Supplying power to outside sheds - couple of questions

[Hallsy]

I know this has been done before and I have searched but after trawling through many threads I just want some clarification on what I intend to do.

First off, I am a qualified electrician, however I am not a domestic spark, I am industrial so do industrial install and machine tool breakdowns/modifications. I therefor have a good understanding of electrcis in general but do not do house bashing day in day out.

I have been asked to put power onto a couple of sheds for my sister-in-law. They have already obtained some SWA, I'm guessing at 4mm (it was dark) but would prefer it to be 6mm.

There is one spare way in the CCU at house, this is on the RCD side.

I intend to bring the cable up the side of the house and into the house next to the CCU, I'll make the SWA off in a metal adpatable box and pass the stripped cores straight through to the CCU. At shed end I will bring the armoured up into the shed, make it off in a metal box and take to a 32A mcb for the ring, and a 6A mcb for the lights. I will either run T+E in plastic trunking and use metalclad sockets, or will use singles in metal conduit.

First question, I would prefer to have an RCD at the shed, but as I am coming off of the RCD side at the house will this cause problems with discrimination? What type of RCD should I use?

The two sheds will be close together but not joined, they want two double sockets in one shed and one in the other, and a light in each. Am I OK using one ring main and one lighting circuit to supply both sheds? I would most likely pass the cables from one shed to the other in adaptaflex (water tight flexible conduit).

For discrimination if I use a 32A mcb at the shed I will need to use a 40A at the house - right? In which case if the cable is 4mm it will be too small. If this is the case could I use 32A at thouse CCU, and use a 25A 30ma RCD in shed ( I have one of these surplus), then 20A for ring and 6A for lights? I don't think they plan to use anything too big in the sheds, it is mainly to supply a freezer.

Finally I am unsure what to do with the earths, I forgot to check what system it is, will do when I pop over this week. I would estimate at around 40M run max. Would it be advisable to use a stake at shed?

 

[Steve]

1. how far is the shed from the house?

2. this is a suggestion:

Run one radial from the house in 4mm² SWA, protected at the house end with a 32A MCB.

Run this to an RCD at the shed, a 40A 30mA should suffice. From the RCD enclosure, go to your sockets in shed 1 and an FCU for the light.

Then run another SWA from shed 1 to shed 2, to the sockets in this shed and an FCU for another light.

Metaclad sockets would be nessecary for this method, in order to terminate the SWA in them, not much dearer than the plastic ones.

3. note exporting TNCS earths (see for reference to see which earth system you have). If you have PME earth, you need to earth the SWA at the house end but not at the shed, and earth from the RCD onwards using an Earth rod, thus making the shed a TT install.

4. note part p

EDIT sorry didnt read post thoroughly. for 40m, i think you need a 20A MCB at the house. Still shouldnt worry since freezers only draw a few hundred watts, if that.

 

[Spark123]

Please be aware of issues using a freezer in an unheated environment: //www.diynot.com/forums/viewtopic.php?t=29820&highlight=freezer

 

[plugwash]

3. note exporting TNCS earths (see for reference to see which earth system you have). If you have PME earth, you need to earth the SWA at the house end but not at the shed, and earth from the RCD onwards using an Earth rod, thus making the shed a TT install.

some recs reccomend you do but there are afaict no requirements to do so.

and installing an earth spike is really not a diy job because you have no idea if its resistance will be acceptable until you actually test it, soil resistivity varies hugely.

 

[Spark123]

The impression I am under re exporting PME earth is it is OK providing there are no extraneous conductive parts which require main eq bonding. If there are extraneous conductive parts requiring main eq bonding and the CSA of circuit protective conductor of the cable supplying the outbuilding is less than that of the PME bonding conductor, a suitable additional conductor needs to be installed or the PME earth not used i.e. go for TT.

 

[Hallsy]

Thanks for the replies guys, I will check the earth system on thursday and decide what I am going to do with the earth then.

I reckon using FCU's for the lights will be the best method, saves on wiring which saves my time.

I don't fully understand what you are saying with the fusing though, initially you said use a 32A mcb at the house then use 40A 30ma RCD at shed. Surely this is wrong as an overcurrent situation in the shed above 32A but less than 40A will activate the mcb at the house rather than shed (well depending whether the 32A shed ring one went first) unless an neutral/phase fault is present. I though the mcb at supply (house) would have to be larger than the RCD.

Hold on, I'm thinking of RCBO's here!! The current rating on an RCD, this is just the maximum rated switching current right, so you don't end up welding the contacts shut? Sorry, I was having a moment then!!

But why would the mcb have to be downgraded to 20A due to the 40M run, I thought volt drop would only be of signifigance for length of run?

As I've said I have a 25A 30ma DP RCD that is surplus and would rather use this if possible as they are a it hard up and I don;t really want to put myself out of pocket.

Would 32A mcb at house CCU (RCD side) to 25A 30ma RCD at shed with 20A mcb for 3 DG sockets and 2 lights be adequate?

I hadn't heard about freezers not operating well below certain temps but it makes sense. I'll mention it to them.

As for Part P, there was already a circuit for a shed in place, I am modifying this circuit. Although, as it's outside it may still be against Part P. If they ever have reason to produce a certificate they will have to get it tested.

 

[Steve]

The 20A MCB in the house is to protect the cable, and the circuit in the shed, from overcurrent events.

The RCD in the shed is to protect you from electric shocks, and happens to be rated at 40A, since this is a common RCD rating, and a cheap one too, however, as you have a 25A one spare, and you are protecting the whole lot with a 20A MCB, you can use this.

