Sub Mains

[Harry7671]

Does a 16mm TWE sub main feeding a consumer unit with 2 x RCD's and a main sw, wired under floor boards need rcd protection ??

 

[Spudnik]

I would say yes.

But that depends on the exact route the cable is taking.

 

[Harry7671]

Thank you for your reply

The cable would run from the mains position (ground floor) up in boxing approx 100mm deep 200mm width then underfloor boards approx 15 meters (joists run the right way so does not need to run through joists but along side) to the other end of the house consumer unit.

i was led to believe Sub mains do not have to be rcd protected as the board its feeding has 1 main sw and 2 x rcds.

does anyone know the regs on this , ive looked but cant find anything .

 

[not clever]

RCD it or use swa.
Or make sure the t/e is below 50mm of all surfaces or clipped direct to surface or enclosed with in earthed trunk.

I'd go swa myself.

 

[Harry7671]

Thanks "not clever" i understand your input to abide for the 17th edition regs but i was led to believe sub main were excempt from this ?

Anyone else got any input this is a real grey area for me.

 

[not clever]

No grey area at all, 7671 is still relevant for submains.
Unless you can be sure that the t/e is surface mounted or in earthed metal enclosure or swa, then you'll be needing an rcd mate.
If the system is TT you'll need an rcd anyway.
I would also run a separate mec.

 

[old dog]

rcd is for additional protection as long as you have mechanical protection and can achieve disconnection times
you do not have to rcd however i would use swa bit more expensive but looks much more professional

 

[ian.settle1]

Thank you for your reply

The cable would run from the mains position (ground floor) up in boxing approx 100mm deep 200mm width then underfloor boards approx 15 meters (joists run the right way so does not need to run through joists but along side) to the other end of the house consumer unit.

i was led to believe Sub mains do not have to be rcd protected as the board its feeding has 1 main sw and 2 x rcds.

does anyone know the regs on this , ive looked but cant find anything .

If you did fit an RCD it would need to be a100mA as if you have 30mA ones upstream of it then the would be no discrimination between them if you protected the 16mm by a 30mA RCD.

 

[Spudnik]

If you did fit an RCD it would need to be a100mA as if you have 30mA ones upstream of it then the would be no discrimination between them if you protected the 16mm by a 30mA RCD.

And this is where the problem arises.

Trouble is, ANY circuit, if not mechanically protected or buried less than 50mm etc etc, must be protected by a 30mA RCD.

So, if the submain is anything other than SWA and not more then 50mm, then its got to have one.

However, if the OP can fix the cable along a joist 50mm or more away from floorboards or ceiling, then i dont see the need.

 

[Harry7671]

Thanks everyone for your input

Looks like i will be using a 100ma rcd with a 63amp 60898 along side.

and yes i will be running a 10mm earthing condutor along side the sub

 

[amp]

Hi
why put an mcb with the rcd just use double pole rcd with feeds in and sub main out saves on mcb

 

[chocolate]

Hi.

15 metres of T&E is a long run from meter to main switch. Also T&E is not an ideal choice for a sub mains especially if its run under floors without any serious mechanical protection. You shouldn't be protecting this cable with a 100mA RCD. Not allowed according to 17th Edition. It's either a 30mA at the meter end and then forget using any RCDs on the consumer unit as this will lead to inconvenient tripping OR you should run a SWA from a switched fuse ( from the meter ) to the consumer unit. The SWA should be three-core 25mm2, and there's no need to run an additional earth conductor as you'll be using the armour and one of the cores to do just that.

Chocolate

 

[markspark]

we;ve encoutered this exact problem at work,( local authority housing), and the answer we got from niccy bod was, unless the cables are buriied more than 50mm ( not likely) or run in trunking ( looks naff) than 30ma rcd is required, as for discriminastion, you can use a time delayed rcd and still achieve disconnection times! hope this helps.

 

[Harry7671]

we;ve encoutered this exact problem at work,( local authority housing), and the answer we got from niccy bod was, unless the cables are buriied more than 50mm ( not likely) or run in trunking ( looks naff) than 30ma rcd is required, as for discriminastion, you can use a time delayed rcd and still achieve disconnection times! hope this helps.

