My shed, when completed will be about 20m from the house.
I intend to have a small consumer unit in it, with one thingy for the light and the other for a double socket.
It will be wired up by a sparky, not me.
I need to get the cable in the ground soon as I'm in the process of laying a patio.
What size of armoured cable do I need?
Thanks
mark
Nice little calculator here
mark wibbled on Sunday 13 December 2009 18:44
3 things are required WRT cable sizing (amongst many other things generally)Voltage drop must remain within bounds (3% max for lighting from a nominal of 230V)
Current carrying capacity of cable
L-E loop impedance, including supply low enough to trip the circuit breaker/fuse in the required time.
On the face of it you are likely looking at around 4mm2, but you might do well to go to 6mm2 (but see the point below re earthing). This is a rough guide, do NOT take my suggestion carte blanche - you need to just run through a few things...
What design current did you have in mind for each circuit? 6A for the lights and 16 or 20A for the socket? And the breaker size at the house CU?
You should be aware that you're not likely to get any discrimination if you actually put an MCB in the shed for the socket. A fault here will take out the breaker at the house as well and thus the shed lights.
Therefore, what is sometimes done is to run basically a socket circuit out to the shed and take the shed lights off a fused spur (or an MCB if you like) locally.
Whereabouts is your RCD protection going? RCBO on whole circuit, reliance on one of the main RCDs supplying several house circuits or local RCD or RCBOs in the shed?
Just to be clear - are you expecting to run high wattage flood lights off the lighting branch? Or just a small tube or bulb or two?
And, subtly, but pretty importantly: Earthing arrangements.
Assuming you were planning on exporting the earth from the house to the shed, does your shed contain any significant metalwork in its structure or do you bring in "earthy" metalwork from the house (eg buried metal water pipes)?
This has implications so it would be best if you can either say "wooden shed" or describe what you have. Were you expecting to provide the earth to the shed via the armour, the armour + a third core or neither and have a local earth rod?
If a sparky is actually doing the work, it might be best to ask them what they expect to be connecting up
This is out of date (16th Edition IEE regs) but it contains much wisdom:
Cheers
Tim
Why not put in a uPVC cable duct with a draw wire so that the cable can not only be installed, but easily replaced or repaired in the future without digging up your new patio?
6mm (£1.75pm) 4mm (£1.31pm)
4mm might be sufficient, but why skimp for a tenner.
The electrician will tell you.
The shed will be brick and block with cavity about 14ft by 11ft inside. The lighting will probably be a fluoresent tube to minimise shadows. No other lighting. The double socket will be used with a 2300w angle grinder as a max load. As for earthing...I haven't a clue! Suggestions sought. No metal waterpipes.
mark
Not really in the spirit of this group though is it? ;-)
A more comprehensive answer can be found here:
Good idea. Any suggestions on where I could get 20m of flexible ducting?
mark
John Rumm wibbled on Sunday 13 December 2009 20:16
I was looking for that article!
Must have had the cloak of invisibility on ;->
To be fair it is a PITA to find if you don't know what it is called. I must create some aliases for it!
Why does it need to be flexible? - why can't you just use 40mm plastic waste pipe, preferably solvent weld
ring is rated at 30A, lighting at 6, so 36A needed. Put in the fattest you can afford. Cable cost is low, digging is not. I'd go for a 60A with
60A RCD on its far end to power the consumer unit.
The chances of being able to pulling 6mm SWA through a bit of buried flexable ducting strikes me as being close to zero. Bung in a bit of
110mm drain though in theory as it's a power duct it ought to be black not brown. Plenty of space to get the SWA through and and space for another smaller duct to carry low voltage phone/network/coax (DSAT or DTTV) etc.
Silly idea. The cable is built to take more than a plastic duct will.
Just get a reel of fat cable and lay it as 600mm down in the ground
Stick a few cat5es in there too, and maybe a hose or rigid waste pipe as well to enable future cable to be added. Cost peanuts, and you'll kick yourself later if you dont.
NT
Shed with brick/cavity/block wall is quite substantial. If this is to house substantial tools (fixed circular saw, fixed router, lathe), then aim for a substantial cable. Conversely if you will only ever need a 2kW fan heater, 2kW power tool & lights then 6mm 3core SWA will be fine.
Earthing depends on construction, any incoming services, any class-1 tools outside (or inside on concrete floor really). So it is possible the spark will want to TT the shed with a local earth tod, however using 3-core over 2-core is little different.
Cable installation.
- Duct - "perforated land drain" of 80-100-110mm.
- Burial depth - Suitable for ground usage, which means 450mm unless cultivated whereupon 600mm+ is better.
- Trench clearing - Clear trench of stones even if you use a duct
- Trench preparation - Base layer of riverwashed sand, duct or cable
- Trench infill - Drop in duct or cable, cover with 150mm of riverwashed sand
- Marking tape - yellow "electricity cable below" tape, splash of sand, fill in
Ebay do suitable black duct, 25m of 80mm is =A318 plus delivery. The proper duct is called "twinwall" which has a smooth inside and is going to cost you over =A3100. You must keep bends very long to facilitate drawing in of the cable. Pick up some polypropylene rope - feed it through the duct via a vacuum cleaner & cloth or flexible cable-rod.
TLC Direct do yellow electricity marking tape, as well as suitable SWA cable & CW glands.
If you lay the duct in advance, take a few digital photo's of a tape- measure on top showing the depth and route. That way there is little concern about the spark signing work off (if he is bothered, most are too trashed from trying to carry immovable 10mm FTE reels to want to go digging a 20m trench in a garden without a mini-digger, dynamite etc).
You can not run other cables in the same duct under 17th.
So if you need other stuff you need another duct. 25mm flexible conduit "contractor pack" in 25m length would do for Cat5/Telephone for example.
I've done that. I was a cheapskate and got about 7 metres of 20mm OD pipe. I managed to thread 2.5 T&E through it. The pipe is buried 600mm deep in the gound and has pieces of wood over the top of it plus a plastic warning tape.
js.b1 wibbled on Sunday 13 December 2009 23:55
I agree. 6mm2 will give the OP the ability to run a 32A circuit, so he has the option of a 32A ring and a 6A lighting circuit (MCB or FCU) - the fact that 6A + 32A > 32A isn't something I'd worry about. The design load (as opposed to the protection rating) of the lighting circuit is actually tiny and the head end of the (house) will be protected at 32A anyway.
The only deviation from a standard circuit would be using a single cable to feed a ring. However, this is logically very little different to a distribution circuit feeding an MCB then a ring, only the MCB is at the feed end and provided the main parameters (L-E impedance, voltage drop, current carrying capacity) are met *in total*, I personally don't see any issues with this.
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