120mm to 25mm connection

tony rowan · Aug 6, 2021

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 · Aug 6, 2021

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 · Aug 6, 2021

Perhaps even 240mm²

Dustydazzler · Aug 6, 2021

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

tony rowan · Aug 6, 2021

westward10 said:
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 · Aug 6, 2021

tony rowan said:
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 · Aug 6, 2021

westward10 said:
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 · Aug 6, 2021

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 · Aug 6, 2021

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 · Aug 6, 2021

westward10 said:
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 · Aug 6, 2021

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 · Aug 6, 2021

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 · Aug 6, 2021

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 · Aug 6, 2021

pc1966 said:
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 · Aug 6, 2021

pc1966 said:
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 · Aug 6, 2021

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

brianmoooore · Aug 6, 2021

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 · Aug 6, 2021

tony rowan said:
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 · Aug 6, 2021

pc1966 said:
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 · Aug 6, 2021

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.

tony rowan

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loz2754

Dustydazzler

tony rowan

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Dustydazzler

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