120mm to 25mm connection

[tony rowan]

Hi, I have a domestic job coming up that requires a 100A supply but it's 260mts from where the metered supply will be installed.
120mm 3 core supply SWA cable (260m long, 120mm 3 core due to voltage drop)
My problem is, how do I connect the 25mm meter tails to the 120mm SWA cable at the meter end & also how would I terminate back to 25mm tails at the consumer unit end?
I know that I could use a300A 3 phase TP & N fused isolator but trying to find a cheaper option for the client.

I have considerd a bus bar chamber or 300A TP&N isolator?

any suggestions would be appreciated

Regards
Tony

 

[westward10]

I assume to get 120.0 you have used 5% for the vd which gives you no tolerance for the final circuits. Submains are often based on 2% but even at 3% you are looking at 185.0.

 

[loz2754]

Perhaps even 240mm²

 

[Dustydazzler]

I’m with WW , I would be looking at 185 cable

 

[tony rowan]

I assume to get 120.0 you have used 5% for the vd which gives you no tolerance for the final circuits. Submains are often based on 2% but even at 3% you are looking at 185.0.

the cable calcs were not done by me, they were done by cable supplier

 

[westward10]

the cable calcs were not done by me, they were done by cable supplier

Sounds like they weren't given enough information on the installation, a 5% drop would equate to 120.0 but as I say you have no leeway on the final circuits.

 

[tony rowan]

Sounds like they weren't given enough information on the installation, a 5% drop would equate to 120.0 but as I say you have no leeway on the final circuits.

my question was not how to calculate voltage drop but for suggestions on how to terminate from one size to the other (120mm to 25mm) I have options ranging from £300 to £1600+ (each end) but was interested in any advice or opinions.
I would think this something that some one has had to deal with?
The cost is unavoidable but there are options & maybe I've not considered them all?
Any advice is appreciated

 

[Dustydazzler]

I have never seen 120mm used domestically , only ever installed into a 3 phase DB

a home fabricated busbar chamber is probably your cheapest solution?

 

[westward10]

I would ask the cable supplier for an alternative method aside from a single cable. Parallel or more cables are a better alternative and although you have more cores you have reduced csa.

 

[tony rowan]

I would ask the cable supplier for an alternative method aside from a single cable. Parallel or more cables are a better alternative and although you have more cores you have reduced csa.

£40K was the cost for mains supply to the source. The client was told that he could save money & build a meter box on the edge of the property & install his own supply from that source. This was suggested by REC. This is not an install that I relish...

 

[pc1966]

It is one of these odd things that the consumer side has 5% on final circuits but the DNO has different volt-drop rules.

You might be able to find a torpedo style joint that allows 120mm to be connected to 25mm or so for ease of wrangling. Ask the likes of Birkett Electric for advice/suggestions.

The other thing is that cable is expensive. You might well want to look at larger but in aluminium to get the VD and cost within budget, but you really have to take care with the joints and bend radius there. Again, as resin filled joint will alleviate the risk of galvanic corrosion between Al and Cu.

Typically Al is 50% larger for the same resistance so a step up (e.g. 185mm Al is approx 120mm Cu)

 

[pc1966]

For example, you might use 2 paralleled cores of 95mm aluminium to get the same as your copper, but cheaper:

Superlec 4X95, SOLID SECTORAL CONDUCTORS,XLPE,PVC,SWA,PVC BS5467 1KV

Please note: If you require advice on which cable to use, how and where to install it, it is recommended that you will need to contact the relevant qualified tradesperson for the job as the Superlec Direct Team are not qualified electricians.

www.superlecdirect.com

There I would be joining the 4 cores of Al to 4 cores of 16mm copper SWA (maybe 5-core so CPC sized OK for TN-C-S), and then finally paralleling at the switches, etc.

But also check the armour works as CPC in terms of disconnection times, etc, or look at an up-front 300mA delay RCD and an all RCBO CU with DP switching (for example Wylex, Crabtree, or Fusebox).

 

[WallaceP]

Junction box lugs and bolt, try to get the lugs with several holes for bolts. If your looking to save money look at a 2 core and a separate earth. But to be honest at the cost of this job what you are saving not putting in an isolator at both ends is probably not much in the grand scheme.
As others said I would query this cable size.

