SWA or else?

[fursdon]

As a result of intended "change of use", I have had a new supply/meter installed in an existing barn conversion.

There is pre-existing (certified) wiring and an existing CU on the opposite side of the barn to the new meter installation.

The logical route between existing CU & new meter is a 7m exposed surface mount run on a roof truss (there is also plenty of Western Power's ducting off-cuts left on site if relevant).

I am in the process of finishing the spec/drawings for tenders for all the associated works and would welcome advice on whether such a incoming connection is effectively mandated as a single run of 2-core 16mm² SWA only (earth on SWA allowed?)that I can leave un-specified or if there are any alternative solutions worth asking for costings, such as using twin long runs of "Tails" for power with parallel 16mm² earth?

I am principally interested in having facts prior to builders quoting their inhouse-spark back to me second-hand.

 

[amlu]

price in materials for a SWA vs "tails" wont be much different, SWA will need around 2 hours more labour to properly terminate, and if properly terminated can carry the earth on the armour.,

 

[bigspark17]

Aslong as tails are not buried <50mm , best way to go for me! Will need a switch fuse on tails at that length away from dno cut out!

 

[davesparks]

The answer on the SWA will depend on where the main peptide bonding is terminated. The armour will be sufficient to use as a cpc but not as a main bond.

 

[fursdon]

Thanks for all the in-puts & it looks like I can initially leave the spec open & then can query any differences that are evident on prices if they don't elaborate.

One immediate feed-back I got from a visiting sparks today was that it will need 3-core 25mm² SWA rather than 2-core 16mm² SWA as he said that the SWA is effectively the meter tails (which now all need to be 25mm²?), rather than just the 16mm² needed by the on-line calculator's nominal max 23kW load of the 100A DNO fuse.

I'm currently working on the assumption that the earthing set-up on the current cu can be used/re-certified as the main bond as it was also recently re-certified when a domestic solar PV array went live, so can't see his point for 3-core SWA.

 

[davesparks]

Thanks for all the in-puts & it looks like I can initially leave the spec open & then can query any differences that are evident on prices if they don't elaborate.

One immediate feed-back I got from a visiting sparks today was that it will need 3-core 25mm² SWA rather than 2-core 16mm² SWA as he said that the SWA is effectively the meter tails (which now all need to be 25mm²?), rather than just the 16mm² needed by the on-line calculator's nominal max 23kW load of the 100A DNO fuse.

I'm currently working on the assumption that the earthing set-up on the current cu can be used/re-certified as the main bond as it was also recently re-certified when a domestic solar PV array went live, so can't see his point for 3-core SWA.

The SWA will not be your meter tails, the meter tails will be feeding the switchfuse which supplies this SWA.

The SWA will need to be 3 core or have a seperate bond run back to the MET

 

[fursdon]

Dave

Thanks for the clarification.

25mm² meter tails to 100A DP switch, then 3-core required (either 3-core SWA or 2-core & separate 16mm² earth as any 3-core SWA is labour sensitive to cost) but 16mm² SWA is OK as per on-line Voltage Drop Calcs for 100A/23kW?

Obviously I can await an entertaining morning when the builders/sparks come to look prior to tendering.....

 

[davesparks]

Dave

Thanks for the clarification.

25mm² meter tails to 100A DP switch, then 3-core required (either 3-core SWA or 2-core & separate 16mm² earth as any 3-core SWA is labour sensitive to cost) but 16mm² SWA is OK as per on-line Voltage Drop Calcs for 100A/23kW?

Obviously I can await an entertaining morning when the builders/sparks come to look prior to tendering.....

It needs an OCPD as well, not just a switch at the origin. A switchfuse is the most sensible option.

You might want to review your cable calculations for a 100A supply.

 

[telectrix]

It needs an OCPD as well, not just a switch at the origin. A switchfuse is the most sensible option.

You might want to review your cable calculations for a 100A supply.

unless he fits a 80A SW/F.

 

[fursdon]

Again, thanks for all the input.

What I failed to mention at the outset was that, as there's been trenching and ducting involved for the new supply, a shed supply was also laid with 3-core 10mm². With a new sub-main for the shed plus the pre-exiting CU both to be connected, the 100A DP switch was only intended to get the meter installation completed without EDF throwing a wobbly.

