Augmentation of existing submain. Complicated layout. Advice please!

[Mark42]

Apologies for the complexity of this question, but there’s no easy way to explain it.

This is a remote Norfolk house and workshops, developed over 30 years.

Due to diversification, building B on the plan needs more power than originally expected, and building D is a recent addition. I’d like the forum’s opinion on whether my proposed upgrade plan is a good way to go.

INSTALLATION NOTES

• All cables are direct-buried 4- or 5-core SWA. Many were existing, with smaller CCA than ideal.
• All DBs are recent Hager three-phase boards, with local earth stakes.
• Most SP outgoing circuits are protected by RCBOs.
• There is good load balancing across phases, especially for lighting.
• When calculating volt drop note that a new pole transformer is shared between only three domestic properties, with the actual no-load voltage at this site never below 250V. I have never metered anything below 242V anywhere, even with everything on at the end of all the long runs, which never happens in normal use.
• The daisy-chaining of sub-mains to sub-mains is not ideal, but is there for historic reasons. Nuisance tripping all up the line has never happened.
• All buildings may be considered as light domestic loads. There is no heavy machinery.
• Disregard the solar panels visible on building F: these we installed by me, and feed back to inverters in the old milking parlour, by taking the DC MV outputs on new SWA runs, not shown.

Buildings B and D are currently supplied as follows:
Main supply at DB1 (Main house) 3-phase x 100A, with direct feed to:
25m x 25mm (red) to DB2 (Barn), then 32A MCB to:
95m x 16mm (org) to DB4 (Store), then 20A MCB to:
96m x 10mm (yel) to DB5 (B-block), then:
Temp direct feed to 25m x 16mm (grn) via JB to DB11 (D-block). Note 10mm in, 16mm out is not a mistake: is was a temporary feed, piggy-backed off the B-block supply, awaiting later upgrade for building D.

Building F (‘Steel shed’) is supplied by a different route:
Main supply at DB1 (Main house) 3-phase x 100A, then 40A MCB to:
12m x 16mm (vio) to DB3 (Old milking parlour), then 20A MCB to:
170m x 16mm (blu) to DB6 (Steel shed)

I’m thinking of tapping into the last (blu) cable above at point X, to provide a branch to feed building D as well, via a proposed new (dashed black) 85m run of 25mm. Overall run DB3 to DB11 approx 200m of mixed 25mm and 16mm

This would have these advantages:
1. To take the load off the red > org > yellow run which is in only 10mm for the last 96m, to supply only building B, as originally intended.
2. Provide a 20A x 3 feed to the new workshop at building D, which is adequate.
3. Remove any need to dig up the formal garden area, or to get another SWA into any existing DB which is already full, or, at the house end, install difficult cable runs in the listed building. The proposed method is an easy trench across a field which can be finished in a day without mess or damage.

Of course it means that both buildings F and D will share a single 20A supply from DB3. I have no idea if this is a standard way of doing things. Technically it’s fine as both buildings are never in use at the same time, with F being only for storage, with a few lights and 13A sockets.

And what about tapping into a four-core 16mm SWA? Has anyone done this with existing cables which might be a bit tight in the ground. I’m hoping that disinterring a few meters of the existing run should give a bit of wiggle room. Plus using a bigger-than-necessary u/g resin joint should give room to work.

What do people think? Am I on the right track here?

I’m a bit stuck as I do need to work with the existing underground infrastructure, and get this done quickly.

Next year a brand new run of 35mm from the main switch to supply to B & D together will finish the job properly.

 

[buzzlightyear]

have looked at the detail map and proposing to tap in to a 16mm and branch then on with a 25ml cable is not good considering the loading to the steel shed ,it needs new cable to where you want it considering loading volt drop .

 

[davesparks]

Next year a brand new run of 35mm from the main switch to supply to B & D together will finish the job properly.

Why not just do this now instead of waiting, it would save wasting money and resources on a temporary repair.

Your proposal is physically possible, but a bad solution in my opinion.

Like anything a breach joint into an existing underground cable is easy enough if you know what you are doing. You just have to strip and connect to the existing cable without cutting it,suitable connectors should be provided with the breach joint kit. The only trouble you may have is that they are normally designed to joint a smaller cable into a larger main rather than the other way around.

 

[Vortigern]

I am not quite clear how you plan to get 3 x 20a from a single 20a supply tap on the blue line.

 

[davesparks]

I am not quite clear how you plan to get 3 x 20a from a single 20a supply tap on the blue line.

The blue line is a 20A TP supply, the OP is going to connect into this via a breach joint.

 

[Mark42]

I am not quite clear how you plan to get 3 x 20a from a single 20a supply tap on the blue line.

I meant 3 x 20A SP, ie 60A all three phases considered. Most loads, apart from some fairly trivial roller shutters, are single phase.

The more I look at this myself, my plan, written on an aeroplane yesterday, looks daft. Perhaps surface-laying 4 core 35mm now would be better, and cheaper in the long run. Get it working both ends then bury it in the summer when the mud has gone.

I just didn't want to spend two grand plus on cable now, what with Christmas just over and all

 

[davesparks]

I just didn't want to spend two grand plus on cable now, what with Christmas just over and all

You'd rather spend almost that much now on creating the temporary solution and then spend it again whne the job is done properly?

 

[buzzlightyear]

The more I look at this myself, my plan, written on an aeroplane yesterday

was you high has a kite when you did the plan ,lol.

