Advice please on burying cables in the garden.

Kissarmy · 25 Sep 2014

We plan on having our Edwardian house totally rewired next year however we are about to have the septic tank replaced with a powered sewage treatment plant and while the terrace and lawn have large trenches dug in them that run from outside the wall that has the DB on it, under the 3ft wall at the edge of the terrace and down to the wall at the bottom of the garden I'd like to get something in the ground to avoid having to trash the garden again.
The plan is to end up with 2 x 13amp sockets and a 6amp lighting circuit on the terrace wall and the same on the wall at the bottom of the garden.
From the supply to the terrace position will be 21m and from the terrace position to the end wall a further 28m
My questions are
1. Do I run SWA (leaving the ends sealed/bagged) or do I run conduit?
2. Do I have a join at the terrace position or do I run seperately from the DB
3. What size SWA or conduit should I use.
Depending on your verdict on joined or seperately run supplies I would be specifying either 1 x 32amp supply (with 16amp breakers at each position) or 2 x 16amp supplies
Thanks in advance

bernardgreen · 25 Sep 2014

Install ducting and then you can pull in the cables depending on what the electrician decides is necessary.

At least 450 mm deep and marker tape 200 mm above the duct.

For the lights and sockets fit double pole isolators in the house so in the event of damage / water ingress the outside equipment can be fully isolated to prevent the RCD being tripped.

Run the sewage equipment from its own MCB and RCD protected circuit from the meter location and not on an RCD shared with other circuits. Install a separate duct to the sewage equipment if control / alarm ( over flow ) cables operating on ELV ( 12 or 24 volt ) are needed.

Also consider an anti-frost heater in the sewage system. Normally sewage is warm enough to prevent pumps mounted on the top of the tank from freezing but our pumped system did freeze up once while we were away during a cold winter.

Kissarmy · 25 Sep 2014

@bernardgreen
Thanks for your quick response.
I shall check with the plant installers to confirm they are using conduit. At the installers request I have already paid building control ready for the inspection before the trench gets filled so I'm sure they will do it correctly. The wiring in the house is ancient, there is quite a lot of rubber upstairs and I've even found a light in an eaves cupboard that still has lead sheathed cable. The only connection point available at the moment is to a modern small unit added when a downstairs toilet was added for the previous owner, it had an rcd and mcbs for a light, shaver socket and a heated towe rail which was removed when we fitted radiators. For the next year the sewage plant is going to have to share this rcd. We are fitting a klargester with rotating disks so there is no pump to freeze.
My amateur calculations give 6mm2 SWA for the terrace and 10mm2 SWA for the end wall, what size conduit would you reccommend I get?
Do I need to cap the unused conduits to prevent water access?
Thanks again.

bernardgreen · 25 Sep 2014

32mm internal diameter electrical duct made by PolyPipe Civils is one option as the entire run can be one un-jointed length.

Or plastic rain water drain pipe 3 inch diameter. Solvent weld the joints and use NO sharp bends, use long radius bends such as

Use a vacuum cleaner to suck a lightweight draw cord through the duct once installed

Kissarmy · 25 Sep 2014

@bernardgreen

Thankyou so much for that - the vacuum cleaner tip is absolute genius, I'd already been wondering how to do that - drain rods etc.

I think I'll go for the electrical duct as I always prefer things to be done properly.

Cheers for the excellent advice

ban-all-sheds · 25 Sep 2014

This sot of ducting:

http://www.drainageonline.co.uk/Ducting/Black-Twinwall-Duct-x-50m-coil.htm

(other suppliers are available - possibly the one you're using for the sewage stuff).

Twin-wall, smooth inside, ribbed outside for strength. Comes with a draw-cord installed so no messing about with a vacuum cleaner (or a hungry ferret and a piece of chicken

).

If you're running cables to somewhere were you might one day want an outbuilding, install two ducts so you can later add comms cabling.

Kissarmy · 25 Sep 2014

@ban-all-sheds

Thanks for the link, having had a look I'll put a blue one in as well so that I can leave a hosepipe down at the gooserun.

Am I okay just having the ducts pop up out of the ground touching the walls?

DetlefSchmitz · 25 Sep 2014

The stuff BAS suggested is what I used and it worked very well. I chose 110mm as it guaranteed an easy pullthrough and wasn't a lot more money.

In my case it was from garage to new shed. I brought the ducting up *inside* the buildings, so I had no worries about water etc. Whatever you actually do, you need to visualise the path the cable will take from the end of the duct to where it is used. And therefore what will happen to the exposed duct end.

By the way, one reason I chose a large diameter is that I was worried about the pull-string. The guys with the digger who buried the cable for me, gleefully told me that they had lost the end of the supplied pull-string when they cut the duct in half ( I had two installed). The large diameter meant that I could push a fish wire through 25m of duct with no problem.

Kissarmy · 25 Sep 2014

@DetlefSchmitz

Good info, I would love to be able to bring the ducts up indoors unfortunately the DB is in the kitchen which has a blown concretr over red ash floor which is going to have to be dug out at some point.
Plan for next years rewire include getting the meter moved outside and have the new CU in the cupboard on the next floor directly above its current location. We are in a flood risk zone and are going to have all the wiring coming down from ceiling. New boiler has gone upstairs too.

Would love to have the service head moved too but it is a very old 3 phase supply (1 phase for normal fuseboard and 1 phase via timeswitch for storage heaters, 3rd phase unused) and I imagine the cost would be prohibitive.

ban-all-sheds · 25 Sep 2014

Kissarmy said:
Am I okay just having the ducts pop up out of the ground touching the walls?

That really depends on your idea of aesthetics. Personally I cannot abide seeing cables on walls. Dunno why - I don't feel the same about downpipes.

The idea I came up with (wouldn't work well with 110mm ducting) was to have the ducting come out of the ground inside a length of drainpipe which runs to an enclosure on the wall, where cables come out of the house.

Don't mind enclosures on walls either. There's just something about cables I can't stand.

The things to watch out for in your scheme are water, debris, and fauna getting into the duct if it finishes at, or near (±) ground level.

Kissarmy · 26 Sep 2014

ban-all-sheds said:
There's just something about cables I can't stand.

The things to watch out for in your scheme are water, debris, and fauna getting into the duct if it finishes at, or near (±) ground level.

I'll cap the ends until they get used and then fashion something that silicones on and around the cable (I'll be using smaller ducting than 110mm)

As long as cables are black and neat I'll be fine seeing them - I think that comes from my job

Robbie Williams

Kissarmy
26 Sep 2014
1

Lighting Dollies containing 600mp 3phase distribution, automation control and media servers.

RF Lighting · 26 Sep 2014

I have a 110mm duct running from under the floor in the house to inside the garage. The end was stuffed with rockwool but I later discovered that mices had chomped a hole through that and were using it as a convenient route between the house and garage.

I ended up shoving the rockwool down the duct a bit and covering it with an inch of sand and cement mortar. This has stopped the mices and can be easily broken out if additional cables need to be installed.

Nice meat rack by the way! I do a lot of work in the same industry hence my user name

ban-all-sheds · 26 Sep 2014

Kissarmy said:
{Robbie Williams}

Can you not get anything to go in some of that free space which will make him able to perform well?

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Advice please on burying cables in the garden.

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