robocog
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| posted on 5/7/12 at 07:39 PM |
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Garage electrics
Hi folks
I have a split load consumer unit in the house
I have an RCD protected and a spare 20A MCB (Type C) fitted and ready :-)
I have some 22A cable (steel protection layer and plastic coated over a nice thick insulation) - its 4 cored - but only intended using 3 cores
I have 2 seperate outbuildings I want to hook up to power (garage and shed)
I was going to buy 2x garage style waterproof insulated consumer units with RCD and 2MCB's 6a for lights and 16a for power and run the cable to
the nearest outbuilding (shed) to the "garage style 2 way consumer unit" and then onwards to the garage to the other 2 way consumer
unit
Have been thinking about this before I go ahead and buy anything or drill any holes
Thought I'd run it past you guys to get ideas
The garage style 2 way consumer units I was looking at have RCD protection (double pole)
Is it daft to go with this style (as its already got an RCD in the house on the split load I'm taking power off)
Or is it a case of belt and braces - and will be random which one will trip first in the case of an issue?
One company offered to do me a pair with a simple 2 pole isolators in place of the RCD
Initially I was looking at fitting 4 metal clad double sockets in each of the 2 outbuildings I need mains power
I understand I cannot use a ringmain in each outbuilding due to only 22a steel overbriad cabling feed from a 20a mcb in the house and the fact I will
also only have a 16a mcb in each building- so socket pairs will each have to be wired seperately (radially) to the garage MCB
I have plenty of black/red twin and earth here :-)
Not running anything with mega power in any of the outbuildings (certainly nothing above 13a at max)
Small lathe - compressor - drill - occasional welding and VERY unlikely I will ever have more than one on at any given time (being I only have one
pair of arms)
Worst case scenario would be radio on loud - lights on and the 130A MIG on full "blow holes in metal" mode
Again I can achieve this from a current outdoor socket and an extension reel with a 13a plug fitted
Also unsure if I should run cabling to the building in conduit or clip it at a low level (incase the fence blows down) or just trail it on the floor
where its sat against a fenceline - surface is concrete so burial is not going to happen
Whatever I do its got to be safer/better than running an extension reel each time I want to do anything and having to keep unplugging stuff and moving
it between the shed and garage
Thoughts pointers and general advice sought
Regards
Rob
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JoelP
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| posted on 5/7/12 at 07:53 PM |
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No point having 2 rcds in a row, just have a main switch in each small unit. Clip the cable up off the ground, no need for conduit as its armoured
cable. May as well put a 20A radial circuit in each building, wired in 2.5mm cable. Consider hammering an earth rod into the ground to get a more
local earth. Its not hard to do and means you arent relying on the imported earth from the house (connect both though).
Make sure you terminate the SWA properly, with the armour clamped in a gland and connected to earth.
Beware! Bourettes is binfectious.
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smudge88
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| posted on 5/7/12 at 08:07 PM |
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I will Be round Saturday after work about 1PM 
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I predict a Riot
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| posted on 5/7/12 at 08:22 PM |
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I've reproduced my post to a very similar thread from a few weeks ago. Your situation seems to be similar.
OK
Firstly you have missed out the most important issue - what is the nature of the distribution circuit to your garage consumer unit?
Also what type of earthing system is installed and are the main earth bonds in place and of the correct size to suit your main incoming supply?
You must not do anything until you know all this
A circuit coming directly from a fuse or circuit breaker is indeed called a radial circuit and no it does not need to be 4mm cable only if you want to
fuse it at 32amps a 2.5mm radial is fused at 20 amps, there is NO limit to the amount of socket outlets you can install, a radial is limited by the
area it covers, a max of 50 metres squared.
If you are not using armoured cables or protecting a PVC cable in conduit or trunking you MUST install a 30ma RCD to every circuit although one can be
used for all circuits.
You do not necessarily need to convert you existing wiring to a ring main, 20amps is more than sufficient for most workshops unless you intend to do
arc welding. Where this is the case I usually install a separate circuit just for the welder and fitted with a type c mcb to cope with the inrush
current.
If you re in any doubt call a couple of local electricians most provide a quote for free.
Yes this job would be notifiable under Part P.
I run my own electrical contracting company and have been an electrical engineer for 25 years so please take this advice seriously.
Regards
Adrian
"Honesty is a very expensive gift. Do not expect it from cheap people ". Warren Buffet on Expectation
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robocog
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| posted on 5/7/12 at 08:47 PM |
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OK have an earth rod as it happens (used it to support a spinning windsock when I seeded the grass - didn't work as intended and I think it may
have actually attracted them and given them better visual clue for landing & takeoff if anything.)
