Shed Electrics

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Can someone help me with their thoughts on shed electrics.
I want to run electric to my shed, and am planning to use 2.5mm SWA cable so its protected. How deep should I bury it - I can do this under a hedge so should avoid random spade attack etc.
I want to run 1 internal double socket, I IP55 external socket, 1 internal strip light and an external PIR light.
Real questions are:-
1) What should the MCB rating be at the house consumer unit and should it be RCD protected?
2) Do I need to add another consumer unit in the shed? If so what should the maximum RCD rating be on this?
3) Should I add a local earth rod? If so what guage of earth wire should I use to connect to it?
4) Is there an electrical book that I can read, that can help me with this kind of question in the future?
Thanks
Quigs
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Try the Which? guide to wiring and lighting. Got it for Chrimbo and its great.
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In an earlier contribution to this discussion,

Until these questions are answered, how do you know that 2.5 mm^2 cable is appropriate?
--
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wrote:

Of course!
What would your recommendation be - by the way the shed is approx 25 m from the consumer unit.
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Have a Google there are plenty of threads on this.

Assuming that is sufficient.

It's not that easy digging near a hedge...
There are no specific regs, If it is an area that is at all likely to be cultivated then go for about 450mm. i did the one up the garden like that, the one from GH to shed runs beneath a path and I put in shallower.

16 Amp should do. Personally I would have an RCD at the shed end in the CU, and instead of an MCB use a switch fuse unit.

You could get away without it I guess, but it would be better with one. an RCD protected one with say 2-3 ways - 16A MCB for the sockets, 5A for the lights. you can get pretty cheap small 'garage' CU from Screwfix for example.

Possibly, there have been discussions on this, unless the shed is near the house I would go for this option.

The Which book is pretty good, though some bits are not covered so much.
One important question to answer is the length of the cable run. This affects the cable size required as on longer lengths such in gardens voltage drop can be an issue. You might need greater than 2.5mm cable.
--
Chris French, Leeds

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If you use SWA cable for the supply, then try to go as heavy as you can without being over the top. If you use a two core SWA with 6mm conductors from the house and have it protected with a suitable breaker, 45amps will do it, within the consumer or sub-mains switch, then you can really take the supply as dealt with and safe for anything, other than welders and massive heating elements, you want to fit and work with in the shed. Burying the cable is the one thing people don't usually do properly and the only advice I give them is to bury it as far down as possible so it doesn't get dug up again accidentally. And that doesn't mean by you because you know it's there, it's also by other people who might live in your house after you do. Having power into the shed is also a great selling point remember.
Within the shed itself you should have a consumer unit with enough ways to take a small ring mains circuit protected at 30amps for wired fuses, or 32amps for an MCB breaker, and run your sockets from this. A lighting circuit will be protected by a 5amps wired fuse, or 6amps MCB breaker,, and this is enough to run even halogen security lights from if you wish. It's also a good idea to have an all weather socket low down beside the door and separately fused from the other circuits and can be used for all manner of things in the garden.
Always make the system separately earthed from the house circuits, but this doesn't mean separating the bonding on the SWA cable between the two points. Use the house earth to protect the outer skin of the cable between the two consumer units, but have a separate earthing rod into the ground locally to the shed and connected directly to the earthing bar in the shed consumer unit. It doesn't interfere with any of the systems proper working profiles and just gives the added local protection needed for this type of installation. Make sure that the impedance between the rod and the ground is as low as humanly possible, so this night mean you have to bang in a two metre length of 15mm copper pipe to have a proper protective local earth.
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Regarding the earth.
So, you take the cable from the house csu to the shed and do you connect the earth to the shed csu or direct to the earthing rod.
The which? book doesn't make this clear and actualy says not to connect the house>shed cable earth wire to the shed csu, connect the shed csu to an earth rod, but what would you do with the house>shed earth?
It's probably obvious to you all but not to me.

