rcd in cu

[oldfart]

Gota nice new shiney split load Cu 6 +6, the rcd is a 63A 30mA, i have the option of changing this to 100A 30mA (got a spare compatable) would this option increase my diversity? or cause me probs!

On the rcd side i would have the following

1x 50amp = shower currently 7.5 (upgrading unit & installing 10mm)
1x 32amp = cooker max working load 7.5kw
3x 32amp = ring mains
1x 32amp = circiut for 2x plinth heaters max load together 6kw.

Service fuse is 100amp

 

[mapj1]

The 100A model will never do any harm, assuming the RCD sensitivity to earth faults is still 30mA in both cases. Given the 100A company fuse I hope the house is small enough that all three ring mains will not be fully loaded at once (i.e. the total floor area is less than 3 times 100 square metres. Actually, given what you are hanging off it I may be more worried if you were retaining the 63A one.

 

[oldfart]

Thanks map you,ve confirmed my thoughts
the third ring is more of a convienience the cu is directly below a new master bedroom and it was easier to give it its own ring, the plinth heaters will probably never be used to full capacity, but if they are there i thought i ought to cater for them, by the way i,m giving them their own radial circiut would 2.5 be ok?

cheers

 

[mapj1]

Don't see why not - but any single radial circuit in 2.5mm should almost certainly have an MCB at 16 or 20A (or a 13A fuse at one end) depending on thermal insulation around cable, all usual caveats about proper routing methods etc, but otherwise fine.
If both heaters share an MCB (even if they don't share a cable) then the cable had better be good for 30A if that is the breaker rating - which probably means using 4mm or splitting into 2 16A circuits each with own MCB or fuse if its more than 20A total. Best to double check those wattages carefully.

 

[plugwash]

how are 2 runs of 2.5mm from a 32A breaker running to two fcus any worse than 2.5mm spurs from a ring?

i remember a post on screwfix from a guy who failed such a setup and when the niceic look at his report they overturned the failure and said it was fine to do it that way/

 

[mapj1]

iI agree ts fine if there are 13A fuses either at load or source end of the 2.5mm - but not if there isn't. It is not clear from the description if there are load end fuses..

 

[Big_Spark]

Oldfart, not all the rings need to go on the RCD unless this is specifically something you want. The Upstairs ring can be on the normal side, and unless you run equipment outside from any of the other two rings then they need not be on it, the obvious exception being your Kitchen ring which should be on the RCD regardless.

 

[oldfart]

thanks for the input guys,
FWL duely noted thanks,
my concern with these damn heaters is the potential power pull. Its a new extension, when i planned the kitchen i took a spur from two ring main sockets at worktop height with 13amp fscu and and sunk sockets below to service these heaters, they in turn have 13amp plugs.

however realising the potential of 6kw on the kitchen ring main prompted me to pose the question about load. General advice was a seperate circiut.

With a new cu going in i have the option of creating their own circiut but concerned over cable size, if the right size cable is used say 6mm could they share one cable and 32amp mcb?

I should say the old fellas a retired sparks, (dark ages) he,s helping me out with the lecy but a bit stuck in his ways, with the help of you guys i,m trying to re-educate him in his twilight years! ( I should have paid more attention all those years ago when he had me chasing out and crawling under floors)

cheers

 

[plugwash]

you have a few options for the seperate cuircuit.

a 32A radial in 4mm or 6mm would be one option but 4mm is kinda borderline for such a cuircuit and 2x6mm and 1x2.5mm in a FCU would be a pain in the arse to terminate especially if the cable runs are in such a way as to have no movement in or out of the box.

personally i think the best way is to run 2 seperate runs of 2.5mm ideally from seperate 16 breakers but its ok to put them on a single 32A breaker if you are short on space.

 

[Big_Spark]

Oldfart, I am assuming that these plimth heaters are thermostatically controlled, as such the full potential load is not likely to be 6kW.

6mm is OTT for this circuit, personally I would do one of two things.