Thanks mark very helpful indeed

 

[Harry7671]

unfortunalty im told no one makes 30ma time delayed rcd's , so the only round the problem to make sure cable is run 50mm away from ceiling and floors along the cable run ....

will be running swa in future as twe distrubution circuit wire in twe looks like a thing of the past!!!

no such thing as a 30ma time delayed rcd

 

[stretchyboy]

The requirement for protecting cables with an RCD when they are less than 50mm from the surface is for walls only though?
I know that I would personally favour an SWA anyday on sub-mains and have a dislike of the use of T&E for this purpose. Also the electricity boards will not accept SWA as a sub-main prior to metering if that is relevant here. But strictly speaking the RCD protection is only necessary for sub-mains within walls. The use of an RCD on the sub-main will also contravene 314:1 All circuits of an installation should not be connected to a single upstream RCD in order to avoid unwanted tripping, avoid hazards and minimize inconvenience in the event of a fault (reg 314.1)

I hope this isn't a spanner in the works ;-)

Plus, even if there was a time-delayed 30mA RCD it wouldn't meet the requirements anyway as 30mA RCD's are for fault protection and to limit the voltage/current flowing into someone during a fault or direct contact, but the time delay would not allow this to happen would it?

 

[MTI]

hi all, just been reading this thread and im not trying to hijack but i have found this really usefull! cheers! i have to do a sub main tomorrow and was wondering the same. i intended on using swa but my questions are..
1)will i need an isolator at the mains cut out possition? my sub is to another room but only 5m in length.obviously length is not important/part of discussion
2) what size swa? i am led to believe 7671 states 25mm tails but will i need 25mm? can i use 16mm as it will more than suffice considering single phase supply only provides 80A?
3) what size isolator at mains to cover appropriate swa size?
4) if swa is 3 core, i take it no seperate earth is needed as mentioned above? plus, surely a main earth as part of a sub would need mechanical protection?

sorry to but in and hope i dont cause us to go off topic! thanks, martin

oh, and another..if cu is plastic, should i just earth sheath at cut out end and tape neatly at cu or should i put clamp on for the sake of it?

 

[electro]

The requirement for protecting cables with an RCD when they are less than 50mm from the surface is for walls only though?
I know that I would personally favour an SWA anyday on sub-mains and have a dislike of the use of T&E for this purpose. Also the electricity boards will not accept SWA as a sub-main prior to metering if that is relevant here. But strictly speaking the RCD protection is only necessary for sub-mains within walls. The use of an RCD on the sub-main will also contravene 314:1 All circuits of an installation should not be connected to a single upstream RCD in order to avoid unwanted tripping, avoid hazards and minimize inconvenience in the event of a fault (reg 314.1)

I hope this isn't a spanner in the works ;-)

Plus, even if there was a time-delayed 30mA RCD it wouldn't meet the requirements anyway as 30mA RCD's are for fault protection and to limit the voltage/current flowing into someone during a fault or direct contact, but the time delay would not allow this to happen would it?

what a good post, i have read the thread from start to finish, here is your answer in a nut shell.
quality

regards electro

 

[MTI]

what a good post, i have read the thread from start to finish, here is your answer in a nut shell.
quality

regards electro

any ideas on my situation please electro? im running sub from original metre and cut out to allow cu in another/next room along so my swa is not a sub for elec board to use as my tails into their metre if you get my drift.

cheers, martin

 

[electro]

hi all, just been reading this thread and im not trying to hijack but i have found this really usefull! cheers! i have to do a sub main tomorrow and was wondering the same. i intended on using swa but my questions are..
1)will i need an isolator at the mains cut out possition? my sub is to another room but only 5m in length.obviously length is not important/part of discussion
2) what size swa? i am led to believe 7671 states 25mm tails but will i need 25mm? can i use 16mm as it will more than suffice considering single phase supply only provides 80A?
3) what size isolator at mains to cover appropriate swa size?
4) if swa is 3 core, i take it no seperate earth is needed as mentioned above? plus, surely a main earth as part of a sub would need mechanical protection?

sorry to but in and hope i dont cause us to go off topic! thanks, martin

oh, and another..if cu is plastic, should i just earth sheath at cut out end and tape neatly at cu or should i put clamp on for the sake of it?

