Upgrading my garage supply.

[Mr_Freeze]

Hi Everyone,
I moved into a new build house 1 year ago.
I’ve got round to investigating the supply to my separated garage.
At the house the builders have fitted a fused spur (non switched) internally onto a ring circuit fused at 13amp.
From this is a short length of 2.5 T&E going externally to a plastic Ip66 mounted external JB.
From this a 1.5 SWA is run underground to the garage and terminated into a metal clad switched fused spur inside the garage.
A double socket is connected to the spur on the incoming side.
The lighting circuit is connected to the load side and the fuse is 5amp. The switched spur therefore acts as the light switch.

The SWA has an earth conductor but the armourings are not earthed separately in the plastic JB. No banjo is fitted at the metal clad spur but the back box is bonded to the earth core.

Questions on existing instal.
Should the internal spur have a switch providing DP isolation before it leaves the house?
Should the armourings be earthed by bonded banjos to the earth core at both ends?

Planned changes.
I would like to replace the metal clad switched spur with a new garage CU with a main isolator with RCD and 2 x MCB’s fitted. 6amp for lighting and 10amp for sockets.
I then plan to wire multiple sockets around the garage in plastic conduit with LNE singles.

Would there be any issues in doing this as in my mind I’m providing DP isolation in the garage with RCD protection and separate rated MCB’s for lights and sockets?

Appreciate any comments?

 

[kenny7askew]

not much point in putting additional RCD in garage because if new build should already be in the house(check),could maybe change spur to double pole so as to be able to isolate in the event of fault in garage,sockets in garage already on 13 amp fuse so not much point in changing to mcb/garage unit,maybe easiest to just extend socket circuit in garage from socket already there,SWA should be earthed.

 

[buzzlightyear]

the non switch feed from you existing circuits should be 2.5mm SWA to the garage not 1.5mmSWA considering you have a light and socket feed from s/fs in the garage
. not bright sparks who put that in, and you want to alter the circuit in that garage .

 

[HandySparks]

Typical cheap option used by builders. OK for a light and the odd small power tool or electric lawnmower.

No point in doing what you're proposing unless you're going to put in a separate non-RCD circuit specifically for the garage.

 

[HandySparks]

the non switch feed from you existing circuits should be 2.5mm SWA to the garage not 1.5mmSWA...

As it's fused at 13A, why would you need a bigger SWA, unless for voltage drop reasons?

 

[buzzlightyear]

its like the film jaws (need a bigger boat) in order for you up grade in the garage a new cross sectional of SWA and separate feed for your garage from the c/u if possible.

 

[Cadgey123]

You are not going to have more ‘power’ in your garage with these changes. You will still be regulated by the 13amp fuse from the house.

 

[Spoon]

Why do you require "multiple sockets"?

 

[Mr_Freeze]

not much point in putting additional RCD in garage because if new build should already be in the house(check),could maybe change spur to double pole so as to be able to isolate in the event of fault in garage,sockets in garage already on 13 amp fuse so not much point in changing to mcb/garage unit,maybe easiest to just extend socket circuit in garage from socket already there,SWA should be earthed.

Hi Kenny, thanks for the first sensible answer.
When you say should be earthed, I thought the same but does it have to be earthed as if it does I can have the builder on that?
I know about the 13a supply but again do they have to provide DP isolation on the spur or is this a nice to have?
The Garage CU I was looking at has a 30ma RCD incuded in the DP isolator, so was just going to use it as its there.
The 6 & 10a are there for further discrimination and I want to wire additional lights in 1.5 and additional sockets in 2.5 singles.

 

[Mr_Freeze]

You are not going to have more ‘power’ in your garage with these changes. You will still be regulated by the 13amp fuse from the house.

Please don't state the obvious, not helpful.

 

[Mr_Freeze]

Why do you require "multiple sockets"?

Why not, its my garage?

 

[Octopus]

Mr Freeze... With the greatest respect unless you plan a new dedicated feed from your fuse box, you are wasting time and money....

 

[kenny7askew]

Hi Kenny, thanks for the first sensible answer.
When you say should be earthed, I thought the same but does it have to be earthed as if it does I can have the builder on that?
I know about the 13a supply but again do they have to provide DP isolation on the spur or is this a nice to have?
The Garage CU I was looking at has a 30ma RCD incuded in the DP isolator, so was just going to use it as its there.
The 6 & 10a are there for further discrimination and I want to wire additional lights in 1.5 and additional sockets in 2.5 singles.

Hi,with double pole spur you will be able to isolate garage in the event of a neutral earth fault in garage to stop house RCD tripping,with more than one RCD on the circuit it will be pot luck wich one trips first if not both,changing to mcbs wont make much difference to discrimination,swa should be earthed

 

[Cadgey123]

If it is easy enough to get a cable from your consumer unit to the garage, install a 4 or 6mm2 SWA and feed it with a MCB to a RCD protected garage Consumer unit.

That will give you plenty juice for garage items, lights, sockets, tools, electric door etc.

 

[telectrix]

Hi Kenny, thanks for the first sensible answer.
When you say should be earthed, I thought the same but does it have to be earthed as if it does I can have the builder on that?
I know about the 13a supply but again do they have to provide DP isolation on the spur or is this a nice to have?
The Garage CU I was looking at has a 30ma RCD incuded in the DP isolator, so was just going to use it as its there.
The 6 & 10a are there for further discrimination and I want to wire additional lights in 1.5 and additional sockets in 2.5 singles.

answer to the above, in red. the steel wire armouring of the SWA MUST be earthed, even if not used as a protective conductor for the cable, the supply end must be earthed, generally by use of a SWA gland and banjo or earth nut. you can pull the builder upon that.

 

[kenny7askew]

answer to the above, in red. the steel wire armouring of the SWA MUST be earthed, even if not used as a protective conductor for the cable, the supply end must be earthed, generally by use of a SWA gland and banjo or earth nut. you can pull the builder upon that.

Thank you you are quite right i should have said MUST not SHOULD

 

[Mr_Freeze]

answer to the above, in red. the steel wire armouring of the SWA MUST be earthed, even if not used as a protective conductor for the cable, the supply end must be earthed, generally by use of a SWA gland and banjo or earth nut. you can pull the builder upon that.

Thanks Tele.
So what about DP isolation at the spur, I know its a nice to have to totally isolate outside, but is it a must that they should've supplied this or not?

 

[Mr_Freeze]

Hi,with double pole spur you will be able to isolate garage in the event of a neutral earth fault in garage to stop house RCD tripping,with more than one RCD on the circuit it will be pot luck wich one trips first if not both,changing to mcbs wont make much difference to discrimination,swa should be earthed

Why not, surely a lighting fault will trip a 6a MCB before a 13amp fuse?

 

[Wilko]

Why not, surely a lighting fault will trip a 6a MCB before a 13amp fuse?

Hi - descrimination between protective devices is an interesting topic. For a short circuit fault where you will have say 500A current, it would be a bit of a race but quite likely the 6A MCB will trip and the 13A fuse will also melt. For an overload fault where lighting was drawing say 18A then the 6A MCB should trip only.

 

[Spoon]

Existing installation has a 5A fuse for the lights and a 13A fuse for the sockets... I don't see the problem.
I have also asked the OP why we are discussing this in private messages and also on the post...