garage/extension wiring queries

[wookey]

OK, so I have a reasonably 'green' extension built, and it could really do with some wiring.

The extension is between house and garage and is 27m2. The garage (split into 'workshop' and 'pottingshed') is 20m2.

All I want to do is put in 'lots' of sockets in the workshop, 'some' in the potting shed, 'some' in the extension, and have some lights (all LED and probably experimental DIY jobbies).

I put in a 50mm conduit to the garage from the house under the extension floor (about 1.8m long - it's just a corridor at that point).

The obvious way to do this was to put one bit of 4mm2 cable through the conduit to a small CU in the garage, to provide the new circuits (routing in there is easy as there is plenty of exposed timberwork and it's only 'garage grade' decoration and the nice bits haven't been plastered yet.

However when I did the sums from the onsite guide I found that I needed 10mm2 cable through the conduit (!). That really doesn't seem like it ought to be right. My mate is wiring his huge new shed up in much the same way but the (20m long) spur to the shed is 2.5mm armoured.

So - here's my sums can someone tell me where I'm going wrong? Or If I'm not wrong do they have better ideas?

workshop circuit: 32A ring (2.5mm)
Potting shed+extension: 20A radial (2.5mm)
lights for all: 5A (1.5mm)

The idea is to have the workshop on a separate circuit because I'm quite likely to trip it messing with something or other from time to time.

Diversity rules (table 1B, appendix 1 in 17th onsite guide) say I should allow 100% of  largest circuit plus 40% of others and 2/3rds of lighting circ. That's  32+20x0.4+5*0.66= 43.3A

If I put some insulation in the conduit end (to avoid thermal bridge into kitchen then that requires derating of 0.78 for 100mm, or 0.88 for 50mm.

The tables for current carrying say: (6E1, appendix 6) that going through an uninsulated conduit 23A is the most a 2.5mm cable can have. 4mm is 30A, 6mm 38A, 10mm 52A. So I actually need 10mm cable(!) for that load, even if there is no insulation on the run. Am I correct to use the diversity rules in this way to calculate the current in the cable to the garage/extension CU, or have I got the wrong end of the stick?

OK, so maybe 32A for the workshop was excessive, even allowing for buying some big toys one day, so make it 20A. That still comes to 20A+20X0.4+5*0.66 = 31.3A. Which is still too fat for 4mm, and would be too fat for 6mm if there is 100mm of insulation.

But I could wire a 20A radial all the way across the house from the CU, through the conduit and round the workshop and extension in 4mm and that would be OK (up to 53m, which is plenty). This doesn't make much sense to me.

The lighting being 5A is a bit silly. This is going to be super-efficient LED - max power consumption is everything is turned on 60W (Allowing 1.8 for power factor of driver units, thats 108W, or just under 0.5A). So a 1A circuit would be plenty. Presumably there is nothing stopping me do that if I put a 1A MCB on the circuit? Table 1A says to use 100W per lighting lampholder, but I won't have any lampholders and it's clear that those numbers are advisory, so I can't see any reason not to do this, except that 1A MCBs seem thin on the ground, and expensive when you do find one (£14+VAT).

That would bring the requirement down to 20+20*.4+1*.66 = 28.66A which does just get us into 4mm2 territory if insulation is avoided.

So, am I doing my sums wrong? Have I got this backwards and should work the other way, saying OK, if I use 4mm to the garage CU then it can only take max current of 30A. Derate *.78 for insulation to get 23A and thus put the whole lot on a 20A MCB (which in practice is a 2kW fan heater plus 2 or 3 power tools all going at once, which will be fine. That gives a rather more pleasing answer, but seems to make the diversity allowances moot.

In note that 0.4s disconnection is needed for normal circuits but 5s for 'dsitribution' circuits. Is a cable to a secondary CU a 'distribtuion circuit'?

Oh, and what about doubling up a 2.5mm cables as I have lot of that but need to go shopping for 4mm? Is that permitted, and then treated as 5mm2, probably derated for grouping ? I suspect it's frowned upon for some reason, but it's quite a lot like fitting a ring main, and we like those in this country.

One other thing. I find the whole onsite-guide way of doing things rather confusing - piles of tables and you have to read over and over (if unfamiliar) to work out whcih table applies with a lot mysterious refs to 'method 103' and 'BSxxxx fusing', so you are forever looking up what they actually mean. Much handier would be a nicew spreadsheet to fill in each circuit/cable section then do the sums for current capacity/voltage drop/disconnection time. Does anyone have such a spreasheet, or shall I carry on making my own?