While you're there on thurs, see if there is already an RCD in the house protecting the CU where you are coming from. If there is an RCD here, you don't need one in the shed.

Also, i dont know what you class as a long run of cable, but 40m is a long way in my book.

 

[Hallsy]

Thanks again,

I realise the breaker at the house is to protect the cable, but you should still have discrimination between over current devcies in circuit. That was all Iwas tring to say.

Anyways, as I have mentioned the house CU has one spare way on the RCD protected side, this is where I will be fitting a 32A MCB (if cable is big enough, if not then 20A). I realise that I wouldn't NEED another RCD at source, but I find it preferable to have the RCD protection as close to final circuit as possible (i.e. at the shed) but am unsure how an RCD protected breaker at the house followed by an RCD at the shed would react in the eevnt of short circuit.

40M is a generous estimate by the way, I'll measure it on thurs properly and work out volt drops, etc.

As I say, the main thing I am worried about is the earthing arrangements, but i can't go any further with this until I go over again on thurs.

Thanks again.

 

[Hallsy]

Something else that is bothering me is the CU in the shed. It's not strictly a CU, I was going to use an adpatable box with a bit of din rail in to hold the RCD/breaker and some terminals. But really I'd prefer to use a proper enclosure, as I've said they are hard up and I initially didn;t bother getting one because it would probably have to come out of my own pocket, but then I saw these:

http://www.tlc-direct.co.uk/Products/HGV304.html

Any good for what I want? Or should it be metalclad in a shed? I could still use a small adpatabel box to make off the SWA before the CU.

 

[Steve]

Having 2 RCDs with the same trip rating (30mA) in a row isn;t a good idea. Either of them could trip, or indeed, both of them, under fault conditions. Discrimination is more important with RCDs than with MCBs. If you have a RCD at the house, dont fit one at the shed. No point.

If you do things the way i described, and you have an RCD at the house, you don't need any enclosure or MCBs at the shed, just terminate the SWA in a metaclad socket.

 

[Hallsy]

Good point, I hadn't thought of that. I was just getting hung up about having an RCD as local to socket outlets as possible.

As you said, coming off the RCD side of the CU with a breaker (20A or 32A depending on cable) then going straight into a radial. Then two lights taken as spurs from two FCU's, job done Although, would it be worthwhile installing some sort of isolator in the shed before the first socket, in the case of an emergency seeing as the nearest point of isolation would be inside the house?

 

[Hallsy]

If only I hadn't looked at this job in the dark!! Went over this morning to get the sockets in and wired at the sheds, also to look at earthing arrangement - which I forgot to do!!

Nightmare, sheds were both full to the brim with crap, and then there was hardly any structual timber inside them, I did well to find places to secure two double sockets in one of them, but the other had nothing!! So not only have I got to wire the sheds, I've got to put my carpentry skills to good use!!

The SWA hadn't been buried although I was lucky with that one as it was only 2.5mm and was in 3 phase colours, brown, balck & grey. I paced it out at around 25-30m, but I'll measure my pace to get a better estimate. I haven;t checked in regs yet but 2.5mm won;t do I'm pretty sure, it will have to be 4mm backed.

As this circuit will be a radial, straight from breaker at house CU > 4mm SWA > first socket > 2nd socket (spur for light here) > 3rd socket (next shed, also spur for light from here) If my long run in SWA is 4mm do I need the rest of the T&E in the sheds to be 4mm? If i had fusing again at the shed then I understand that I would use 2.5, but coming straight from the 4mm SWA radial leg does it need to stay 4mm?

Hope someone can give me some advice before tomorrow as I'm going around early armed with some 2x2 and everything else I need to hopefully get the job sorted!!

 

[Hallsy]

OK, had a quick look in on-site guide, seeing as it is a radial of at least 25m to the first shed, then another 5m-10m worth of cable after that then table 7.1 shows that I would need to use a 4mm cable throughout backed up by a 20A MCB, allowing 43m at 0.4s disconnection - right?

I realise this is a guide and that I may get slighty different answers after working out the volt drop based on deisgn current, etc but I'm guessing it won't be far out.

So for a radial as decribed I will need to use 4mm conductors throughout - yes? Still OK to spur from it?

So now I've got to lay my hands on some 4mm SWA, that'll go down well with the in-laws. I hate jobs like this
Unles anyone can think of a way of using the 2.5mm SWA (without moving the shed closer ) safely? But then again I don;t really want to use it as it isn't in single phase colours anyway.

EDIT: Just realised that you don;t get single phase plus earth colours in SWA, so it seems that brown is phase, grey taped blue as neutral and black is taped yellow/green for earth. Great idea this harmonisation - yeah right!!

 

[plugwash]

EDIT: Just realsied that you don;t get single phase plus earth colours in SWA, so it seems that brown is phase, grey taped blue as neutral and black is taped yellow/green for earth. Great idea this harmonisation - yeah right!!

it can be obtained but its not common or reasonablly priced.

 

[Spark123]

OK, had a quick look in on-site guide, seeing as it is a radial of at least 25m to the first shed, then another 5m-10m worth of cable after that then table 7.1 shows that I would need to use a 4mm cable throughout backed up by a 20A MCB, allowing 43m at 0.4s disconnection - right?

Yes,
Volt drop is the limiting factor in this case.

So for a radial as decribed I will need to use 4mm conductors throughout - yes? Still OK to spur from it?

You may use any conductor rated 20A+ as long as it is installed correctly and as long as it is within the volt drop and efli spec.
You can extend a 20A radial circuit to cover a floor area of up to 50m².