Sub mains for extended distance from meter to CU

[Notasparky]

As the user name states, I am not an electrician but I am trying to be an informed customer when dealing with a friendly local sparky and the DNO/supplier whilst undertaking some major electrical works on my property and some advise on the best solution would be welcome.

The works to be undertaken:

• Move of meter from high level on an interior wall to a low level external surface mount box on the other side of the same wall
• Move and replace the fuse wire fuse box with a metal clad CU
• Improve the earthing situation of the property
• Complete rewire creation of new ring mains & lighting circuits to provide seperate circuits for differnt parts of the house.

The situation:

• The house is a 1950s bungalow with solid block walls
• Supply is a TT system from overhead cables that are clipped to the side of the house and the soffits before enter the loft breify and dropping down the the 100A DNO fuse and the meter.
• Current earthing arrangement is completely inadequate with 1x 1.5mm cable on the water pipe before it changes into plastic to the street and 1x 1.5mm to an earth stake in the front garden.
• The preferable location for a new CU is in a cupboard approx 6m of cable run from the proposed meter location and would require running in the loft above insulation that is yet to be fitted.

Suggestion I am hoping to put to SSE and local contractors include:

• Changing the earthing arrangement from TT to a from of PME if availble
• Installing a fused switch at a location close to the new meter to keep the new 25mm meter tails <3m and a new cable run from there to a CU.

Options I am considering for the meter to CU connections that I would like advise on please:

1. Short meter tails to an externally mounted box containing a fused switch or large double pole RCD and then run either SWA up the outside of the house into the loft and down to the CU or come out of the back of the box into a void behind a false wall and up into the loft that way.
2. As above in SWA tails to a fused switch located in the loft and then in SWA or other more flexible cable if mechanical protection is not required to the CU
3. Mount an RCD in the meter box and run the cables out the back of it into the false wall void and up into the loft.

This void is being created to hide dental heating pipes located on the opposite side of the door frame to the proposed electrical runs and would create a space approx 50-60mm deep and 70mm wide in a safe zone next to the front door.

Taking people's better understanding of the regs into cinsideration and the bend radius of the swa that I am expecting to need to use at some point in this (I use it a lot at work and know it can be a pig)

A) are any of these suggestions valid
B) which would people recommend I suggest to a qualified electrician.

Thanks

 

[Midwest]

The first part, changing the supply, you will need to enquire with your DNO to see if there's scope to change, and how much it will cost.

I would approach that aspect first before proceeding further.

 

[Notasparky]

The first part, changing the supply, you will need to enquire with your DNO to see if there's scope to change, and how much it will cost.

I would approach that aspect first before proceeding further.

Thanks, I have done so this week and raised a change of supply request with SSE, they are due to be in touch next week, I asked them about the PME and the relocation of the meter and all seemed ok. they could not stipulate the length of the meter tails over the phone but did say they will need to be 25mm.

What do you think about the options for getting from their fuse to a new CU?

 

[Pete999]

Thanks, I have done so this week and raised a change of supply request with SSE, they are due to be in touch next week, I asked them about the PME and the relocation of the meter and all seemed ok. they could not stipulate the length of the meter tails over the phone but did say they will need to be 25mm.

What do you think about the options for getting from their fuse to a new CU?

By 25mm I think they mean 23mm2 the CSA of the tails. As you have already approached SSE regarding moving the supply and costings.
I think your next step should be to contact electricians in your area to give you a costing of the other works you are intending.

 

[westward10]

Have you enquired about moving the supply near to the location where you want the new consumer unit.

 

[Midwest]

Thanks, I have done so this week and raised a change of supply request with SSE, they are due to be in touch next week, I asked them about the PME and the relocation of the meter and all seemed ok. they could not stipulate the length of the meter tails over the phone but did say they will need to be 25mm.

What do you think about the options for getting from their fuse to a new CU?

Have you enquired about moving the supply near to the location where you want the new consumer unit.

That would be my first option.

 

[Notasparky]

Have not spoken to the distributor about moving the supply that far and am unsure if that would be possible.

The thought process for moving it to the outside wall close to where the meter is now is that I believe there is a difference between moving it over and under 1m from its current location and to move it closer to the preferred CU location would mean running the overhead supply into the loft for 3-4m before coming down into a cupboard with no outside walls and locating the meter in there.

Besides the additional space required is that a feasible option when planning the layout of the new system? I have in my head that the supply should terminate at the entrance to the property?

 

[Midwest]

Have not spoken to the distributor about moving the supply that far and am unsure if that would be possible.

The thought process for moving it to the outside wall close to where the meter is now is that I believe there is a difference between moving it over and under 1m from its current location and to move it closer to the preferred CU location would mean running the overhead supply into the loft for 3-4m

I subbied on a job, where the DNO (SSE) did just that. You'll have to see what they say.