OK when I put the 2 breakers and MCB consumer units in - best practice to join the 2 outbuildings?
Via a termination block and tails to the middle one or perfectly OK to use the consumer unit input to make the join?
(if physically partical with the cable I have - which it should be)
Earth rod - best pratcice at the far end or in the middle - or no difference as far as anything goes? and use just the most practical place?
I do not have any earthing glands yet but will use the steel sheath as an earth as well
Is there a neat / best practice way of getting the cabling through walls?
I did see "faceplatey" type things - but think they were mainly for TV aerial leads
Not sure if the cable I have would bend nice enough to use one of these though
It's going to have to exit the house quite low down on the outside wall - its going to exit into a narrow ish alley that I use to get the
motorbikes in and out and don't fancy a trip / catch hazzard if poss
Could drill at 45 degrees to assit the exit angle (but possibly bad practice from a walls mechanical point of view is it?)
maybe I'll need a box on the outer wall to keep it all neat and come through the house in something more flexy and use a join in the box?
Grant - Not sure if we are going to be about on Sat afternoon yet
Got visitors from Ooop North coming this way - and we haven't made any meet up arrangements yet
Would deffo appreciate another pair of eyes on it though and have a new toy to play with - which I think you will approve of (you may heve already
seen the vids?)
If the cars on the drive - were in and the kettle will be doing overtime
If not - are you about on Sunday?
Regards
Rob
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robocog
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| posted on 5/7/12 at 10:04 PM |
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Don't understand the "what is the nature of the distribution circuit to your garage consumer unit?" - sorry
I'm not thick -I'm sure I could answer that if it were multiple choice and had Google available ;-)
I rewired the house to the new split load consumer unit
I then did the done thing and got it inspected / tested / signed off and got him to swap the tails and earth over from the old consumer unit to the
new one when he was satisfied with my handywork
I'm sure he would not have put his name on it if the main earth bonds were not of the correct size to suit the main incoming supply
As for type - possibly PME...dunno - not something which I have ever looked into or asked about
I know our last house was PME - which is why I have an earth rod "hanging round spare"
The incoming has a clamp on it and an earth wire come off that to a junction block that the house bonding and the bus bar in the consumer unit goes to
(in quite chunky wiring throughout)
Bonding - we are on plastic plumbing throughout but the (literally) few cm of copper we do have is bonded as well as is the sink/bath AFAIK
The plumbers who put the waterworks in and comissioned the boiler dealt with this side - and they also got someone in to sign their electrical side
off (part P was well and truly in by now) - so again if the main earth bonding was not up to snuff I'm sure it would have been noted and either
changed or condemned having been inspected indipendantly by two different people twice
in the split load consumer unit
Any sockets that are capable of being used to anything outside the building are protected by a 30ma RCD
(curently 2 exhisiting outside sockets - one single at front and one double at back - both seperately radially wired to a single 16a MCB) which is
what I currently plug the extension reel into without any problems - bar a welder that occasionally tripped the MCB on inrush
I have a 20A type C MCB in there on the 30ma RCD protected side solely for the garage all ready and waiting
No conversion or addition to any exhisting physical wiring layout
(glad I thought ahead when I got the consumer unit now :-) )
Regards
Rob
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dhutch
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| posted on 6/7/12 at 10:51 AM |
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Lots of threads on the DIY NOT forums about this sort of thing.
On one level, from what I understand what you have suggested isnt stupid and proberbly better than a lot.
On the otherhand however, your proberbly not qualifited to make the changes, and to spec it exactly right.
As said, there is no really point in having a second RCD, although there is an argument that the armoured cable could come out of the house pre RCD,
and they have an RCD in Garage/Shed should you get a trip you then done have to go back to the house. But although Ideal this is proberbly overkill
for most, and more than my parents in there garage.
Daniel
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jamesbond007ltk
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| posted on 6/7/12 at 02:09 PM |
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Length
Sorry if this has already been covered but had a quick scan and didn't see it.
What sort of distances are you talking for the two cable runs?
Although your cable is rated to 22A (I assume 1.5 mm2?) the voltage drop could be quite significant given the small conductor size. The could lead to
problems some items of equipment in future.
Quick calc based on a 1.5mm 70C SWA and a run of 25m (not allowing for internal wiring, ambient temp, grouping etc.) with a drop per ampere per metre
of 29mV/A/m and assuming 13A usage: (29x22x25)/1000 = 15.95.
Even if your the voltage at your consumer unit was 230V your resultant useable voltage would only be ~214V. This would be made even lower by any
internal wiring. It would also be greater than the voltage drop allowed in BS7671.
Depending on the distance involved this may not be a problem but for the above example you would need 2.5mm2 minimum, but even that would be very
close.
Rich
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