Sub Main Connection From Meter Box

[JSGreen]

Hello All, new to the forum. Would be grateful for advice on whether the following proposal is an acceptable solution or open to any suggestions for improving.

The brief is to supply a detached garage with incoming power and CU. Power will be used to supply a couple of sockets, lights and an EV charging point. The house CU is not suitable to take the supply from due to location and lack of available space.

The proposal is to run a 16mm SWA sub main cable buried underground from the meter box to the garage. The sub main cable length will be approximately 30M. The tails in the meter box will be split between the isolator and house CU and a Henley block installed. 25mm tails will then be run from the Henley block to a separate plastic enclosure next to the meter box. It would be possible to run the tails in the cavity and enter the back of the enclosure. The chosen enclosure is an empty 8 module IP66 CU with din rail, earth and neutral connector bar. Within the enclosure it is planned to install a 100amp DP Main Switch and 63 amp Type B MCB. External glands and earth nuts will be used to bond the SWA to the earth block within the meter box.

Advice is also sought on the suitability of a 63 amp Type B MCB for protecting the sub main? DNO fuse is 100amp, PME system.

Does the above sound reasonable?

 

[timhoward]

Welcome to the forum. That's quite a significant project you are asking for help with!
Being blunt it's a slightly unusual first post as you clearly know (or have researched) some of the principles, but your profile has you as "homeowner / DIY" and this certainly isn't a DIY task.
So I think many will read your post and wonder if you should be entertaining doing this work, and also wondering about how you will notify building control of the new consumer units and new circuits.

Don't take this the wrong way but can you tell us a little bit more about your previous electrical experience? It might help break the confused radio-silence!

 

[westward10]

Note: This was moved from DIY for more responses.

 

[timhoward]

Note: This was moved from DIY for more responses.

Ok. In that case I'll share what I was thinking:

Before constructing your own enclosure I'd exhaust all options that are out there first, including any options to supply the sub-main from inside using a simple fused switch.

Ideally it would be better to use a fuse rather than an MCB to protect the distribution circuit, as you can't guarantee a B32 will not also take out a B63 at same time for most manufacturers (or at least you'd have to look it up)

If the CPC of the distribution circuit is sized by table 54.7 (as opposed to calculated) then the steel wire armour equivalent copper size of 16mm 2 core SWA isn't big enough to use the armour as the CPC according to my table. So I think 3 core SWA would be needed.

If the protective device is 63 amps then it looks like 10 sq mm SWA (3 core) would also be adequate.

If the EV charge point is connected to the PME earthing it needs to be chosen with a little care.

Need to remember surge protection.

 

[pc1966]

As above, a switched-fuse with a 63A fuse is a much better way of protecting the sub-main as you will get total selectivity with the 100A DNO fuse (so practically no risk of losing the house supply due to any down-stream fault), and you will get fairly good selectivity with down-stream 32A MCB as well (so fault on one garage circuit unlikely to take out the 63A fuse and so other circuits).

As the end you will need a small CU with SPD and RCD protection (or RCBOs) for all of the final circuits.

A bit more effort is needed to check the cable specs are acceptable (not at home, regs not to hand) but the above suggestion of 3C 16mm SWA sounds very reasonable and likely to meet all requirements., where as 2C might not due to the lower conductivity of the steel armour as a CPC (and adding a 10mm CPC along side might be more effort & practical trouble than any potential cost savings).

Of course the SWA needs to be properly terminated with cable glands, etc, and to reiterate this is not a DIY job by any means.

 

[JSGreen]

Thank you very much for the two replies above. All very helpful and informative information which is very much appreciated

To briefly answer the first posters question. I am engineer. More mechanically biased than electrical. My current role involves managing the M&E installation of industrial test equipment. So yes, I have an understanding of the principles but I am not a qualified electrician by any stretch. Besides not having the required competencies to complete the work or provide the required certification, I also do not have the required equipment to be able to prove the circuit dead after isolation. So I will not be putting my fingers inside the meter box and will appoint a competent electrician to complete the termination and testing. I hope this has alleviated any concerns.

I do however have an interest in the circuit design, its operation and all things techie. As I said in my original post open to any suggestions for improving the installation? Would there be a benefit to installing SWA with data for example? (I don’t have Solar…yet). I intent to do some of the prep myself. Mount the enclosure and run the cable. To lay the sub-main involves a 30m trench dug across lawn and garden and lifting patio slabs! So I want to get it right.

Regards the SWA. My intention was to lay a 16mm SWA XLPE 3 core cable and use the earth / third core as the earth conductor. My reasoning for earthing the armour was to prevent this becoming live in the event that someone was to dig through the sub-main?

Point noted regards the use of a 63amp fuse rather than MCB.