 

[R-fur]

I would want to get the SSE cables off my house. Would it be possible to run a cable from the nearest pole? If it is in or adjacent to your garden you could do the excavation which would considerable reduce the cost. The meter box could then be placed back to back with the CU, drill through the wall and the tails will be less than 3m so no switch fuse required.

 

[Notasparky]

The pole is in my back garden and my and my attached neighbours supplies are fixed to my house, at some point the previous owners of next door have moved their meter to the outside wall like my original thoughts but I believe they had kept the consumer unit in the kitchen back to back with the meter box. layout changes in my property mean this is a less feasible option.

I had not considered changing the cabling to an underground setup but I would still have the issue with locating a meter in the centre of the house or a longer run to the CU.

Thanks for all the responses people, This is giving me a lot to think about. It's only these 2 houses on the street that have an overhead supply, all the newer properties have an mordern underground (TN-C-S?) supply. I had previously dismissed contacting the DNO regarding the possibility of connecting to this system instead - should I put that back on the table in discussions with them next week?

I am intrigued by the possibility of the supply terminating in the centre of the property, This is a potential solution that I did not think was an option. Time will tell if SSE will allow me to request this.

 

[Midwest]

I am intrigued by the possibility of the supply terminating in the centre of the property, This is a potential solution that I did not think was an option. Time will tell if SSE will allow me to request this.

The only thing to be aware of, is their supply cable has no steel armouring like swa cable. Careful consideration needs to be taken on them routing their cable. Loft insulation, and mechanical damage from suitcase, Scalextric sets & Xmas decorations could be an issue

 

[R-fur]

Most DNOs have rules that say service cables in customers houses should be protected by a 100A fuse in in a fireproof pipe, but we have all seen loads of cases where this is not the case.

 

[DefyG]

You seemed quite well informed for a notasparky yet you say

(I use it a lot at work and know it can be a pig)
with reference to swa cable! so if notasparky, how come you use it alot at work?

 

[Notasparky]

SSE have been out, had a look and given me a quote to alter the supply.

There are a few details in the quote I need to discuss with my builders electrician but extending the supply into the property does not appear to be an option. They are stipulating 25mm tails of no more than 3m.

After reading what SSE have to say the currenty preferred solution would be for an externally mounted meter with 3m SWA tails to a 100A fused switch in the loft and then a further 5m run of 25mm swa clipped direct to a CU located in cupboard in the habitable space of the house.

Do you professionals out there fore see any issues with this as a plan for me to put forward to my builder who will be installing the meter box and his electrician who will be doing the works?

I am suggesting the tails are clipped to the external masionary (on a shaded side of the house) before entering the loft space.

As for temperature de-rating of the cables in a "cold roof" loft - all insulation is now currently below the cable runs for the existing wiring and shall remain that way for the house re-wire for the time being. Would that mean:

30degC ambient current carrying capacity, referance C for clipped direct = 146A?

An estimated maximum ambeint loft temperature = 50degC due to lack of insulation and therefore Ca = 0.82?

Would this still give a current rating of 119A for the cable after the 100A fused switch?

The loft is border with lights and a fixed ladder for access as this is where the boiler is located. I am not proposing that I suggest to the electrician the CU is located up there only the fused switch for the sub main, the operation of which should very rarely if ever be needed.

Are there any regulations which state this could not be done this way?

 

[Pete999]

SSE have been out, had a look and given me a quote to alter the supply.

There are a few details in the quote I need to discuss with my builders electrician but extending the supply into the property does not appear to be an option. They are stipulating 25mm tails of no more than 3m.

After reading what SSE have to say the currenty preferred solution would be for an externally mounted meter with 3m SWA tails to a 100A fused switch in the loft and then a further 5m run of 25mm swa clipped direct to a CU located in cupboard in the habitable space of the house.

Do you professionals out there fore see any issues with this as a plan for me to put forward to my builder who will be installing the meter box and his electrician who will be doing the works?

I am suggesting the tails are clipped to the external masionary (on a shaded side of the house) before entering the loft space.

As for temperature de-rating of the cables in a "cold roof" loft - all insulation is now currently below the cable runs for the existing wiring and shall remain that way for the house re-wire for the time being. Would that mean:

30degC ambient current carrying capacity, referance C for clipped direct = 146A?

An estimated maximum ambeint loft temperature = 50degC due to lack of insulation and therefore Ca = 0.82?

Would this still give a current rating of 119A for the cable after the 100A fused switch?

The loft is border with lights and a fixed ladder for access as this is where the boiler is located. I am not proposing that I suggest to the electrician the CU is located up there only the fused switch for the sub main, the operation of which should very rarely if ever be needed.

Are there any regulations which state this could not be done this way?

I think SSE mean that the Tails should be no more than 3 Mtrs from their service head to the connections to the CU