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

 

[ayjay]

if the future part is to have an electricity board meter and fuse, it will have to be run in electricity board concentric armoured aluminium core cable , in a 2" duct from the existing service i would think-- thats what i used to have to do when i worked on flat conversions
the ducting tube couldnt have more than 2 -90 degree bends and it had to be rewireable

 

[telectrix]

when you say " through the floor structure" what do you mean? ground floor or 1st floor, floor? is the ground floor concrete ?

 

[Colin33]

if the future part is to have an electricity board meter and fuse, it will have to be run in electricity board concentric armoured aluminium core cable , in a 2" duct from the existing service i would think-- thats what i used to have to do when i worked on flat conversions
the ducting tube couldnt have more than 2 -90 degree bends and it had to be rewireable

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.

 

[Colin33]

when you say " through the floor structure" what do you mean? ground floor or 1st floor, floor? is the ground floor concrete ?

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.

 

[Spudnik]

Split con could be used, however, this must be treated the same as twin and skin if its buried.

 

[Colin33]

Split con could be used, however, this must be treated the same as twin and skin if its buried.

Yes, but with this in mind then surely using twin/&earth is cheaper and easier?

 

[AJP]

I'd go for 16mm t&e personally. Don't forget your volt drop. I'd fit a KMF cut out and come straight off the tails. If it's over 32A you don't have to have a RCD, but it's always good too IMO. That then could be used as a permanent supply. As Long as your cable is proected, your service provider shouldn't mind!

 

[Spudnik]

Yes, but with this in mind then surely using twin/&earth is cheaper and easier?

It is cheaper, but it certainly aint easier.

 

[Colin33]

I'd go for 16mm t&e personally. Don't forget your volt drop. I'd fit a KMF cut out and come straight off the tails. If it's over 32A you don't have to have a RCD, but it's always good too IMO. That then could be used as a permanent supply. As Long as your cable is proected, your service provider shouldn't mind!

I was assuming that all unprotected cables had to be rcd'd now, are you saying that if it's over 32amp it doesn't have to be? Do you think that rather than a KMF and an rcd, a double pole rcbo would suffice? It's a bit tight so need to keep as compact as possible!

 

[johnboy6083]

I'd go for 16mm t&e personally. Don't forget your volt drop. I'd fit a KMF cut out and come straight off the tails. If it's over 32A you don't have to have a RCD, but it's always good too IMO. That then could be used as a permanent supply. As Long as your cable is proected, your service provider shouldn't mind!

I don't know where the ommitance of RCDs for circuits over 32a comes from. If you bury any cable in a domestic situation that has no earthed metal protection, and is buried less than 50mm, then it definitely needs to be RCD protected.

 

[Colin33]

I don't know where the ommitance of RCDs for circuits over 32a comes from. If you bury any cable in a domestic situation that has no earthed metal protection, and is buried less than 50mm, then it definitely needs to be RCD protected.

This is what I thought... anyone else care to comment?

 

[mogga]

if the future part is to have an electricity board meter and fuse, it will have to be run in electricity board concentric armoured aluminium core cable , in a 2" duct from the existing service i would think-- thats what i used to have to do when i worked on flat conversions
the ducting tube couldnt have more than 2 -90 degree bends and it had to be rewireable

concentric isnt "Armoured" btw

 

[ayjay]

my mistake there mogga,all i know is we had to use the same concentric cable as is used in normal supplys that come in from outside,this was supplyed by the electricity board free of charge,- it can be looped off of the existing service head via a cut out and a service head and meter installed at the far end,we just ran the cable and they connected it- fixed onto the correct mounting board
this was a few years ago mind, its prob changed now also different authorities have different rules

 

[mogga]

Yes, but with this in mind then surely using twin/&earth is cheaper and easier?

16mm twe doesnt have 16mm earth sp con does

 

[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.