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It seems that some people isolate the cable earth at the shed, whilst others connect it to the earthing terminal. Obviously, there is always a connection to the earth rod.
I can see advantages to both. Using the cable introduces a second earth to the system, in case one earth fails. However, using the cable could result in current flowing from the house to the shed under earth fault conditions.
However, I suspect any such flow would be shortlived and limited in current, preventing the small earth SWA earth overheating. Also, the supplier's earth is probably much better anyway.
In any case, there should be an RCD in the shed covering the entire installation. The "best" solution is a time delayed 100mA RCD with 30mA RCBO for sockets and 6A MCB for lights. However, unless you are using dangerous machinery in the shed, this is overkill and you could get away with a 30mA "whole shed" installation. The difference is (apart from being around 40 quid cheaper) that when you chop the power cord, the lights go out (which isn't a good idea if you are holding a still rotating dangerous piece of machinery!)
Christian.
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Do you think having a 2nd earthing source is good practice? My query was do you feed house supply cable to earth rod or to the shed csu first and then hook the earth rod into the shed csu.
"The "best" solution is a time delayed 100mA RCD with 30mA RCBO"
Isn't a RCBO a MCB and RCD combined? Which just plugs into the CSU (v. expensive though). Is it possible to get different mA's for the RCBO?

others
connection
conditions.
current,
earth
RCBO
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The "best" solution would have the following
incoming SWA | +---> 100mA -------+---> 6A MCB ---------> lights time delay | RCD +---> 20A/30mA RCBO --> sockets
The RCBO could be a different size (i.e. 16A/32A) depending on the size of your main feed cable. The above is total overkill for a shed that isn't really a workshop with dangerous machinery. The solution above is better, because in the event of an earth fault on the sockets, the RCBO is guaranteed to blow before the 100mA time delay unit, keeping your lights on.
For normal shed purposes, the following is cheaper:
incoming SWA | +---> 30mA -----+----> 6A MCB ---> lights RCD | +----> 20A MCB --> sockets
You may be able to use the RCDs as replacements for the switch incomer on the consumer unit. You can almost certainly buy the second solution off the shelf as a preloaded consumer unit under the "garage" consumer unit section. It will be much cheaper.
i.e. http://www.screwfix.com/app/sfd/cat/pro.jsp?ts (438&idc138
Has a 16A MCB, instead of a 20A. However, at 25 quid, it is cheaper than just a single RCBO.
Christian.
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I though it was good practice to have the shed earth separate from the house earth (if there was an earth at the shed end) The supply goes to the CU not the earth rod.
My shed/GH supply is fed with 2 core SWA. The steel sheath of the SWA is earthed at the house end. It is not connected to earth at all at the shed end. The same would apply if the cable was had an earth as well.
The SWA cable terminates at the shed end in a small plastic CU (if you used a metal CU it would need an isolating gland). The CU earth terminal is then connected to the earth rod.
Shed CU has a 30mA RCD protecting all the circuits.
At the house end the supply is connected to a Switch fuse unit (Wylex make them) into the supply via a Henley block.
Did you work out the cable size needed?
--
Chris French, Leeds

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"Matt Pearson" wrote in message