 

[Mark42]

OK, say I forget the black run and breach joint. It's unprofessional. Run a new 4-core 35mm or 50mm as below in white?

It's a crazy run but is the only machine-diggable routing which does not involve damage to a finished garden, or crossing deep ditches.
This is probably worth doing as I could also put an SWA fibre in the same trench to get decent internet to B & D which has always been a problem.

But working with cables that size and length is beyond my experience.

I'd disconnect and abandon the old 10mm yellow run.

The new supply would end inside B in a big isolator, with two parallel 16mm 3P feeds out: one set to DB5 locally (for building B), and one set to the green final run to DB11 in building D.

Question 1: Is it OK to reduce CSA like that locally?

Question 2: Conversely, may I 'supply' the feed end of the (say) 50mm SWA via a switchfuse fed from the main switch with say 4 x 25mm D/I singles? If so, could these come from the supply side, via a new hole in the RHS of the existing isolator, and be protected only by the DNO's 100A cutouts?

Pic of the main switch below. There's no way of terminating another big SWA into that, as the top already has 2No. 25mm 4-core SWAs going out, and there's not much space around.

Question 3: Or is it permissible to 'back-protect' a poorly-protected cable run by having the switch fuse only at the much easier-to-install far end, say at 40A per phase? Mechanical damage to the cable would blow the board's cut-outs, long-term overload from buildings B and/or D would trip the local switchfuse. I'm warming to this idea. Might that be OK?

What would others do? (Apart from say they don't want the job )

 

[Wilko]

Hi - just an after beer thought, but farms in my day often had overhead supplies.

 

[davesparks]

OK, say I forget the black run and breach joint. It's unprofessional. Run a new 4-core 35mm or 50mm as below in white?

It's a crazy run but is the only machine-diggable routing which does not involve damage to a finished garden, or crossing deep ditches.
This is probably worth doing as I could also put an SWA fibre in the same trench to get decent internet to B & D which has always been a problem.

But working with cables that size and length is beyond my experience.

I'd disconnect and abandon the old 10mm yellow run.

The new supply would end inside B in a big isolator, with two parallel 16mm 3P feeds out: one set to DB5 locally (for building B), and one set to the green final run to DB11 in building D.

Question 1: Is it OK to reduce CSA like that locally?

Question 2: Conversely, may I 'supply' the feed end of the (say) 50mm SWA via a switchfuse fed from the main switch with say 4 x 25mm D/I singles? If so, could these come from the supply side, via a new hole in the RHS of the existing isolator, and be protected only by the DNO's 100A cutouts?

Pic of the main switch below. There's no way of terminating another big SWA into that, as the top already has 2No. 25mm 4-core SWAs going out, and there's not much space around.

Question 3: Or is it permissible to 'back-protect' a poorly-protected cable run by having the switch fuse only at the much easier-to-install far end, say at 40A per phase? Mechanical damage to the cable would blow the board's cut-outs, long-term overload from buildings B and/or D would trip the local switchfuse. I'm warming to this idea. Might that be OK?

What would others do? (Apart from say they don't want the job )

What is the design current of this distribution circuit and the length if it follows the white route?

Q1. You can reduce the cable size to make the final connections from a larger cable size as long as the smaller is suitably protected from overcurrent by the ocpd for the circuit. Alternatively a suitable ocpd may be installed after the reduction in cable size as long as certain conditions are met


Q2. Yes you can use smaller cables at the supply end if necessary, subject to the same requirement for them to be protected from overcurrent.
There shouldn't be 2x 25mm outgoing cables connected in that isolator anyway, the terminals are designed for one cable only.

Q3. No you cannot use the cutout fuses to protect a circuit nor can you install the ocpd at the far end of a distribution circuit. This is a very basic part of the wiring regulations.

Some of the questions you are asking are very worrying from someone who is designing electrical installations.

 

[Vortigern]

I suppose a new supply for the B/D building is out of the question?

 

[Mark42]

What is the design current of this distribution circuit and the length if it follows the white route?

Good questions - I forgot to write that down. The white route is not exceeding 275m including drops.
We need about 20A per phase peak demand in both buildings, so design at 40A per phase. We'd never need 120A overall, but there's always the danger of things being plugged into the 'wrong' sockets and overloading a single phase.

... Some of the questions you are asking are very worrying from someone who is designing electrical installations.

Yeah, sorry, stupid questions. I'm well jet-lagged and not thinking straight. I'm going to sleep on this one.

I can think of no obvious way of connecting a long and hence heavy-sized SWA to that effectively domestic supply in a little cupboard in a tiny domestic hallway. In my day it would have been from a great big bus-bar chamber on an open wall. But we just don't have the real estate.
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I suppose a new supply for the B/D building is out of the question?

Sadly, no.
A good suggestion, but all that would do is to transfer the same problem onto the supplier. The transformer is up by the house. It would need to be a similarly long LV run, at DNO prices!
There's no HV much nearer either.
Welcome to rural England
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Hi - just an after beer thought, but farms in my day often had overhead supplies.

Yup, they still do round here. Often a bit of T&E hanging off some fencing wire as a catenary Horrible in so many ways.
Naa, no overheads please: Ugly poles, maintenance issues, dangerous with machinery moving about.
I spent the last 30 years getting rid of overheads here, both LV and HV. I'm definitely not putting any back!