* 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  

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.

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.

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.

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!  )

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

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.

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.

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!