"tapping off" a garden feed

[Stringman_uk]

Hi all
I have a summerhouse at the end of the garden which has a power supply coming from the house via a 16a breaker and armoured cable leading to a consumer unit in the summerhouse.
We are building a home office in the middle of the garden.
I am hoping to run ( or rather have run) power to it.

At the moment the armoured cable runs up the garden on the left hand side, and obviously the office is on the right hand side.
What i was hoping to do is split the cable near the house, run the new cable under the decking and along the right hand side of the garden to a new consumer unit in the office.
Is this feasible?
The alternative is to still split the cable but put either a mini (external) consumer unit with one feed to the office the other to the summerhouse.
or even as above but with a couple of external 13a sockets.
Obviously I would get it signed off or a sparky do do all the work but I wanted to check my ideas are viable first.
Thanks in advance

 

[flameport]

With only a 16A supply, there is no point in installing any consumer unit at the end of it or anywhere else.

 

[securespark]

What is the size of the SWA and what is length of the run?

 

[Stringman_uk]

With only a 16A supply, there is no point in installing any consumer unit at the end of it or anywhere else.

Can I ask why is that. The main reason in installing the consumer unit was to be able to isolate the lights and power circuit and in the hope that should anything trip, then it will trip the board in the office rather than in the house. and since you have to "convert" swa to twin and earth at some point why not use a consumer unit?

 

[Stringman_uk]

What is the size of the SWA and what is length of the run?

That is a very good question I will have a look and see if I can measure the fitted one.
The existing run is about 20m
Which actually brings me to my next question. If the cable is of sufficient rating could the main breaker be swapped out for a 32a?

 

[crystal ball]

With only a 16A supply, there is no point in installing any consumer unit at the end of it or anywhere else.

Plenty of instances where this is done, for good reason too

 

[Harry Bloomfield]

Plenty of instances where this is done, for good reason too

Quite! My garage is fed from a 16amp armoured, MCB circuit, due to lack of forward planning on my part. That then feeds into an RCD, then on to a consumer unit. The CU then feeds via multiple MCB's, my hut, summerhouse, around 20 sockets around my garage and workshop, three separate lighting circuits inside the garage and workshop, plus several outdoor sockets for the garden. It's handy, to be able to isolate individual supplies, at the CU.

Were I planning it now, the main supply would have been 32amp, but as it is, it works just fine.

 

[flameport]

isolate the lights and power circuit and in the hope that should anything trip, then it will trip the board in the office rather than in the house.

Whether that happens depends on what protective devices are used, what their specifications are and whether they have been selected appropriately.
With a 16A circuit, achieving that will be next to impossible.

Installing a consumer unit increases the cost and makes it notifiable work.

 

[ericmark]

I have put joints in SWA cable, but the joints are not cheap, so normally one would run the new cable from either the house or the existing out building, one would not join two cables.

Today we want RCD protection, either using a stand alone RCD or a RCBO which is a RCD and MCB combined, using the RCD method it is common to have many MCB's from one RCD, if this is already the case in your house, then adding further RCD's or RCBO's is a bit pointless, OK with a 10 mA RCD fed from a 30 mA RCD in theory the 10 mA should trip first, but often simply pressing the test button will trip both, so start point is what have you already got in the house?

RCD's as seen here have test buttons.

The next point is building construction, with a brick building a TN-C-S supply is normal, with the supplier (DNO) providing the earth, however with a metal building this may not be safe, so a metal shed will likely need a TT supply, {url=https://electrical.theiet.org/media...s-a-supply-to-a-detached-outbuilding.pdf]this tells one how to wire an out building[/url] although a little out of date and this explains the problems with supply type but it does not end there, clearly we have services already under many gardens, be it water, gas, drains or electric, so knocking in earth rods not only can they hit existing services, but where the existing are metal, they can also cause problems with the earthing.

So when an electrician visits he looks at the area and works out the best option to use, I would not expect a problem with my garden, but that does not mean no problem with yours.

I have at work been approached by some one asking how to do XYZ, and have given advice, then at a latter date visited their home, and realised my advice was not really appropriate for their home, so would suggest you get an electrician to quote for the work, and ask what will need to be done, one it may be cheaper than you think to get it done for you, and two you will get a better idea to if there is anything in your home which may impact on the work required.

 

[Stringman_uk]

Thanks for the replies
I am now wondering if its possible to put another breaker into the consumer unit and then run the swa from that to the office?
It does mean a longer run but it saves splitting the cable and running it under decking etc

 

[Stringman_uk]

Edit
I have just checked and the breaker in the house is actually 32a

 

[SUNRAY]

With only a 16A supply, there is no point in installing any consumer unit at the end of it or anywhere else.

A point I agree with generally.

Plenty of instances where this is done, for good reason too

A point I also agree with.

Such as a friends property, some 20+ years ago he cut the cable on his Flymo and started making a repair assuming the fuse had blown... However the fuse only blew when he touched the wires together while making the repair.
The circuit consisted of 20A fused switch of the Henley block, app 20m 2.5 T&E (7/0.029) feeding 4W Wylex in workshop with all fuses 5A lighting and 15A sockets in workshop and same again for shed. After the incident I added an RCD in an enclosure and 15/16A plug in MCB's for the power circuits

Realistically CU not required as sockets could happily run on the 20A supply and the lights would only require a FCU as the light switch.

Ever since he has got into the habit of switching off the MCB's when not in use after leaving a thermostasic fan heater running. So yes as Harry says it's handy to be able to easily turn off sections.

 

[ericmark]

In my old house I looked at spiting the circuits further so to reduce the chance of loss of power to freezers, there is really nothing else in the house which will cost should I lose power, rest was just a nuisance, but the type of fuse box, mean to do that, I needed to swap my fuse boxes for consumer units, or unit, and I never got around to doing it. All I did was swap fuses for MCB's. Each fuse box was fed from it's own RCD both 30 mA, so just two circuits in essence. Although those two circuits were further divided into eight.

I would get a couple of weeks of tripping, then it could go years until next time.

As it stands we have no idea what @Stringman_uk has other than a 32 amp overload. We don't know if that overload is a fuse/MCB/RCBO so really can't advise what he should do.

This house I have gone all RCBO, but back in the early 90's when I fitted the RCD's in my last house, we did not have RCBO's for domestic fuse boxes, the consumer unit was not invented back then.

 

[crystal ball]

Whether that happens depends on what protective devices are used, what their specifications are and whether they have been selected appropriately.
With a 16A circuit, achieving that will be next to impossible.

Installing a consumer unit increases the cost and makes it notifiable work.

£40 for a CU

 

[Harry Bloomfield]

I have put joints in SWA cable, but the joints are not cheap, so normally one would run the new cable from either the house or the existing out building, one would not join two cables.

That depends on the cost of the alternative methods...

https://www.amazon.co.uk/BRANCH-Underground-Armoured-1-5-6mm-Cable/dp/B071JRW7B5