I've rambled on there rather, but I hope the basic issue is reasonably clear. Clues welcome as I really don't want to get told I've done it all wrong when it comes to getting it certified.

And finally: I note that separate of low and high voltage circuits is needed. I specified a 70-mm conduit but the builder failed to find one so put in 50mm. I wanted him to do 2 but that never happened so now I have a problem getting the ethernet/1-wire cables to the garage down the same 50mm conduit as the mains. It'll be bad for signal quality as well as against the regs. is there such a thing as metallised flex conduit of say 12mm ID down which I could put the LV cables (which fixes the regs problem). If it was metallised that might fix the interference problem too - anyone know of such a thing?

Gah electrics is a faff, especially not it's got rules coming out of its ears.

[Baz]

My eyes or brain glazed over halfway through that but I'm wondering if there is an additional allowance for a 'per room' load. A 32A ring is intended for a whole floor of a reasonable house (there is an area factor) not just one room. 20A is more than a single room is going to take. In practice you have to allow for a 2.7KW fan heater but nobody is going to put two of them on in one room so you could nudge it down ot 15A.
Is the conduit required to be 50mm? If there were two 20mm conduits would that be ok ( which might perchance run next to each other inside a 50mm tube)?

[pb]

Clipsal (& probably others) do Cat 5 cable with a higher insulation rating on the outer sheath which allows you to run it in the same partition as mains cables.  That might also fix your regs problem.  Not sure if you can find an equivalent for Cat 6 or higher.

Conduit-wise, you can certainly get small flexible (or semi-flexible) stainless pipes but they are expensive.  Could consider using flexible plastic insulation and wrapping it in aluminium tape for screening, perhaps.

I also didn't quite have the strength to follow your calcs all the way through at this early point in the morning, but I think your suggestion of starting with an allowable current, fitting a suitable MCB and working "backwards" is fine.  The whole diversity thing is intended to allow you to calculate an appropriate cable/breaker size for a "normal" circuit so that the breaker doesn't pop under foreseeable domestic loading.  If you can live with a more constrained supply, and understand that the MCB will trip if you start putting a fan heater on every other socket, then there is no reason you can't use a smaller supply than the diversity calcs would suggest.

I believe doubling up cables for a larger effective size is fine although I've never done it.  I'll have to get the regs out and check.

[Iain]

Hi
They do a variety of flex metal conduits.
One such
https://www.easupplies.com/LIQUATITE-BR-1050-Flexible-Metal-Conduit-p/el12i9.htm
 Or could you put the mains in as SWA cable through the conduit? Would that be acceptable for the regs?
Iain

[pb]

By the way, do you have a copy of the actual regs (not the OSG)?  If not then you are welcome to borrow mine for a while.  They are sometimes a bit easier to follow than the ready-reckoner advice in the OSG.

[A.L.]

If I put some insulation in the conduit end (to avoid thermal bridge into kitchen then that requires derating

Do you not only have to de-rate if more than a certain length of cable is insulated? You are probably only insulating 100-200mm?

[wookey]

Sorry my screed was so mind-numbing for you all. Reading though all that OSG guff is worse (especially in a slow PDF viewer), I assure you. It's taken about 2 days to come to the same conclusion that was 'obvious'. ("4mm will do", I'm just a little surprised how far I have to turn the MCBs down.).

pb, no I don't, as they are not online anywhere and quite expensive. Borrowing yours would indeed be most handy. I'll mail you.

A.L. - Correct, but derating applies even down to only 50mm of insulation (0.88).

Iain, thanks for that. I discovered that CPC will sell me 10m of 12mm ID stuff like that for £17, which looks like it will do the trick. I think I can get 3 cat5 cables through there which should be sufficient.

