Shed Electrics

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

Reply to
Quigs
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Try the Which? guide to wiring and lighting. Got it for Chrimbo and its great.

Reply to
Robert Irwin

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

Reply to
Set Square

In message , Quigs writes

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.

Reply to
chris French

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.

Reply to
BigWallop

Of course!

What would your recommendation be - by the way the shed is approx 25 m from the consumer unit.

Reply to
Quigs

formatting link
help

-- Adam

adamwadsworth@(REMOVETHIS)blueyonder.co.uk

Reply to
ARWadsworth

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.

Reply to
mp

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.

Reply to
Matt Pearson

The cable you use as the supply should be connected at both ends regardless. The only thing you need then is to connect the earth bar in the consumer unit to an earth rod in the ground local to the shed. The requirement is not to rely on the length of cable your using as the supply as the main earthing point of the shed, because if this breaks down in any way it is not supplying a proper earth point to the shed.

Reply to
BigWallop

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.

Reply to
Christian McArdle

How deep should I go, I heard 2m but that seems a lot for 2 or 3 double sockets and 1 or 2 lights.

Also what, in your opinion, supply cable mm2 size should I use 4 or 6mm2? Should a 4mm2 use 30/32A and 6mm2 use 40A. Plus what earth cable to rod should i use?

Many thnx

Reply to
Matt Pearson

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?

conditions.

Reply to
Matt Pearson

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.

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a 16A MCB, instead of a 20A. However, at 25 quid, it is cheaper than just a single RCBO.

Christian.

Reply to
Christian McArdle

What value do you put on your life ?

Dave

Reply to
Dave Stanton

If you go with a 6mm csa' SWA cable with a 40amps breaker in the house, then you can literally fit and forget it, because it is heavy enough to take any sort of load you put on, within reason of course, and the difference in the price between 2core X 4mm and 2core X 6mm isn't that much for a more safety conscious supply that is going to last for many, many years.

The earthing cable between the CU and the rod should be taken in 16mm which is the heaviest needed by any installation and introduces the least impedance (resistance) to any fault current. This 16mm cable combined with a 2mtr length of copper pipe hammered into the ground, and a proper earthing strap wrapped around it, is going to give you a very safe supply which will keep its integrity through many a large fault condition if it needs to. What you want from a local earthing point is that the impedance is as low as possible between the consumer unit and the ground. So taking 2mtrs of copper into the ground is about the right amount to keep good contact between rod and ground in almost all conditions.

The consumer unit should also have an RCD main switch fitted which will drop the supply at the slightest hint of a local fault in the outbuilding without causing nuisance tripping back at the house.

Reply to
BigWallop

In message , Matt Pearson writes

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?

Reply to
chris French

As per the regs then?

From the fixed wiring in the shed.

Kind of true, actually entirely true, however the regs don't allow you to do this.

That is assuming there is a suppliers earth and it is all correctly connected to the installation.

I personally favour an RCD at the house to cover the cable also in the event of a spde related incident or similar!

As you say, a prebuilt 'garage unit' with 30mA RCD, 16A and 6A MCB in one pvc enclosure would be the cheapest solution.

SJW A.C.S. Ltd.

Reply to
Lurch

many thnx for your advice, it's all clearer now

Reply to
mp

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.

  1. 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 >----------

Reply to
Andy Wade

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