Hi MTI

1. you will need a fused switched isolator as over 2M - 3M rule dependant on DNO
2. i would use 25mm or you will need to do calcs for 16mm, (and single phase can be 100amp)
3. it needs to be metal or use an earth tag to earth the outer armour. 80amp overcurrent if this ties in with your MD.
4. not sure of question? more info please (you do not need a seperate earth, mark cable both ends)

regards

 

[MTI]

cheers electro for the info so far. ignore the seperate earth question-was just checking. as swa is 3 core and not relying on sheath as main earth then all is well.

single phase 100A? correct me if wrong but although some propertys have 80 or 100 amp slow blow cut out, you can only physically draw 80A from any single phase supply? an engineer told me this and i just took his word for it.

think ill go with 25mm for what its worth and no questions asked. price is good so not bothered really and will avoid calcs as you mentioned

if i have seperate earth conductor combined within swa (3 core) and its earthed via nut, banjo and gland on isolator, surely its not mandatory to bond at plastic cu? may do it anyway but any advice most appreciated mate.
all companys i have worked for, have fitted 2 core subs and seperate earth to mutli story appartments etc but not bonded swa sheath at final cu.same goes with 3 core subs too. suppose its good practice but not as necessary with 2core and seperate earth

also, wont swa of say 5m take 100A? correct me if wrong? still going with 25mm but just out of interest

sorry, meant 16mm swa taking 100A+ ?

 

[electro]

cheers electro for the info so far. ignore the seperate earth question-was just checking. as swa is 3 core and not relying on sheath as main earth then all is well.

single phase 100A? correct me if wrong but although some propertys have 80 or 100 amp slow blow cut out, you can only physically draw 80A from any single phase supply? an engineer told me this and i just took his word for it.

think ill go with 25mm for what its worth and no questions asked. price is good so not bothered really and will avoid calcs as you mentioned

if i have seperate earth conductor combined within swa (3 core) and its earthed via nut, banjo and gland on isolator, surely its not mandatory to bond at plastic cu? may do it anyway but any advice most appreciated mate.
all companys i have worked for, have fitted 2 core subs and seperate earth to mutli story appartments etc but not bonded swa sheath at final cu.same goes with 3 core subs too. suppose its good practice but not as necessary with 2core and seperate earth

also, wont swa of say 5m take 100A? correct me if wrong? still going with 25mm but just out of interest

sorry, meant 16mm swa taking 100A+ ?

Hi MTI

A 100amp fuse will take 100 amps +
bonding a Cu if its plastic No,you will need to terminate the earth into the MET
25mm swa 145amps
16mm saw 110amps
these are rough guides no CF taken into account, as i dont know your exact run.

regards

 

[MTI]

hi electro, sorry if we got our lines crossed-bonding plastic cu? all i meant was protecting/earthing the sheath of swa at both ends. obviously with plastic cu and swa, there is nothing to terminate swa into. nobody i know would put a gland on swa at cu just to finish the job off nicely. i dont mean glanding it to plastic cu obvoiusly but just 'leaving loose' within a plastic 50x50 trunking or something as it comes out of floor as an example.
right, so metal clad isolator at mains cut out,
swa glanded into isolator,
25mm d. insulated tails from metal clad isolator into metre,
banjo on gland too with flying lead onto met in isolator box?
earthing conductor within swa onto met in isolator box or straight to met at supply and one back to metal box?
other thing is, i have a tns with clamp on incoming feed to house-no pme-bummer!
plus to make it worse, as normal in my area, next doors house is daisy chained into my job! would you link both clamps? i think supply company say no.

i ended up with this on a job last week and had to use a spike as no incoming earth

 

[electro]

Hi

you need the DNO in to remove the clamp and sweat a new eathing connectionTNS, or upgrade to TNC-S

regards