Baz - the conduit of 50mm is what is now embedded in concrete. I don't have much choice about that. I'd be a lot happier if I had two of them - one for data. I did ask for that, but they builders were in a terrific hurry as the concrete lorry was coming and I foolishly said that I guessed one would suffice, forgetting about the data stuff :-(

Oh and whilst I'm here I'd like to know why my building control fee for the extension doesn't include electrics for some reason, just 'everything else'. They'd want another 200 quid for that (when the 'whole extension' fee was £150 IIRC). There is something badly skew in the system here: electrics should not cost more to certify than structural calcs+ foundations+drainage+roofing+timberwork+ventilation+glazing, especially when it's a ring, a radial and a few lights. Somebody went slightly crazy in regs-world and we're all stuck with the fallout. I would kick up a massive fuss on principle and force the buggers to give me a certificate but the missus won't let me :-/
 

[pb]

Oh and whilst I'm here I'd like to know why my building control fee for the extension doesn't include electrics for some reason, just 'everything else'. They'd want another 200 quid for that (when the 'whole extension' fee was £150 IIRC). There is something badly skew in the system here: electrics should not cost more to certify than structural calcs+ foundations+drainage+roofing+timberwork+ventilation+glazing, especially when it's a ring, a radial and a few lights. Somebody went slightly crazy in regs-world and we're all stuck with the fallout. I would kick up a massive fuss on principle and force the buggers to give me a certificate but the missus won't let me :-/

That does sound rather nuts.  Did you try getting a quote for building control from anybody other than the council?

Admittedly the electrics in my extension are all being self-certified by the sparks (just because it's easier and, since he's on site anyway, basically free) but I don't recall the BCO asking for anything like that much money to sign them off.  We're currently using RHBC because they are a bit cheaper than East Cambs and seem easier to get on with.  And I'm fairly sure that in my previous dealings with South Cambs they told me that, although they preferred not to do electrics, they would sign them off if I insisted and wouldn't charge any more for it.

Also note that Part P para 1.26 specifically forbids building control from charging extra for any electrical testing which they might feel is necessary.  It wasn't obvious from your description whether the £200 was a test fee or just an administrative one so this might or might not help in your case.

[artful_bodger]

When I was in Oxford, I had a similar fee problem with BC.  Seems they didnt have a qualified inspector, so the local sparky charged them £150, they charged me £200.  I kicked up saying they should have qualified people, after all, all the other trades are covered.  In the end they backed down, but only after they accepted that I was competent to issue my own IEE completion certificate.  Before you rush and look at the certificate, you'll need to scrounge an RCD tester, an ELI meter (I think - need to check that) and if you're on overhead line, an earth rod tester (often part of ELI meter)  You can hire an all in one.
Or, you could ring up a local sparky and ask him to test it for you and issue a cert.  You can pick his brains on sizing etc as he'll be signing it off.  It'll probably cost you well less that £200 and you get advice.  Cost me £50 (1 hour) last year for a house, when I couldnt scrounge the gear.  Well worth it frankly.

A 20A/2.5mm radial in the garage + potting shed will be fine.  Dont even think about a RM.  Extra cable for little gain, and much harder to wire.  And if you follow the OSG for testing (you should) it takes about twice as long on install.  Not relevant here, but in industry, the 5/10/20 year retest time adds up.  Ive run a welder plus heater off a 20A radial. 

For the distances and ELI you're talking a 4mm/20A radial from the house to garage sounds ideal.  Put in a 5A lighting circuit, and a 10 (if you want) 20A radial circuits.  You'll need to fit 32A in the house, and you havent mention RCDs.  Compulsory for the sockets, but you need to make sure the upstream ones grade correctly ie dont fit 30ma in the garage and house.  Ultimately, the diversity if for installed appliances, like cookers.  If I connected 3 x 3kw heaters on my ring, it would trip.  The user has to be responsible for the appliances on a socket outlet. (NB amazingly, the regs state the sparky is responsible for a ring in a kitchen and then recommends radials for large appliances - also good advice - I hate rings...difficult to test, difficult to calculate in industry)

You could install 2  x 2.5, as you probably have a drum of it.  It doesnt exactly current share, but it would be > 4mm.  The sparky will look at you as if you're mad ('cause he has a drum of 4mm), but on LV feeders, particularly where space is tight, it is common to install 2 x35mm say for a 70mm.

You're right about the cat5 and insulation resistance and sharing.  I'm pretty sure the (standard) reel I've got is 500v rated, so is suitable.  If you lived in France, you get 20mm flexible conduit off any DIY shed shelf for peanuts.  It is the same as drain hose on a washing machine.  Cant get it in the UK - ok you can but expensive.  Have a look at pvc sheath - like heat shrink without the shrink.  No good, tho if you want to add stuff at a later date.  Just a thought here.  Dont install it untill after BC have given the OK.  Clearly the mains cable is safe in its own right, otherwise you couldnt hold it.  Hosepipe is plenty insulative enough, but it doesnt come with a certificate.  Turning the idea on its head, put in armoured cable for the power, and the second layer of insulation on that covers the (lack) on cat 5?  Effectively a comms duct with a extra insulated power cable.