Option 1: Install a radial circuit specifically for the plinth heaters using 4mm T&E feeding Switched Fused Spurs. These Spurs then feed Flex outlet plates, using 2.5mm T&E, with the Plinth heaters hardwired into the F/O Plates.

Option 2: Install a pair of radials, one for each plinth heater, using 2.5mm T&E. Again feeding Spurs and F/O plates as above.

Option 3: Install a small ring main using 2.5mm T&E feeding the pair of spurs with all other connections as in option 1 above.

Honestly, I would recommend Option 1 as it deals with all the potential loading issues, keeps the circuit simple and the cable size is easily terminated in spurs, however the decision is yours.

I would recommend that these heaters are either protected by an RCBO (or pair for two circuits) or from the RCD side of a split load board. The high risk that exists for these heaters to be splashed or otherwise exposed to liquids is high and the necessary safety concerns need to be addressed sensibly.

 

[Big_Spark]

you have a few options for the seperate cuircuit.

a 32A radial in 4mm or 6mm would be one option but 4mm is kinda borderline for such a cuircuit and 2x6mm and 1x2.5mm in a FCU would be a pain in the a**e to terminate especially if the cable runs are in such a way as to have no movement in or out of the box.

personally i think the best way is to run 2 seperate runs of 2.5mm ideally from seperate 16 breakers but its ok to put them on a single 32A breaker if you are short on space.

Plug, this is rubbish advice mate. 4mm can easily handle the loading of these two plinth heaters so for you to imply otherwise is not warranted.

Suggesting that two seperate circuits can be connected to a breaker that exceeds the current carrying capacity of the cable of those circuits is simply stupid, sorry but no other words seem to fit, this is a breach of the Regs, good practice and simple common sense. It is practices like this that cause the type of electrical problems that has caused the knee jerk reaction of the ODPM in creating Part P.

 

[securespark]

Jim

how are 2 runs of 2.5mm from a 32A breaker running to two fcus any worse than 2.5mm spurs from a ring?

i remember a post on screwfix from a guy who failed such a setup and when the niceic look at his report they overturned the failure and said it was fine to do it that way/

I take it Plug was using this to back up his theory.

 

[Big_Spark]

Well the NICEIC are WRONG as it is a clear breach of BS7671 and so if this report is correct then this simply highlights the fact that the NICEIC think they own the electrical Industry and are some form of God to it.

Further, two spurs from an existing circuit are not the same as two radial circuits from a common breaker.

The ring the spurs come from has been designed to operate in these conditions, and the load for the individual spurs is limited by the loads likely to be placed upon them. Two individual circuits running from a common breaker that exceeds their CCC do not have these limiting factors, further, as these are on a 32A breaker, it is possible, and reasonable to assume, that at some point in the furture an individual could believe it to be safe to load the circuit(s) to capacity. On a ring this electrical stress is seperated along the legs of the ring and the cuit conductors do not actually carry the full load of the circuit for this reason, in this situation you could easily load one radial of this setup to beyond the capacity of the cable to cope, thus intorducing a potential for a dangerous failure of the circuit.

If both were run from a 20A breaker, whilst still questionable, at least the circuits could not be overloaded, however the breaker could not cope with the full potential load of the plinth heaters.

 

[ban-all-sheds]

NICEIC think they own the electrical Industry...

They are getting there.

Sickening how after years of banging on about the qualifications needed to be a NICEIC contractor, and how this made them all nice and safe (we'll ignore the small print of the way it actually worked with qualified supervisors), as soon as the Part P Competent Person scheme came into being they were right there at the front with their "electrician-lite" Domestic Installer set-up designed to rope as many kitchen/bathroom fitters, plumbers etc into their camp as possible.

 

[oldfart]

Thanks all, i really appreciate your help, its interesting to see the usual names appearing when viewing and searching topics.

Unknown (or probably not) to yourselves, using the search engine you,ve all helped me in the past, thanks for dedicating your spare time to this site (and to my extension!)