The first thing after the DP switch will be a "standard" domestic 2-way CU (to take 25mm² in-coming tails & a second with 100A double pole switch & the relevant trips for "old" main & new sub-main circuits) and then split it out to old CU and new shed 10mm² SWA.

I leave the detailed design to you guys as the 'competent person' , but sadly, experience has left me with enough curiosity to make sure I know what to expect. Some times it's what I forgot to say (eg here - the shed supply) but sometimes it can be ".... priced to get the job...." without thought for future-proofing. Either way, I don't like surprises, so little tools (like TLC's cable calculator) are useful as a pointer to where the "challenges" may lie. I can foresee handling problems with 3-core 25mm² SWA as it's the best part of 1" diameter, and a sparks is expensive to pay as a labourer, so if the TLC calc suggests 2-core 16mm² with a separate earth may work or as ("amlu" suggests) 25mm² across a rafter may work, it gives me something to discuss with the 'competent person' when it comes to pricing.

Life's little challenges make it interesting and your comments give me something to ask "intelligent" questions about.

 

[Leesparkykent]

Again, thanks for all the input.

What I failed to mention at the outset was that, as there's been trenching and ducting involved for the new supply, a shed supply was also laid with 3-core 10mm². With a new sub-main for the shed plus the pre-exiting CU both to be connected, the 100A DP switch was only intended to get the meter installation completed without EDF throwing a wobbly.

The first thing after the DP switch will be a "standard" domestic 2-way CU (to take 25mm² in-coming tails & a second with 100A double pole switch & the relevant trips for "old" main & new sub-main circuits) and then split it out to old CU and new shed 10mm² SWA.

I leave the detailed design to you guys as the 'competent person' , but sadly, experience has left me with enough curiosity to make sure I know what to expect. Some times it's what I forgot to say (eg here - the shed supply) but sometimes it can be ".... priced to get the job...." without thought for future-proofing. Either way, I don't like surprises, so little tools (like TLC's cable calculator) are useful as a pointer to where the "challenges" may lie. I can foresee handling problems with 3-core 25mm² SWA as it's the best part of 1" diameter, and a sparks is expensive to pay as a labourer, so if the TLC calc suggests 2-core 16mm² with a separate earth may work or as ("amlu" suggests) 25mm² across a rafter may work, it gives me something to discuss with the 'competent person' when it comes to pricing.

Life's little challenges make it interesting and your comments give me something to ask "intelligent" questions about.

so the distribution circuits to each sub board are going to be protected by MCB's by the look of your post?

 

[fursdon]

Yes - one for old "main" board circuit (8-10m away across the barn) and the other for the new "shed" sub.

 

[davesparks]

Well that's just asking for trouble then isn't it.

Did the TLC online calculator not warn you about the discrimination issues?

 

[fursdon]

I started this post really only looking for a guide as to whether or not SWA would now be mandated for the surface mounted connection of a new supply across (an unheated space) to an existing CU some 8-10m away as the new connection will still need a "main switch" and "main board" to fit into the DNO's meter cabinet. Getting it into the meter cabinet is to avoid any possible "wobblies" from the meter installer and to allow the work to be split into before/after meter installation.

I had intended to use words in the spec such as:

"...Within the meter box ... provide the tails, main switch and main distribution board ... for the installation of a domestic meter by the supplier ... suitable for the connection of new sub-mains to the existing domestic Consumer Unit and to the new 3-core 10mm² SWA external supply...."

With the advent of 17th edition, I had rather assumed that the "...3m tails..." rule(?) indicated that I would be expecting designs to come back with 3-core 25mm² SWA and labour costs to match for handling a thick & unwieldy cable to go through a couple of 'S' bends so as to terminate in the meter box (which amlu etc seem to confirm).

The "old" cu shares the DNO's TN-C-S earth with a solar PV installation that will be unaffected, so, subject to re-testing, presumably negates the need for the SWA sheath as an earth. Even assuming the need for a separate earth, would not a separate 16mm earth run with 2-core 25mm² SWA be easier/less labour to handle?

As I see it, this leaves SWA only for physical protection on a surface mounted run. Davesparks comment made me think about what sort of design I might see submitted (and if my "ask" was feasible) for the small two way main board within the meter cabinet, which all seem to come with a DP-switch and couple of "free" mcbs - presumably these can be swapped out for down-rating RCBOs as sub-main cut-outs to meet the "...discrimination issues..." for the old CU and new shed sub-boards. Hopefully that could down-rate the 10m long run to 80A & hence viable in 16mm² SWA or old fashioned Double Insulated surface cable?

