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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Consumer Unit Upgrade / Installation in Garage https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1338382682 Message started by Smithy2000 on May 30^th, 2012, 1:58pm

Title: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on May 30^th, 2012, 1:58pm

* Consumer Unit Installation in a Detached Garage *

Hi All,

I've been looking at upgrading the electrics in my Garage and wondered if anyone could help or advise on what I would like to do is actually possible...

I currently have a Garage Consumer Unit with a 25amp RCD and a 16amp + 6amp MCB. Currently powering x3 Two Gang Sockets and x1 Ceiling Rose Mounted Bulb

It's a large Garage that I intend to turn into a workshop / one car garage after all the clear up work is completed.

So what I would like to see happen is all the present electrics stripped out and the following installed:

x5 Two Gang Metal Clad Sockets Internally

x2 External Sockets - x1 One Gang and x1 Two Gang (Both IP Rated and Breaker Protected)

x6 Internal Four Foot Single Flourescent Light Fittings (38w each)

x2 External Halogen PIR Flood Lights (150w and 300w)

The cable supplying the garage as been replaced with approx a 25 Metre Length of SWA Cable: 4mm sq 3 Core T+E with a current rating of 42Amps.

What I would like to see is the Consumer Unit replaced with say a 6 Way Consumer Unit consisting of:

40A 30MA RCD

Fitted with x4 MCB’s as below:

16A: Internal Sockets (For x5 Two Gang Sockets)

10A: External Sockets (For x1 Two Gang, x1 One Gang)

6A: Internal Lights (For x6 Four Foot Flourescent Tubes)

6A: External Lights (For x2 Halogen Floodlights with PIR)

Now here is the question/s:

Is the above Consumer Unit configuration OK, viable or even possible?

I have been looking at Consumer Units in Screw Fix etc but when you go over more than 2MCBs the RCD seems to jump up to 63amps + and also has a 100amp DB Switch?

I have no basis for the CU layout I've listed above, I just thought it would make it neat and safe having dedicated circuits for each.

Or should I just stick with the Consumer unit I've already got and just have all the sockets run through the 16amp circuit and all the lighting through the 6amp circuit?

Any help or advice would be much appreciated so that I'm not going over the top or more importantly, not spending too much!

Cheers :)

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Lectrician on May 30^th, 2012, 5:31pm

The current rating of the RCD incomer needs to be equal or larger than the protective device protecting the submain to the garage. If you have fused your 4mm SWA at 40amp in the house, an RCD of 40, 63, 80 or 100 would be required. Any of these would be fine aslong as the OPERATING current is 30mA.

4mm PVC SWA is rated at 38amps clipped direct/buried.
4mm XLPE SWA is rated at 49amp clippe direct/buried, but should be treated as PVC due to it's terminations. Not sure where your 42amps comes from?

25m of 4mm SWA also exceeds the maximum permissible voltage drop at 40amps (even at 38amps). Even reducing the submain size to 32amp the voltage drop is exceeded. The cable really is too small for any such loading.

Looking at your circuits, you would be better as below:

20amp 2.5mm radial for the internal sockets (even a 32amp 2.5mm ring if the submain was a 40amp or above).
20amp 2.5mm radial for both outside sockets.
6amp for the inside lights.
6amp for the outside lights.

There is no requirement for the final circuits to not total more than the incomer.

There is no requirement for "breaker protected" outside sockets - you are intending to fit an RCD to the consumer unit, so job done. Infact, fitting "breaker protected" sockets would not comply with discriminations and could be problematic with the main RCD.

The light fittings would be better as 5ft fittings - these are the most economic in terms of light output per watt.

Good choice on metal clad accessories. A metal clad CU would be best too so you can gland the SWA in correctly, and the wiring would be best done in a fully constructed PVC conduit system using single core PVC cables.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on May 31^st, 2012, 11:47am

That's great and many thanks for the quick and educating response.

So what size / rate of SVA Cable would you recommend installing for the length I require and my needs?