 

[pc1966]

For example, you might use 2 paralleled cores of 95mm aluminium to get the same as your copper, but cheaper:

Just looked, not necessarily cheaper. Check you calcs and run all options!

 

[tony rowan]

It is one of these odd things that the consumer side has 5% on final circuits but the DNO has different volt-drop rules.

You might be able to find a torpedo style joint that allows 120mm to be connected to 25mm or so for ease of wrangling. Ask the likes of Birkett Electric for advice/suggestions.

The other thing is that cable is expensive. You might well want to look at larger but in aluminium to get the VD and cost within budget, but you really have to take care with the joints and bend radius there. Again, as resin filled joint will alleviate the risk of galvanic corrosion between Al and Cu.

Typically Al is 50% larger for the same resistance so a step up (e.g. 185mm Al is approx 120mm Cu)

Thanks for the reply, The client has taken delivery of the 120mm 3 core SWA. The REC are due on site in about 2 weeks to install their metered 100A supply at the boundry.
No hurry for power as the dwelling is not there yet
I did consider a resin torpedo joint (this may be an option?) but I don't think this option would work with the meter box/cupboard

 

[freddo]

I've used this sort of thing in an enclosure before to change from a larger to smaller cable:

 

[brianmoooore]

I did a similar job to this several years ago, using 90mm2 SWA. May have been 4 core.
I fitted ring terminals to the 90mm2 conductors and the 25mm2, bolted them firmly together inside a plastic adaptable box, then filled the box with resin. The adaptable box was smaller than you might think.

 

[DefyG]

300A TP&N isolator

Did you mean switch-fuse or thinking of relying on DNO's cut out fuse for protecting the distribution cable?

 

[westward10]

Just looked, not necessarily cheaper. Check you calcs and run all options!

Wasn't looking at cost but the ease of smaller csa. That aside the vd has not been assessed correctly.

 

[JD6400]

We use these with heat shrink , either in an adaptable box or a bit of 4 / 6 inch trunking as a chamber .
They are set to shear off at the correct torque.

 

[Rockingit]

For that kind of money, three things strike me as worthy of investigation:

1. Ask the DNO to price for a closer installation point;

2. Conductors in parallel;

3…. slightly out of the box thinking here, but, use a matching pair of LV to MV transformers and do the long run in 11kV

 

[Yozza19]

Done something similar before got two units made by ec services in Bradford double pole mccb front end and a at dwelling end was small bus bar set up got both for around a grand to my exact spec

 

[pc1966]

Thanks for the reply, The client has taken delivery of the 120mm 3 core SWA.

OK so they are kind of committed to the cable sizing now!

I did consider a resin torpedo joint (this may be an option?) but I don't think this option would work with the meter box/cupboard

I really doubt the resin joint would fit in there at all!

But you can bury it and just take some 25mm (or 35mm if saving a few mV drop) from a buried resin joint in to the DNO cabinet.

 

[pc1966]

3…. slightly out of the box thinking here, but, use a matching pair of LV to MV transformers and do the long run in 11kV

I suspect going above LV is a whole lot of trouble for most folks and would really push the job back to the likes of the DNO who have HV approved persons.

It all seems a done-deal now, but they might have been better to have gone for a smaller 3-phase supply. After all 40A/phase would match the total 100A single phase supply and less issues of volt drop. However, no idea what the additional DNO cost or standing charges would have been.

 

[Rockingit]

I suspect going above LV is a whole lot of trouble for most folks and would really push the job back to the likes of the DNO who have HV approved persons.

It all seems a done-deal now, but they might have been better to have gone for a smaller 3-phase supply. After all 40A/phase would match the total 100A single phase supply and less issues of volt drop. However, no idea what the additional DNO cost or standing charges would have been.

Would be an interesting conundrum for Part P / DI’s, though! Not even a departure from 7671 as 7671 wouldn’t apply to that bit….. Interesting.

 

[pc1966]

Would be an interesting conundrum for Part P / DI’s, though! Not even a departure from 7671 as 7671 wouldn’t apply to that bit….. Interesting.

Indeed!

You could step up to something like 400-0-400V for the link and still be within sane normal LV voltages for most switchgear, etc, but it would really take a bit of computing for the transformer/installation cost versus the cable costs.