 

[shanky887614]

I started this post really only looking for a guide as to whether or not SWA would now be mandated for the surface mounted connection of a new supply across (an unheated space) to an existing CU some 8-10m away as the new connection will still need a "main switch" and "main board" to fit into the DNO's meter cabinet. Getting it into the meter cabinet is to avoid any possible "wobblies" from the meter installer and to allow the work to be split into before/after meter installation.

I had intended to use words in the spec such as:

"...Within the meter box ... provide the tails, main switch and main distribution board ... for the installation of a domestic meter by the supplier ... suitable for the connection of new sub-mains to the existing domestic Consumer Unit and to the new 3-core 10mm² SWA external supply...."

With the advent of 17th edition, I had rather assumed that the "...3m tails..." rule(?) indicated that I would be expecting designs to come back with 3-core 25mm² SWA and labour costs to match for handling a thick & unwieldy cable to go through a couple of 'S' bends so as to terminate in the meter box (which amlu etc seem to confirm).

The "old" cu shares the DNO's TN-C-S earth with a solar PV installation that will be unaffected, so, subject to re-testing, presumably negates the need for the SWA sheath as an earth. Even assuming the need for a separate earth, would not a separate 16mm earth run with 2-core 25mm² SWA be easier/less labour to handle?

As I see it, this leaves SWA only for physical protection on a surface mounted run. Davesparks comment made me think about what sort of design I might see submitted (and if my "ask" was feasible) for the small two way main board within the meter cabinet, which all seem to come with a DP-switch and couple of "free" mcbs - presumably these can be swapped out for down-rating RCBOs as sub-main cut-outs to meet the "...discrimination issues..." for the old CU and new shed sub-boards. Hopefully that could down-rate the 10m long run to 80A & hence viable in 16mm² SWA or old fashioned Double Insulated surface cable?

if you call a local electrician for a quote most would be quite happy to do the work if you installed the cable for them under there instructions.

25mm swa isnt a size that can be easily installed on your own over long runs so keep that in mind

 

[fursdon]

25mm swa isn't a size that can be easily installed on your own over long runs so keep that in mind

Thanks for the confirmation - in old money 3-core 25mm SWA must have all the flexibility of a 1" pipe & that's my potential problem whether DIY cable-monkey or not.

Finishing and connecting 25mm SWA will also not be cheep, hence the hope of finding enough information to be able to "...discuss..." any possibility of alternative designs in 16mm (or surface mounted double-insulated) if the ultimate electrician's full design will necessitate 25mm.

The "old" CU that's for re-connection was designed & last certified at 60A, so designing a down-rating of the new supply should be theoretically possible, but until actually surveyed by the electrician that's going to do/certify the work I'm working on the possibility that changes to regs etc may only allow a down-rating to 80A - again hopefully possible through a new small "main board" at the meter-cabinet with appropriate RCD/RCBO as 60/80A control cut-out switching so that I'm left with something less than 3-core 25mm SWA to handle.

 

[shanky887614]

Thanks for the confirmation - in old money 3-core 25mm SWA must have all the flexibility of a 1" pipe & that's my potential problem whether DIY cable-monkey or not.

Finishing and connecting 25mm SWA will also not be cheep, hence the hope of finding enough information to be able to "...discuss..." any possibility of alternative designs in 16mm (or surface mounted double-insulated) if the ultimate electrician's full design will necessitate 25mm.

The "old" CU that's for re-connection was designed & last certified at 60A, so designing a down-rating of the new supply should be theoretically possible, but until actually surveyed by the electrician that's going to do/certify the work I'm working on the possibility that changes to regs etc may only allow a down-rating to 80A - again hopefully possible through a new small "main board" at the meter-cabinet with appropriate RCD/RCBO as 60/80A control cut-out switching so that I'm left with something less than 3-core 25mm SWA to handle.

if the mainswitch is rated to 60amp that is the most you will have. fuseboards are not expensive why dont you get someone to actually look a it?

 

[fursdon]

why dont you get someone to actually look a it?

That's my intention, but as I live in the boonies, I think it's only considerate to give a flavour of what I want done, to save a wasted journey to quote for a job that's either impractical or too expensive to quote for. I'm also worried that a just any implied potential for a 3-core 25mm SWA job will reduce the number of people responding (to nil?).

