Submain ?

[Colin33]

Hi all. I'm new here and after some help please...

I'm currently wiring an old house as complete renovation progresses. The house is to be divided into two some time in the distant future, so two separate consumer units are being fitted. But, for now, there is to be just one (existing) 100amp supply to the property. I have wired the first 'half' of the house and need advise on the best way to run a submain to the next CU. The client does not wish to use SWA prefering not to have cables visible. Therefore, I'm considering running 10mm twin/earth through the floor structure of the building. I realise that this will need to be RCD'd but need advice on the best way to do this. Could the submain be run from a 40amp 30ma RCBO in the first consumer unit or does it need double pole isolation as well? I realise that there will be no discrimination from the CU at the other end of the submain but not sure there's anything I can do about this!

This is a TT system, and the demand is very low. No showers or electric heating. All energy efficient lighting. I am sure that the 100amp supply is sufficient for the two installations.

Any advise much appreciated

 

[MAGICIAN]

Remember and use mechanical protection

 

[malcolmsanford]

At some point in the future, the submain will be removed and a new supply will be brought in from the pole outside by the supplier.

The existing first floor (wooden) is to be raised a couple of inches in order to level, so the cable can run under the new floorboards. There will be no insulation to run through and a length of about 12 meters.

Hi all. I'm new here and after some help please...

I'm currently wiring an old house as complete renovation progresses. The house is to be divided into two some time in the distant future, so two separate consumer units are being fitted. But, for now, there is to be just one (existing) 100amp supply to the property. I have wired the first 'half' of the house and need advise on the best way to run a submain to the next CU. The client does not wish to use SWA prefering not to have cables visible. Therefore, I'm considering running 10mm twin/earth through the floor structure of the building. I realise that this will need to be RCD'd but need advice on the best way to do this. Could the submain be run from a 40amp 30ma RCBO in the first consumer unit or does it need double pole isolation as well? I realise that there will be no discrimination from the CU at the other end of the submain but not sure there's anything I can do about this!

This is a TT system, and the demand is very low. No showers or electric heating. All energy efficient lighting. I am sure that the 100amp supply is sufficient for the two installations.

Any advise much appreciated

As this seems to be a temp/permanent situation as you say and it's a TT installation and it is just 12 mteres have you thought about 4mm MICC that will give you 50amps as a temp supply, it is only 15 metres so not going to break the bank, and it can be buried and not need the 30mA RCD, though for TT it most likely need a 100mA one.

 

[Colin33]

No, I haven't considered MICC, but from what I remember the terminations will be quite bulky?
The main issue here is the cable protection and shock protection. If I use swa, then I don't need an RCD at supply end but will need KMF. With this I will also need a box to terminate the swa before it goes into the kmf !
If I use twin/earth I need KMF plus RCD.
It seems that what ever way I do it I need some bulky fittings which simply wont fit neatly within the space available next to the CU.
The most space saving solution perhaps would be a double pole RCBO on the end of some twin/earth, does anyone see any issues with this arrangement? (other than RCD discrimination which seems unavoidable here?)

 

[Des 56]

No, I haven't considered MICC, but from what I remember the terminations will be quite bulky?
The main issue here is the cable protection and shock protection. If I use swa, then I don't need an RCD at supply end but will need KMF. With this I will also need a box to terminate the swa before it goes into the kmf !
If I use twin/earth I need KMF plus RCD.
It seems that what ever way I do it I need some bulky fittings which simply wont fit neatly within the space available next to the CU.
The most space saving solution perhaps would be a double pole RCBO on the end of some twin/earth, does anyone see any issues with this arrangement? (other than RCD discrimination which seems unavoidable here?)

Use this
100 Amp Metal Clad Switch Fuse, Main Switches, UK Electrical Supplies

 

[Deleted member 26818]

30mA RCD protection is only required for cables concealled in walls at a depth less than 50mm, which don't have any other form of additional protection.
You haven't yet indicated that the cables will be concealled in any walls.
SWA isn't the only cable that has an earthed outer sheath, you could also use a BS8436 cable such as Flexishield or Guardian.

 

[malcolmsanford]

Spin doesn't BS 8436 still have issues with the size of protection device it can utilise and it's fault current limit of less than 200amps? Or have these issues been recitified

 

[Deleted member 26818]

I believe that many of the issues have been adressed yes.
Although I believe the advice is still that the cable is only suitable for use with Type B MCBs/RCBOs.

 

[Colin33]

Use this
100 Amp Metal Clad Switch Fuse, Main Switches, UK Electrical Supplies

Yes, but what about the RCD protection??

 

[Colin33]

30mA RCD protection is only required for cables concealled in walls at a depth less than 50mm, which don't have any other form of additional protection.
You haven't yet indicated that the cables will be concealled in any walls.
SWA isn't the only cable that has an earthed outer sheath, you could also use a BS8436 cable such as Flexishield or Guardian.

I have said that the cables will be concealed in the floor, under floorboards...which has the same implications as being concealed in the wall.
Flexishield is great, but only upto 6mm (32 amp) which is not big enough. 40A is required here.

 

[malcolmsanford]

Electrical & Physical Data | Flexishield not according to their data a 6mm can take up to 63amp on a tray and I would reckon you wouldn't be far off for erection methods F and G.

 

[Deleted member 26818]

I have said that the cables will be concealed in the floor, under floorboards...which has the same implications as being concealed in the wall.
Flexishield is great, but only upto 6mm (32 amp) which is not big enough. 40A is required here.

No it doesn't have the same implications as being concealled in a wall.
It is only cables concealled in a wall, and in safe zones that are required to have 30mA RCD protection.
6mm² BS8426 has a current rating of 58A when clipped direct, and you can now obtain 10mm² cable.

 

[Colin33]

Electrical & Physical Data | Flexishield not according to their data a 6mm can take up to 63amp on a tray and I would reckon you wouldn't be far off for erection methods F and G.

If I can persuade them not to stuff the floor around the cable with insulation then maybe this will be possible! I've never used this stuff, how is it terminated with regards to the aluminium tape? Surely this has to be earthed in order for it to be suitable for use without the need for RCD protection?

 

[malcolmsanford]

They do either a brass stuffing gland that will earth the sheath or a normal plastic/pvc similar to the type used for flex.

 

[Deleted member 26818]

It is terminated in a similar manner to T&E or FP200.
There is a bare earth conductor through the cable which is in contact with the shield.
With regards to the RCD protection, I suggest you refer to either Regulation 522.6.8 or 522.6.100 (amendment 1), and you will not that RCD protection is not an acceptable means of protecting the cable.
As such, the use of one without any of the other allowable means of additional protection would be a departure, and you as the designer would have to attest that the same degree of safety will be provided as would be achieved by compliance with the Regulations.

 

[Colin33]

No it doesn't have the same implications as being concealled in a wall.
It is only cables concealled in a wall, and in safe zones that are required to have 30mA RCD protection.
6mm² BS8426 has a current rating of 58A when clipped direct, and you can now obtain 10mm² cable.

This surprises me. I have always assumed that being concealed in a wall, or under a wooden floor, would carry the same degree of risk. I have just confirmed with niceic guy who agrees that it seems a little silly, but you're absolutely right.