 

[westward10]

Cable that size you are either looking at an mccb in an enclosure at the source or a Glasgow switch-fuse. Personally it would be a 315A switch-fuse with an extension and down rate the factory fitted fuses to 100A. Same the other end but replace the fuses with links.
I have just been on a site where a 400A supply was required to a new building but due to cost the new head was fitted to the perimeter of the site, here a Schneider NSX board was fitted and parallel 150.0s connected from a 400A mccb.
Unfortunately big cables require big switch gear.

 

[pc1966]

I have been impressed by the Schneider Square D Quadbreak fused-switches, but they are fairly compact considering so as you say you really would need an extension box to get the cores fanned out and aligned with the terminals.

You might get away with a 250A one down-rated with 100A fuses, but having taken a quick look I'm really struggling to see any specifications from Schneider on the max cable capacity or BS88 size to see what choices would be supported.

From memory the 100A uses M8 bolts, but by 250A it would have to be something like M12 and matching lugs / crimp tool, or those shear-bolt ones shown in one of the above posts?

EDIT: Found this:

https://www.rselectricalsupplies.co.uk/images/webupload/products/downloads/Quadbreak%20tech%20and%20dimensions-31-10-2016-11293.pdf

Has 200A & 250A as M10 and max 120mm, then 315A is M12 and up to 400mm.

I just can't begin to imagine how hard 400mm is to wrangle!!!

 

[pc1966]

This sort of thing:

250A TP+N Metal Fused Switch

Rated for 240V and 415V

www.cef.co.uk

You could use one line and the non-switched neutral link for single-pole isolation, but for DP use then use two of the line terminals but have one with a link instead of the fuse.

 

[pc1966]

Come to think of it, if you bought a TPN fuses switch for the feed and (e.g. the SQB2503K above), and the switch-only version for the consumer end (SQB2503L), you would have a spare link from it for such a trick without buying any more.

 

[pc1966]

Found the fuse sizes here:

What type of replacement fuse links can be used with Quadbreak fuse switches? | Schneider Electric UK

Looking at one of the data sheets for LV fuses (Mersen BS88 LV Fuse Links & Fuse Holder) it looks like you can downgrade a B3 size fuse with a B2, B1, or B1C giving you ratings all the way down to 32A (though I would really double-check the fuse physically fits, etc, before trying it).

 

[brianmoooore]

Overcomplication going on here. It may be a cable rated at 230A or so, but it will be running at an absolute maximum of 100A, so change at each end to a short length of 25mm2 in a resin filled buried joint, then terminate in 'normal' domestic gear at each end.

 

[westward10]

I'm not seeing anything complicated and less joints the better.

 

[JD6400]

Although the Glasgow range or the Schneider boards look the dogs dangles , don't forget he is only dealing with 100 amps at the end of the day.
Those inline reducers with heat shrink in a suitable enclosure with the joint coming down to 25 mm tails for the final connections , are likely to be at least £££s cheaper if not possibly ££££s.

I have just realised that I did not mention that they can handle large difference's in CSA between sides.

 

[brianmoooore]

Don't disagree that the less joints in a circuit, the better, but properly made, resin encapsulated joints are at the higher end of the reliability spectrum.

 

[westward10]

I think the OP is busy rolling out his 260m of cable?

 

[westward10]

Don't disagree that the less joints in a circuit, the better, but properly made, resin encapsulated joints are at the higher end of the reliability spectrum.

Can imagine doing a periodic on it without any info on the cable except a 25.0 coming out of the ground each end and wondering why your Zs is so low for an imagined 260m of 25.0??

 

[pc1966]

Can imagine doing a periodic on it without any info on the cable except a 25.0 coming out of the ground each end and wondering why your Zs is so low for an imagined 260m of 25.0??

Well if it is for a certain Dr Who then maybe there is tardis technology involved

 

[pc1966]

I think the OP is busy rolling out his 260m of cable?

Hard work I'm sure! A quick check has 120mm 3-core as about 5.1kg/m so total of just over 1.3 tonnes of wire to be laid out.

 

[freddo]

We recently pulled in 185M of 4 core 120mm swa on high level ladder. Certainly not an easy task!