Hence the OP & subsequent clarification to a possible wording and the idea that new supply/main-board and existing CU are 10m apart:

"...Within the (new) meter box ... provide the tails, main switch and main distribution board ... for the installation of a domestic meter by the supplier ... suitable for the connection of new sub-mains to the existing domestic Consumer Unit and to the new 3-core 10mm² SWA external supply...."

 

[davesparks]

The time saved by using 16mm vs 25mm will be in the order of a few minutes and is a ridiculous thing to base a specification on.

 

[shanky887614]

it depends on the distance to what size cable you need.

25mm is not a large cable maybe for domestic but most pubs and comerical places have bigger supplies for example i was doing some work on a pub and was pulling 70mm cables in


25mm is around 1.6kg a m so it wont be that bad to pull in

 

[fursdon]

The time saved by using 16mm vs 25mm will be in the order of a few minutes and is a ridiculous thing to base a specification on.

The load calculations are for the electrician to calculate/certify and seem indeed to be irrelevant to the 10m cable run (25mm²) it's back to the original dilemma - will SWA be mandatory for physical protection (as I hope the existing Solar PV DNO's TN-C-S earth or separate run will meet earthing requirements ) or is there an alternatives of Double Insulated surface cable "tails" (either clipped to the surface of a roof truss or ducted with the left-over DNO underground ducting)?

 

[davesparks]

The load calculations are for the electrician to calculate/certify and seem indeed to be irrelevant to the 10m cable run (25mm²) it's back to the original dilemma - will SWA be mandatory for physical protection (as I hope the existing Solar PV DNO's TN-C-S earth or separate run will meet earthing requirements ) or is there an alternatives of Double Insulated surface cable "tails" (either clipped to the surface of a roof truss or ducted with the left-over DNO underground ducting)?

Load calculations are not irrelevant, they are the basis for selecting the cable size!

I don't understand what you mean about the PV earthing? The means of earthing will be the TNCS connection at the new supply

 

[fursdon]

Load calculations are not irrelevant, they are the basis for selecting the cable size!

Sorry Dave - didn't mean to imply the load calculations were totally irrelevant - only that (fortunately) I don't have to worry about them, so are now "irrelevant to my question" as you've convinced me that 100A/25mm is the way to go.

I don't understand what you mean about the PV earthing? The means of earthing will be the TNCS connection at the new supply

The existing barn cu & PV installation are currently connected to the original incoming supply (via house back to transformer pole) so have an existing TNCS earth circuit.

"Change of use" means that house supply and PV (both in my "ownership" on existing house-to-barn supply) remain, with only the 2 barn flats and shed re-connected separately to the new supply. The PV installation (on the barn roof) runs back via the house and will be retained by the house, including its earth circuit on the current house-to-barn supply. As the total cable distance to transformer pole will be much the same either via house or via new service cable, and as the existing barn CU is already earthed via the house, I was just wondering if the barn could be earthed to either or both, not just via the new supply.

 

[NDG Elecs]

Have scan read through this, and to me it sounds like it is all being over-thought a touch. It does not take long to gland and terminate SWA so that should not be a massive consideration in my view.

I should not imagine many electricians would be put off due to it being SWA, consideration to the route may require a labourer to assist, but it really is not that complicated.

 

[davesparks]

Sorry Dave - didn't mean to imply the load calculations were totally irrelevant - only that (fortunately) I don't have to worry about them, so are now "irrelevant to my question" as you've convinced me that 100A/25mm is the way to go.



The existing barn cu & PV installation are currently connected to the original incoming supply (via house back to transformer pole) so have an existing TNCS earth circuit.

"Change of use" means that house supply and PV (both in my "ownership" on existing house-to-barn supply) remain, with only the 2 barn flats and shed re-connected separately to the new supply. The PV installation (on the barn roof) runs back via the house and will be retained by the house, including its earth circuit on the current house-to-barn supply. As the total cable distance to transformer pole will be much the same either via house or via new service cable, and as the existing barn CU is already earthed via the house, I was just wondering if the barn could be earthed to either or both, not just via the new supply.

No you can't mix and match between two different supply earths! All earthing and bonding in an installation needs to connect to the means of earthing for that installation.
Any extraneous parts which are common to both installations will need to be bonded to at point of entry to each installation.