I dont want to worry you, but if your garage is detached, and you've got PME, I'm pretty sure there is something about not exporting the earth ie you need a rod, but this might be for livestock only - not something I normally cover!

[djh]

Probably a silly idea but is it possible to run the data cabling in the roof somewhere? That would keep them separate.

[pb]

There's no actual prohibition in the regulations against exporting a TN-C-S earth to a generic outbuilding.  But...

If the workshop contains any extraneous-conductive parts (eg has a metal water service pipe) then you would need to bond them back to the main earth terminal, and in practice it would probably be easier/cheaper to make the workshop be TT if there is any appreciable distance involved.  Even if there is nothing extraneous-conductive in the workshop, I would personally make it TT if it's more than a few metres from the house to avoid any possibility of significant potential differences between the protective conductor and the local "true earth".

If the "outbuilding" is a caravan then TN-C-S is explicitly forbidden and that part of the installation needs to be TT.  In practice I think this probably applies to anything with a metal body, e.g. greenhouses as well.  There are various similar provisions for building sites and, as you say, places where livestock are involved (apparently because some animals are particularly vulnerable to even small potential differences between their front and back legs).

[psutherland71]

AS pb says 'If the workshop contains any extraneous-conductive parts (eg has a metal water service pipe) then you would need to bond them back to the main earth terminal' using 10mm2 cable, if not then the 10mm2 cable isn't needed.
For diversity, the OSG is only guidance and you can apply any diversity you like to a circuit. For your new consumer unit I would run 2 x 20A radial circuits for sockets and a 6A radial for the lighting and feed the C.U. from a 32A MCB in the house. Diversity could be justified, for example -

Max load on radial 1 - welder 13A
Max load on radial 2 - heater 8A
Lights - 4 x 5' fluorescents 2A

Total - 23A

The socket circuits will have to be RCD protected and I would run a 6mm2 cable to the C.U.

MCBs and cable sizes are calculated using the design current for the circuit so you could put in a 1A MCB on the lighting circuit just as you could reduce the other MCBs to match the anticipated loads, the cable sizes can then also be reduced, but normally some spare capacity is added in to allow for future alterations/additions.

If we were to calculate diversity for our houses in the way the you have from the OSG most of us would probably find we need a 150A DNO supply fuse (or larger).

Regards

Peter

[artful_bodger]

Thanks for clearing up the PME bit, pb.  Had something in the back of mind, but had forgotten about the water (etc) services.  Thankfully plastic pipe has saved a lot of bonding copper.

Apparently in Saudi, you need to bury so many earth mats around substations built on sand so the camels dont get a belt from touching the metal fences, it gets a bit silly.  And you dont want to fry a camel.... much better roasted.

Thinking about what psutherland posted, if you do think you'll have unusual load eg large 3kva/110V transformers or possibly welding sets, think about type C MCB.  Bit more pricey, tho.

[wookey]

psutherland. Right, as you say that seems entirely sensible. However, you say to put a 32A MCB on the 4mm cable. Table 6E1 still says that '4mm2 multicore cable having PVC insulation'  'enclosed in a conduit' will only take 30A, not 32A. Table 6F seems better as it's for 'PVC insulated and sheathed flat cable with pretective conductor' which is what I'm using, but it doesn't give 'in conduit' - only in ceilings and stud walls with/without insulation, in a conduit in an insulated wall, and clipped direct. Only the last of those can take >32A, and I don't believe that applies.  I don't understand why ref method B ('enclosed in conduit') is not listed for T&E, only other (unsheathed) cables? So it seems that a 32A MCB would be in danger of not protecting the cable adeqately, especially with any insulation involved. 

Anyway. I take the point. Next widely available MCB is 20A which is a bit on the low side.

[Ted]

Wookey, do you have the 17th OSG or the 16th?

In the 17th BS7671 twin+earth cable is covered by Table 4D5 - which is Table 6F in the matching 17th OSG on p134.

For 4mm in conduit in insulation (method A) this gives 26A.  Table 6E1 gives 25A for the same method.