The 42amp for the present cable came from the Cable Suppliers Notes as detailed below:

600/1000V for domestic and industrial environments; does not require to be enclosed in trunking or conduit; can be used in power networks; buried underground; indoor or outdoor applications

BASEC approved; 6943X type
Standard BS5467
Stranded copper conductors to BS EN 60228:2005
3 core XLPE (Cross-Linked Polyethylene) Insulated
PVC (Polyvinyl Chloride) Sheathed
Galvanised Steel Wire Armor (used as earth conductor)
Max current rating 42Amps
Temperature range (fixed installation) -40C to +90C
Overall Diameter of cable (for gland sizing) 15.3mm

I think I should've come here first, before I purchased the replacement cable!

Oh well, I guess I can always use it for supplying one socket and a bulb in the shed ;o)

Thanks again in advance.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Lectrician on May 31^st, 2012, 4:40pm

For 40amps over a distance of 25m, you would need to go for 10mm².

This would give a voltage drop of 4v at 40amp, leaving a little head room for the final circuit volt drop.

6mm² drop 6.57 volts, leaving only 0.33 volts for the final circuits, a little too tight!

The chances of drawing 40amps is fairly low in a standard garage - It really depends on what you intend to run in there, and how much will be running at once.

Cable manufacturers seem to produce their own current ratings based on use in "free air". Cables clipped direct and buried need to be de-rated as they cannot dissipate their heat as quickly. If the same cables are installed in areas of high ambient temperature, grouped with other cables, run in thermal insulation etc, further de-rating factors need to be applied too.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on Jun 1^st, 2012, 12:56pm

Many thanks again Lectrician, you're advice is very much appreciated.

10mm it is then. Sounds like one meaty cable to run but so be it.

I'm guessing this cable would ideally need to run direct (without breaks) from Garage CU to the Main CU in the property but would it not harm or affect voltage drop if there was say a suitable mid-way junction box?

Reason I ask is because I'm guessing at 10mm, the cable will not take too kindly to a 90 degree bend? As on one section of the cable run, it would need to go up an external wall of the house (fed inside conduit) before entering the house at 90 degrees via a pre-drilled hole leading into the ceiling void?

Any recommendations on how to get around this or what appropriate Terminals / Junction Box to use at this point would also be appreciated.

NB: The property has concrete floors so entering below floor level at a lesser angle is not an option.

Also and hopefully finally, what amp of MCB would you recommend for the dedicated Garage Supply circuit in the main CU of the House? (I noted last night that it currently as a 20amp MCB).

In response to usage, I don't believe I would ever be drawing the maximum from it. I could only imagine at most, maybe the use of lighting, a radio, a power tool and possibly a compressor running at the same time.

However I would prefer to spend extra time and a bit more money in making sure that the system was safe, capable and future proof than barely manageable of it's potential.

Many thanks again.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Lectrician on Jun 1^st, 2012, 3:08pm

10mm 3core SWA is not that bad to bend or install. It can be cleated to the wall, no need to run in conduit, but you can place it in some if you wish. U capping is also an option.

The hole into the loft should be done at a suitable angle upwards to allow the cable "to flow" into the hole, and it also keeps the water out.

You can joint the cable, and could do this at the transition from inside to outside. If there is a loft void, I would joint it in the loft I think - Using a large-ish metal adaptable box. If jointing it outside, I would use a gewiss type IP rated plastic enclosure.

You can run 10mm Twin and Earth PVC cable insie the house, and SWA outside. Remember, if you run the SWA all the way (preferred), you would need to be able to correctly gland it into the house consumer unit - this is not always possible (depending on the location and make). If you cant gland it into the house CU, you would need to gland it into an enclosure and then run twin and earth to the CU, so this joint could go anywhere (where it is accessible).

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on Jun 1^st, 2012, 11:20pm

Once again, many thanks Lectrician.

Just one more question (I promise!), I think you may have missed it amongst my last writing.

What amp would you recommend for the MCB to protect the dedicated Garage Supply in the main CU of the House?

I noticed last night that it is currently protected by a 20amp MCB but guessing this will not be large enough?

Cheers.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Lectrician on Jun 2^nd, 2012, 8:37am

I thought we had agreed on 40amp at the beginning of the thread ;)

That gives you a nice bit of head room.

20amp is very tight and would really only allow the use of one thing at a time - no expansion etc.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on Jun 2^nd, 2012, 11:52am

That's great and thanks for the confirmation.

40 amp it will be then.