This thread seems to be generating more heat than light and not much in the way of signal-to-noise ratio.
Briefly, there are two main earthing options:
1. Exported house earth ----------------------- Simply 'export' the house earth via the armour of the SWA cable (the 'submain'). This will be fine for a dry timber shed with its floor raised well off the ground, so that it remains dry. With this option the shed installation has TN-S earthing and RCD protection is only essential for sockets likely to be used for feeding portable equipment outdoors. If non-RCD protected circuits are used then you need to be sure that the earth fault loop impedance (Zs) is low enough to operate the fuse or MCB at the house end within 5 seconds in the event of an earth fault at the far end of the submain cable, and to operate the protection for any socket sub-circuits in the shed within 0.4 s.
If Zs is too high then the options are: (a) use a larger size SWA cable, and/or; (b) use a 3-core SWA cable with one core as a CPC (earth) in parallel with the armour, or; (c) use an RCD at the house end (a 100mA Type-S RCD if you want discrimination with any RCDs in the shed itself).
With this option there is absolutely no point in using a local earth electrode; its resistance simply will not be low enough to do any good.
Do NOT use this option if the house earthing is TN-C-S (PME) and either of the following apply:
- the shed floor is damp, or the 'shed' is in fact a greenhouse;
- you plan to use Class I (earthed) appliances outdoors on a regular basis (most portable tools etc. are Class II ('double insulated).
If there are any metal service pipes entering the shed (water, etc.) then they should be bonded to the incoming earth near the point of entry.
2. Separate 'TT' installation ----------------------------- This option uses independent local earthing and is ultimately safer, but only if the RCD(s) are correctly selected and regularly tested, and the earth electrode is properly installed and maintained. The rationale is that by limiting the current dumped into the local earth, its voltage will not rise to a dangerous voltage relative to the ground.
The armour of the SWA cable is earthed at the house end, but MUST be isolated at the shed end. This can be achieved by using a special isolating gland (expensive) or by terminating the cable into a plastic housing via a plastic cable gland. The armour should be treated as being live, so should not accessible or connected to accessible metalwork.
ALL circuits in the shed must have RCD protection. For a comprehensive workshop installation follow the advice in the OSG and use at least two RCDs, or separate RCBOs, but for a simple garden shed a single 30mA RCD will be quite OK. This can be at either the house or shed end, but it migh be inconvenient to have to walk back to the house if it trips. If the RCD is at the shed end then you still have to make sure that the submain cable is fault and earth fault protected - see remarks about Zs, above.
An earthing system is required. In most soils a single 8 ft. earth rod (two 4 ft. sections screwed together) will be OK, but the earthing resistance should always be measured. The BS 7671 requirement is that the product of the earth electrode's resistance to earth and the highest rated RCD must not exceed 50 volts - or 25 V if it's a 'horticultural installation' (greenhouse). This allows quite high earth resistances to comply with regs - 1,600 ohms with a 30 mA RCD, but note that the OSG recommends a maximum value of 200 ohms. The connection to the earth rod should be in 16mm^2 wire (unless protected in conduit, etc.) and the joint should be accessible, unless welded. Using a proper 'earth pit' from the electical wholesaler is the professional way to do this.
~ ~ ~ ~ ~ ~ ~
Going back to the original enquiry: this is only two lights and 'small power' in a shed, probably just for running portable power tools and maybe a fan heater, with the outside socket being for a lawnmower or hedge trimmer, etc. It's hardly a comprehensive workshop installation.
Design ------ The first step, as always, is to establish the maximum demand and I'd guess that about 4kW would be adequate, or 5kW at the most. If that's the case then a 16A (3.7kW max.) or 20A (4.6kW max.) circuit from the house is all that's required -- unless the OP wants to provide for future expansion. There's no real need for a separate consumer unit here. To keep things simple the sockets could be connected directly to the circuit cable and the lights fed via switched fused connection units, fitted with a 5A fuses and used as the lightswitches. You could also add a 20A control switch to act as a main isolator if desired. Something like this, perhaps (or re-arranged to suit the desired physical layout):
DP SW FCU DBL SKT FCU 20A 5A 13A 5A __ __ __ __ From house >----------<|__|-----|__|-----|__|------|__| + | ^ *| | | | | | | | Shed <-- | --> PIR light | light | __ ---|__|* Outside socket
RCD options (if no RCD at house end): + use RCD as main isolator (essential if using local TT earth option) ^ insert RCD here (for 'exported' TN earthing only), or * use RCD sockets
For a tidy looking job use metalclad wiring accessories mounted side-by-side and coupled together using 20mm conduit bushes and lock-rings. Terminate the incoming SWA onto the first accessory (the DP switch in my sketch). The only additional wiring is then that to the lights and outside socket.
Cable size ---------- The max. circuit lengths in 2.5mm^2 cable are 33m for a 16A circuit and 27m for 20A (voltage drop limited). This makes the use of 2.5mm^2 SWA rather marginal and I'd strongly recommend using 4mm^2 to keep the voltage drop down. The difference in cost is tiny.
HTH
--
Andy



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It is the "MUST" part above that needs explaining. We established in a previous thread that in a TN supply/TT generator switchover system, that the TT earthing rod required for generator TT operation may be connected to the TN supplier's earth. Why does the same not apply here, provided that the earth rod is tested to below 200 ohms and the RCD requirements are met? Is it some sort of earth loop current issue?
Christian.
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"Christian McArdle" wrote in message

