SWA only or T+E joined to SWA for new garage supply

[Donquick]

Hi,

I need to put a new supply to garage as old supply is a 2.5mm T+E spur off of ring main which runs overhead outside!

Is it standard to a) run the SWA all the way from the house CU all the way to the CU in the garage in one continuous run or b) run T+E from the house CU to a junction box inside/outside the house wall and then continue with SWA to the CU in the garage.

If a) then is it ok to just drill a hole in the wall and use a bit of silicon or is a box required where the SWA appears on the outside of the wall. If b) is it ok to use a junction box inside the house to join the cables or should the junction box always be on the outside of the house where it can be accessed easily.

I would prefer to do b) as it is cheaper and easier to lay the T+E in the house.

Thanks.

I am going to put the cables in as it is a long way and I need to lift a floor etc. Once done I will leave all connections/junctions and garage CU to an electrician next year.

 

[sxturbo]

My dads consumer box is in the middle of his bungalow, and he wanted power to his shed

So he done 6mm twin and earth through the loft and into an ip67 junction box mounted close to the soffit. He then done 6mm swa from this junction box to the shed.

Edit: calcs were done prior, of the potential load and distance required.

 

[Notch7]

ideally SWA from house CU to garage CU

if you need to swap from SWA to T+E then you need a suitable enclosure which can satisfactorily terminate SWA cables -and if outside then of suitable IP rating too

the only way you can be sure the cable type and size etc is ok, is to get the electrician you will be using to specify

 

[oldbutnotdead]

Big disadvantage of the t & e nowadays is if any of the t & e run is chased into a wall < 50mm from the surface the cable will require RCD protection at the cu. Which is a nuisance when you're in the garage, cause an N-E fault and have to traipse all the way into the house to reset.
EDIT As @Notch7 re getting your electrician involved at design stage, it's up to him/her to specify cable size, type, route etc

 

[Taylortwocities]

ideally SWA from house CU to garage CU

Actually, ideally, not from the house CU at all, but from the incoming supply, to the garage via a separate switch fuse at the supply end. This removes any issues of circuit problems affecting the house consumer unit.

If running T&E and changing to SWA at an outside wall, this is my preferred method.

Sorry for blurred image!

As above. engage with your electrician before you start!

 

[Luckyphil]

I fit supplies to lots of Sheds / Garages , imo always best to have minimal joints in a Supply Cable , so for me it’s SWA all the way , at the Con Unit either gland it onto the Con Unit or terminate into a Wiska or Metal box & carry the Cable through to the Con Unit

cheers
Phil

 

[briwire]

As others have said, it is best to avoid joints if possible, so a continuous cable from the CU to shed would be the best option. However, SWA cable can be a bit of a pain to run through joists and restricted space if there is a long run inside the house. So it is perfectly acceptable to run the correct size T&E inside the house joining to SWA in a box. If you have the box inside, the bend radius when you go through the wall and down to the ground can look a bit rough. Far better to come into the back of a wiska box on the outside, then straight down with the SWA.

 

[Taylortwocities]

. So it is Far better to come into the back of a wiska box on the outside, then straight down with the SWA.

Ah. You chose to ignore my “far better” solution pictured above.

 

[Donquick]

Thank you all very much for the valuable help. I think I will focus on installing SWA from the supply in the house all the way in one run. This being the case, a couple of quick follow on questions please:
a) It is mentioned that if T+E comes within 50mm of surface then it must be protected by an RCD, may I deduce from this that SWA does not carry that requirement. I am thinking that the electrician will probably go for a Henley block near the board and then a separate switch fuse at the supply end (as suggested in one answer) - would this then be ok with SWA even if there are areas where I am < 50mm? Moreover, what is the problem with providing a RCD protection, apart from a small addition cost in components?
b) What is the accepted way of passing the SWA through a cavity wall. [There are a couple of solutions in answers if I am joining, but none for a straight thought SWA run]. I am thinking about a diagonally downward hole from inside to outside to decrease the bend radius on the cable and then a simple plastic cover outside to hide the exit and provide some water ingress protection. Is this the standard solution?

Thanks again in advance.

[I want to future proof the garage by providing for the possibility of a 7.4kW car charger which required a 32A MCB. I have used an online voltage drop calculator over my distance of 16m assuming reasonable separation from other conductors and it says 4mm SWA is the correct cable. I intend to use 6mm as a precaution.]

 

[Taylortwocities]

Cable calculations need to be done by the electrician who is going to have to certify that he has designed the installation.
Cable routes need to be agreed by the electrician who is going to have to certify that he has installed the new circuit.
Decision on practicality of switchfuse, etc need to be done by the electrician who is going to have to certify that he has designed the installation.
SO, really.....
Per my post no. 5

engage with your electrician before you start!

PS

It is mentioned that if T+E comes within 50mm of surface then it must be protected by an RCD, may I deduce from this that SWA does not carry that requirement.

Correct, providing that the SWA is correctly installed, earthed, etc.

 

[oldbutnotdead]

Big advantage of the Henley blocks, separate overcurrent protection for the SWA etc etc. is that a fault in the garage won't affect the supply to the house and if the fault in the garage is caused by human intervention (ie careless use of MIG welder and large electric heater) the fault can be cleared in the garage without having to traipse down to the house.
Cable size and type- talk to your electrician. Budget for 10mm or maybe 16mm- you might actually want to do some work in the garage while your electric car is charging so don't go for the minimum. Yes cable is pricey but if you write the cost down over however long you plan on living there it fades into background noise

 

[Donquick]

Thanks both. Yes, point taken, I will get the electrician in before i do anything. Can anybody answer the question about how to run continuous SWA through a wall though - as a curiosity. (I need to plan around this).

 

[flameport]

Moreover, what is the problem with providing a RCD protection, apart from a small addition cost in components?

A fault in the garage will trip the RCD in the house. Meaning the garage will have no power at all including no lights.

Whether that's a big problem depends on what's being used in the garage.
A socket for an occasional use with a lawnmower in the garden - probably not.
Multiple devices being used in the garage including power tools when it's dark outside - another matter entirely.

how to run continuous SWA through a wall

Drill a hole slightly larger than the cable at an appropriate angle in the same direction of the cable, dress the bricks/masonry at the edge of the hole to provide a smooth surface, pass cable through.

 

[SimonH2]

Actually, ideally, not from the house CU at all, but from the incoming supply, to the garage via a separate switch fuse at the supply end. This removes any issues of circuit problems affecting the house consumer unit.

I disagree.
You need OCP anyway, so just use an MCB in the CU. If you can't have a non-RCD protected way in the CU then it's not a great design of CU.
As long as you don't share an RCD with anything in the house then nothing you do will affect the house supply - unless you manage to blow the main fuse in which case it matters not whether the garage is off the CU or split off the tails.
BUT, having the garage off an MCB in the CU means you have one single main switch that will isolate everything.

IMO the only advantage of a separate switch fuse is that it's easier to have a fuse rather than MCB for improved discrimination against faults.

A fault in the garage will trip the RCD in the house. Meaning the garage will have no power at all including no lights.

Yeah, that's a right p.i.t.a. My brother made the mistake of allowing some T&E that needs RCD protection in the supply to his detached garage. When the RCD trips (he had some water in a light fitting), you're left not knowing if the light didn't come on due to a problem in the garage or something in the house - and it's a right p.i.t.a. finding the fault - circuit by circuit, many trips back to house to reset RCD