Sub mains for extended distance from meter to CU

Notasparky · Jan 25, 2020

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:

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.
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
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

westward10 · Feb 1, 2020

You can locate the switch-fuse where you wish provided it has suitable access and does not exceed 3m from the service head. Best person to ask is your builders electrician.

DefyG · Feb 1, 2020

I personally do not like the idea of a switch fuse isolator in the loft, although have seen it before!
I would recommend an externally mounted IP rated enclosure (or another meter box) adjacent to the meter box housing the S/Fused Isolator and run the swa from there. Meter tails will be less than a meter in that case. Explore other routes for the swa!

As for calculating current carrying capacity, I would leave that to the professional electrician doing the work depending on installation method he/she suggests.
The DNO OCP device is likely to be rated at 100A irrespective of you calculating 119A, or the tabulated CCC of the cable.

davesparks · Feb 1, 2020

Notasparky said:
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.

25mm is big and awkward to work with in a domestic setting, and usually completely unnecessary.

Unless youve got an exceptionally high power demand a 63A supply to the CU will normally be more than enough.

Notasparky · Jun 6, 2020

Resurrecting an old thread now, but things have been on hold for a while.

Am keen to get the meter move sorted out and the full house rewire that follows done but before that can happen there are a couple of issues that I would welcome the forums advise on please, though first some history:

Just before lock down work was being done on my place whilst I was away for work myself. 2x new recessed meter boxes had been installed adjacent to each other in a location agreed with SSE, one for their meter and cut out and the other for a fused switch.

The builder who had kindly taken charge of organising everything whilst I was not around arranged for an electrician to:

Install the fused switch
Mount the new consumer unit in the new location (old hot water tank cupboard, I put a combi in so this is now an empty space)
Run SWA between the 2x locations and terminate ready for the meter to be moved.

Prior to leaving I put notices on the walls for the preferred location of the new CU and spoke with the builder about the preferred route of the SWA in the loft.

All appeared to have gone well until I returned home into lock down to find the new consumer unit had been installed in a different location to where I had asked and to do so they had taken out the plumbing to my shower to fit the CU in and the route in the loft was not what I was expecting either.

I had asked for the CU to be at high level on the left hand wall of the cupboard, just below the celling with a second option of at mid level on the same wall if there are any regulations say I cant have it up high?

Instead it was put at mid level on the back wall of the cupboard, making it hard to access over the machine machine that will be located in there and disbanding the shower plumbing. I had also asked for the SWA to be clipped to the purling or other way of keeping the cable off the ceiling rafters as only half the final roof insulation is currently in place to make the wiring and plumbing works easier. Instead it was neatly clipped to the celling rafters and would be sandwiched between the layers of insulation when the works are complete.

The first thing I had to do when returning home was disband much of what had been done and pull back the SWA so that I could plumb the shower and this is the currently state of play.

I am considering my options, either asking the sparky to come back and redo it or just redoing it myself and getting them to pick up from where they left off but with everything in a different location.

The 25mm SWA that was used is too short to be rerouted to the preferred location in the cupboard if it remains clipped to the celling rafters and would need to cross over several water pipes and over roof timbers that tie the rafters together and does not appear to be a sensible route to get it to where it needs to be, run higher up as per my preference it would be even shorter and is 3-4m shy.

A couple of questions here please:

Are there any regulations in the new book that stipulate the mounting locations of a CU which would prevent it from being mounted close to the celling?
Are there any issues with clipping SWA direct to purlings onto which T+E is already mounted for some lights and sockets?

I would very comfortably pull in and terminate 25mm SWA myself, in fact whilst this work was going on in my house I was in a factory in Scotland working with the same size cable for the VSD supply to the new motor I was installing and pulling back/re-terminating 95mm SWA for the associated control panel we were moving and re-wiring. I am an industrial burner engineer and the equipment we install often uses some chunky cable. (just to give some context picture of the burner in question is attached - there is more to go on top still from the scaffolding)

Which brings me on to my second set of questions, when looking at the works in the new meter boxes the SWA was not terminated on the outside of the box like I would have expected, I would have expected to see it glanded off on the outer box and run the inner sheath through a stuffing gland on the fused switch with a separate earth from the banjo to the casing of the fused switch.

Instead a large hole was knocked in the meter box entry location and the full cable was glanded off on the fused switch - this must have been a challenge given the bend radius of the cable and result in it being incorrectly terminated with the sheath showing.

The meter tails to the adjacent box are also a point of concern, additional holes had been knocked (none of these holes were made with a hole saw or a die punch but look to be a hammer and screwdriver affair) towards the rear of the box to enable the tails to be passed though the cavity wall between the boxes. I would have at least expected cut holes and rubber grommets to prevent the chafing of the outer insulation that is evident on the meter tails.

Would it be correct to gland off the SWA on the outer box?
Is it ok to run the meter tails though the cavity between the boxes and should it be glanded?

I am not really planning on doing too much of the works myself, nor am I after a stick to beat the professionals who have done the works so far but rather some advise on the best practices in this situation please so I can continue with the renovations.

And now for some pictures:

Random photo of my day job.