Sorry for a rather delayed follow-up. If you don't break the earth then you have effectively exported the house earth and your shed is a TN system, not TT. And, unless you've sunk a massive earthing system, your local earth electrode will be bugger-all use in preventing a rise in voltage above the local ground potential if a supply network fault is trying to pull it up toward 230V.
The need to get away from the supplier's TN earth is relevant where the equipotential zone concept is difficult or impossible to apply - hence the reference to greenhouses and Class I equipment used outdoors. Remember that TN earths can rise to potentially dangerous voltages due to (for example) cable faults on TN-S systems, or the classic broken service neutral on a PME supply. OK, these things are pretty rare, but they do happen, which is one reason why equipotential bonding is required. The big advantage of TT earthing, properly implemented and maintained, is that your earth really is at the local ground potential, and will stay there. Even if the earth electrode has the maximum recommended resistance value of 200 ohms (see OSG), then to trip a 100mA RCD shouldn't lift the earth by more than 20V.
The downside of TT is the reliance on RCDs, which don't always fail safe, and the need to ensure that the earth system is maintained. Hence in dry conditions it's probably best to stick with TN.
An alternative protective measure for PME installations now being recommended by the IEE [1] is to provide your own earth electrode and connect it to the main earth teminal in the _house_ installation - i.e. in parallel with the supplier's multiple earthing. The recommended maximum earth resistance in this case is, for domestic installations, 20 ohms - not necessarily easy to achieve with DIY efforts in some soils. Then, in the event of a break in the supply neutral, there is a path to earth which will tend to reduce the touch voltage on your earthing. If you only have a few lights switched on this will be a quite effective safety measure, but it won't help a great deal if an electic shower is in use at the time of the fault - that's when you really rely on the bathroom's supplementary bonding to save your life.

I hope that's clearer now from the above. A standby genny on your own premises can always be operated as a TN system - why would you want use TT for that? You do have to provide your own independent earthing of course - and the parallel earth electrode approach referred to above would kill two birds with one stone.
[1] See, for example, section 12.5.4 of the current /commentary/ on BS 7671, ISBN 0852962371. (Recommended reading for all wiring regs geeks.)
--
Andy




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In an earlier contribution to this discussion,

Would you care to explain that a bit more?
I am considering a small genny to power my central heating plus a couple of freezers and some lighting in the event of a power cut. All devices fed by the genny would be unplugged from their usual mains sockets and plugged into the genny instead - and would thus be totally isolated from the house supply. [I have wired my CH through a 13A plug rather than a FCU to facilitate this.] What do I need to do about earthing these appliances when they are being powered by the genny?
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the
[...] <snip a whole load of "signal">
I would vote for the inclusion of that in the electrical section of the uk.d-i-y FAQ, if it is sufficiently generic to deal with a lot of the export/don't export earth questions.
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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On Tue, 20 Jan 2004 09:54:16 -0000, "Andy Wade"

Nice write up. As this comes up a lot, how about sending to Phil to go into the FAQ....
I've used both of the described methods for different applications.
Do you have any particular suggestions where the provisioning is for a workshop and heavier equipment is in use - say 30A single phase, where the workshop is dry and the house supply is TN-C-S?
.andy
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"Andy Wade" wrote | Briefly, there are two main earthing options: | 1. Exported house earth | ----------------------- | Simply 'export' the house earth via the armour of the SWA cable (the | 'submain'). This will be fine for a dry timber shed with its floor | raised well off the ground, so that it remains dry. ...
| Do NOT use this option if the house earthing is TN-C-S (PME) and ^^^
Should that 'and' be an 'or' (PME earths should never be exported)?
| For a tidy looking job use metalclad wiring accessories mounted | side-by-side and coupled together using 20mm conduit bushes and | lock-rings.
And I'd suggest running a green/yellow earth wire from the SWA gland through each box and connecting to the earth terminal in each box and on each faceplate, rather than relying on conduit-box-faceplate touching contacts. Maybe you assumed this would be done, but better to be explicit.
Owain
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"Owain" wrote in message

No, I meant "or" - but you are free to disagree ;-). I wouldn't say "should _never_ be eported" - "never, without due consideration" perhaps.

Yes I agree entirely - Reg. 543-02-07 demands that anyway.
--
Andy



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