 

[JSGreen]

Also pc1966. is there any reason that the SPD couldn’t go in the enclosure next to the meter box as opposed to the garage CU? Would this not afford more protection with it being further down stream?

 

[pc1966]

Thank you very much for the two replies above. All very helpful and informative information which is very much appreciated

To briefly answer the first posters question. I am engineer. More mechanically biased than electrical. My current role involves managing the M&E installation of industrial test equipment. So yes, I have an understanding of the principles but I am not a qualified electrician by any stretch. Besides not having the required competencies to complete the work or provide the required certification, I also do not have the required equipment to be able to prove the circuit dead after isolation. So I will not be putting my fingers inside the meter box and will appoint a competent electrician to complete the termination and testing. I hope this has alleviated any concerns.

Thanks, that is very helpful background and you seem to be approaching this in a sensible way.

I do however have an interest in the circuit design, its operation and all things techie. As I said in my original post open to any suggestions for improving the installation? Would there be a benefit to installing SWA with data for example? (I don’t have Solar…yet). I intent to do some of the prep myself. Mount the enclosure and run the cable. To lay the sub-main involves a 30m trench dug across lawn and garden and lifting patio slabs! So I want to get it right.

Yes, network cable is a good idea. You can get SWA with network built in just for EV systems, but for the more general case if you put in some suitable twin-wall duct (maybe 50/63mm as tolerable price for 50m coil and easy for such cable combinations to feed through) you can run other cables along side the SWA for anything you want/need.

It is far easier/better to put them all in at the same time, but if you have a draw rope alongside the SWA you can add alter as needed.

As a minimum get the black UV tolerant network cable for outdoor use, but you can also get armoured CAT cable as well as armoured firbe if you fancy something needing multi-gigabit speeds in the future.

Regards the SWA. My intention was to lay a 16mm SWA XLPE 3 core cable and use the earth / third core as the earth conductor. My reasoning for earthing the armour was to prevent this becoming live in the event that someone was to dig through the sub-main?

The armour must be earthed for that very reason. You can use it as the CPC if all electrical parameters are met, but as steel is around 8 times less conductive than copper it is often not low enough to get a high enough fault current so the supply protection disconnects sufficiently fast.

Point noted regards the use of a 63amp fuse rather than MCB.

MCBs have many practical advantages over fuses for domestic application but for something that ought not to go a fuse has a better combination in terms of both fault disconnection impedance (Zs) and selectivity with other protective devices. While not usually a concern for domestic systems, they usually can interrupt for higher fault currents as well.

Old idea, simple idea, but details are quite complicated!

 

[pc1966]

Also pc1966. is there any reason that the SPD couldn’t go in the enclosure next to the meter box as opposed to the garage CU? Would this not afford more protection with it being further down stream?

Yes, in the ideal case you would protect house & garage with a SPD at the source, but if not doing so then at least put it in for the sub-board.

The issue for good SPD performance is usually cable inductance, not resistance, so they need short and direct connections to the supply, easier to do inside a DB. Having said that you can get isolator switches with SPD built in but not very cheap:

Wylex 100A DP Supply Isolator c/w Type 2 SPD | Wylex (REC2SPD)

If you do not have any isolator fitted then it might be worth having that fitted when the fused-switch is done as then any further work on either your main house CU or the fused-switch can be done without the trouble of getting the DNO to pull the main and re-seal the cut-out fuse, etc.

 

[davesparks]

I intent to do some of the prep myself. Mount the enclosure and run the cable. To lay the sub-main involves a 30m trench dug across lawn and garden and lifting patio slabs! So I want to get it right.

Regards the SWA. My intention was to lay a 16mm SWA XLPE 3 core cable and use the earth / third core as the earth conductor. My reasoning for earthing the armour was to prevent this becoming live in the event that someone was to dig through the sub-main?

Most decent electricians will be very wary of connecting cables which have been installed by someone else because they have no way of knowing how well they have been installed or if there is any damage.
You either need to speak to the electrician beforehand and find out what they are happy for you to do and what inspections they would want to make, or install a twin wall utility duct with drawstring so that they can install the cable later.
Duct has many advantages as you can add another cable later if needed for data or something and it reduces the chance of the SWA being damaged when installed.

If you are directly burying SWA cable it would ideally be bedded in, and covered with soft sand to avoid damage.

Mounting an enclosure isn't really going to save anything as the electrician is likely going to need to take it off again to do some part of the work and then put it back again.


I personally would install whatever switchgear you are using in an IP rated GRP (or metal) enclosure rather than using an IP rated DIN rail enclosure as they generally don't fare too well outside regardless of their IP rating. Sunlight and cold weather are very good at making plastic enclosures brittle.