Thanks for all your advice and help on this one, it's very much appreciated.

Have a great Bank Holiday! :o)

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by CWatters on Jun 2^nd, 2012, 1:32pm

Lectrician,

Perhaps worth adding a comment about earthing this set up and the number of cores in the SWA ?

I'd post a reference to that IEE article but I think somone said it was out of date.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by The_Trician on Jun 3^rd, 2012, 11:50am

Agreed.
10.00mm twin and earth has a much reduced cpc (earth wire) size.

Far better to stick with 10.00mm 3 core SWA all the way to the house consumer unit.

Also, make sure you run a earth wire from the brass earth tag on the armoured cable glands at each end of the run.

Another thing might be a problem with fault discrimination if using a 40A mcb at the house-end and a 32A mcb for the Ring at the garage-end.

Might be better to fit a switched fused isolator with a 40A cartridge fuse instead?

TT

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Lectrician on Jun 3^rd, 2012, 1:59pm

Yea, a 10mm T+E cable has a 4mm earth, and so would need to be supplemented with a run of earth if the submain was used to bond extraneous conductive parts in the garage (if there are any).

It used to be frowned upon to take a PME earthing system out to an outbuilding, but these days it is not, as long as any extraneous parts are bonded (hence the 10mm).

Extraneous conductive parts being anything "earthy" from outside the garage such as water or gas pipes, or any metalic structure of the garage which introduces an "earthy" potential from outside to in.

A 40amp MCB will struggle to discriminate against even a 6amp MCB with a direct short type of fault. A fuse would provide discrimination, but in a domestic situation like this you need to decide if you would prefer to have clear discrimination in the (unlikely ish) event of a fault, or have an easy to reset type of protection.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on Jun 5^th, 2012, 12:47am

Thanks Guy's, a few more things to take into consideration.

It's funny you should mention earthing, has I have noticed that the current wiring system also as an additional single earth wire (housed in a green / yellow sheath) running into the current 2 way CU.

I traced this earth wire back and out through the rear wall of the garage. It then appears to connect to what I believe is a an earth rod going straight into the ground.

Should I be checking this out further and maybe connecting the system I intend to install on to this too as a secondary earth?

Cheers again for any advice.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by The_Trician on Jun 5^th, 2012, 2:13pm

Personally, I'd leave it in, and re-connect it to your new set-up.
You can't have too much earthing!

I did mine the other way round - a 10mm 3 core armoured cable between the house and outbuilding, with the third core used as the main earth, then I earthed both ends of the armouring at both ends of the run, then I also put a supplementary earth rod in, outside the outbuilding.

It won't do any harm to leave the earth rod and earth cable connected up.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by MikeB on Jun 6^th, 2012, 4:47pm

I don't have the regs handy but I'm sure I read somewhere that connected earths should have the same csa, so if the house main earthing conductor is 16mm any connected TT earth should also use 16mm cable? Apologies if I am totally wrong I wouldn't be surprised.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Lectrician on Jun 6^th, 2012, 5:05pm

Kind of.

If you have a TN-S or TN-C-S earthing system, you should (on a typical domestic) have a 16mm main earthing conductor and 10mm main protective bonding conductor.

If you then also had an earth rod for some reason connected to the same earthing earthing system (and not isolated) it is essentially an extraneous conductive part and as such would require main protective bonding, so a 10mm minimum would be required to the rod.

If the rod is being used to create a second earthing system (in an outbuilding for example) then you would not be required to do this, and a minimum of 4mm would be fine for the rod, although most folk would still fit a bit 10mm minimum.

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on Jun 7^th, 2012, 3:53pm

Cheers The_Trician,

It was my initial thought too that it wouldn't do any harm to have multiple earths but thought it best checked / confirmed all the same. I'll be sure to add that into the run then.

Thanks again all and at least I can continue with a more confident approach in what products to buy and actions to take now.

Cheers! :)

Title: Re: Consumer Unit Upgrade / Installation in Garage
Post by Smithy2000 on Jun 7^th, 2012, 4:12pm

Apologies MikeB and Lectrician,

I've just noticed your last postings too and thanks for the info also.

I'd rather over engineer this project than just about meet the requirements, so 10mm it will be.

Thanks again all :)