <= 0.35 ohm with the TN-C-S earth a maximum cable length of 84 meters is given. Is 84 meters the correct figure? (without considering de-rating for thermal conditions etc).

The reason why I’m asking: The final ring installation I’m planning is 17 meters from the consumer unit to the first / final socket in the ring. The supply from the CU has to go via a single story building (above bathroom & kitchen) prior to entering the main body of the house where the two ring mains are. The route of the cable shall be in the ‘single story’ loft space so I will take into account thermal de-rating as the temperature is liable to rise above 30C in summer.

Any help on this one appreciated.

Supply Information:
Supply 240V, Single Phase, PSCC = 16KA via BS1361 cartridge fuse in termination.
Earthing Type: PME terminal, Maximum earth loop impedance = 0.35R.

Installation Details:
Ring final circuit protected by Type B Single Pole MCB (MK 5932s, 32A).

 

[Adam_151]

Its limited either by volt drop or EFLI (assuming worst case Ze for either TNCS OR TNS). Of the top of my head, I think the 88m figure is from volt drop, because in this case I think that bites first.

If you want a longer rfc, do your own design calculations, chances are you'll need a bigger cable (though not always, you you pick a standard arrangement that happens to limited first by EFLI, and you measure and find your Ze is lower than assumed, then an increase in cable size is not always needed)

And please measure your supply chararistics, 16kA, and TNC-S with 0.35ze is straight out the mouth of someone who wouldn't know what an EFLI meter was if they found one in their cornflakes, I'm willing to bet you havn't got a 16ka PFC unless you have a 11kv transformer right next door, anyway measurement is far better as it proves the company earth hasn't dropped off (more relevant to TNS though, but sometimes they manage to install a TNCS earth terminal and forget the link* )


*and if you're really unlucky they'll have connected it to phase (ask dingbat about that one!)