Supply to Garage and Shed

[Flinstone]

Hi there,

I would like to break into an existing 4mm SWA cable currently running from my house to a detached garage in order to add a 2-way metal socket and flourescent lamp to my humble 8x6 shed which is sited along the existing cable route.

The house-to-garage route is 25m and the shed is approx 6m from the house in the same direction.

Currently the garage supply is wired from the house meter via a 30A FCU (tbc) and 63A 30mA RCBO. Installation is I think circa 1985-1990 (by previous owners). The house uses a TT earthing arrangement.

A couple of questions:

I had hoped I could fit a junction box at the rear of the shed to terminate the two SWA cables together i.e., in & then back out again to continue to the garage. The box would conveniently sit pretty much immediately above the existing cable run, and act as the terminating point. From here I could drill thru' the back of the box & into the shed in order to to use PVC to the shed's 2-way CU.

1) Is it acceptable to spur off to the shed in this way rather than provide separate feeds from house to ea building? I don't think I would want 2x SWA cables running down my house back wall (the existing comes out of the soffit from a kitchen extension and then down the entire wall ht).

2) Could anyone recommend a junction box, if any, to fit on the outside of the shed such that it is robust enough to accept 2x SWA glands and will be weatherproof too?

3) Should the shed CU be made of plastic or metal? I ask because metal seems more robust but you can get plastic ones with a higher IP rating (I think the air in the shed will be damper compared to the garage). I won't need to terminate SWA here if I can use PVC as described.

4) Is it usual to locate the CU high up on the shed wall to limit the knocks or could it be located at floor level and serve as a terminal for the 2x SWAs too, e.g., bring SWAs up through timber floor and then into CU box? I like the sound of this because the SWA is covered & out of site plus no external box required! Though I don't feel too comfortable about a CU at floor level either!

5) Is it correct to fit RCD-based CU to both the shed and garage or keep the one existing RCD back at the house instead?

Thanks for any advice

Regards

 

[ban-all-sheds]

I would like to break into an existing 4mm SWA cable

Why on earth did you put such a small cable in? 6mm² or even 10mm² wouldn't have cost much more, and would have given you much better flexibility/future-proofing....

I had hoped I could fit a junction box at the rear of the shed

there is a junction box at the bottom of the shed's back wall which is right next to the existing cable route.

1) Is it acceptable to spur off to the shed in this way

As long as the cable size is OK, but I don't see why you need a CU when all you've got is a socket and a light, just put the socket onto the cable from the house and use a switched FCU for the light.

2) Could anyone recommend a junction box, if any, to fit on the outside of the shed such that it is robust enough to accept 2x SWA glands and will be weatherproof too?

Any reputable make.

Get one larger than you think you'll need, and check that they don't have any internal ribs that would get in the way of the gland nuts, like the Wiska ones have.

And use earth nuts rather than banjos.

But why not take the SWA into the shed? Why introduce a JB? Please don't say that you went to all the trouble of installing a new cable and didn't plan the whole job properly, so that you got it right first time...

3) Should the shed CU be made of plastic or metal? I ask because metal seems more robust but you can get plastic ones with a higher IP rating (I think the air in the shed will be damper compared to the garage). I won't need to terminate SWA here if I can use PVC as described.

If it's so damp that you think you need an IP rated CU then maybe you shouldn't have any electrical accessories in there at all..

And as I said - why bother with a CU?

4) Is it usual to locate the CU high up on the shed wall to limit the knocks or could it be located at floor level and serve as a terminal for the 2x SWAs too, e.g., bring SWAs up through timber floor and then into CU box? I like the sound of this because the SWA is covered & out of site plus no external box required! Though I don't feel too comfortable about a CU at floor level either!

Quite right, particularly in a shed.

But it's a shed , not your living room - nothing wrong with having SWA running up the wall.

Into and out of a metalclad RCD socket, with a spur from there to a switched FCU for the lights.

5) Is it correct to fit RCD-based CU to both the shed and garage or keep the one existing RCD back at the house instead?

Neither.

Have the RCD protection local, i.e. an RCD socket in the shed and an RCD in the garage CU, nothing at the house end. If you're going to be using power tools anywhere then install emergency lighting - you don't want to be plunged into darkness whilst holding Mr Fingerchoppy.

 

[Flinstone]

Wow! I appreciate you taking the trouble to give such a comprehensive reply... thanks

Why on earth did you put such a small cable in?

I'm sorry I fibbed!! Truthfully , it's MICC at present and I intend to replace it but I was scared at the time of writing that an authority such as your good self would mentally hurt me over cable sizing questions!

I had thought 4mm/7.5kW max. load seemed adequate to me, and because the OD of the 10mm is >6mm>4mm>Pyro, I was nervous of an ever growing big black cable coming down my wall. Now you have made me think it over yet again.

there is a junction box at the bottom of the shed's back wall which is right next to the existing cable route

Ambiguous writing I'm afraid it is the wall that is next to the cable run, not a junction box.

As long as the cable size is OK, but I don't see why you need a CU when all you've got is a socket and a light, just put the socket onto the cable from the house and use a switched FCU for the light

So I don't need, say, a separate 16A MCB protection for the socket then? I had thought it was going to be needed to protect the shed related wiring; so my described 30A fuse from the meter will make the MCB redundant? Would there be any justification to fit a CU if one wanted to maintain the supply to the garage in the event of a relevant fault in the shed?

If it's so damp that you think you need an IP rated CU then maybe you shouldn't have any electrical accessories in there at all.

I was considering the CU box likely to rust in a simple garden shed over the medium term rather than any serious affect on the electrical components. What typical application is there for a IP56 CU box by the way? And are they intended only where electrical accessories are not fitted locally?

Have the RCD protection local, i.e. an RCD socket in the shed and an RCD in the garage CU, nothing at the house end. If you're going to be using power tools anywhere then install emergency lighting - you don't want to be plunged into darkness whilst holding Mr Fingerchoppy.

So in summary for the shed, the junction box is out, the cables can come in thru' the floor & terminate directly in the 2-way metalclad RCD socket using earth nuts on the glands. Use min 6mm SWA; remove the existing house RCD; keep the existing house 30A FCU, and add separate RCD protection to each outbuilding (I think I will need to change the garage fused CU in order to facilitate a RCD there).

And thanks for the pointers ref. the junction box, earth nuts & emergency lighting.

Thanks indeed!!

PS: It might seem silly to ask, but should the SWA glands (now to be located in the shed) be an internal or external type?

 

[ban-all-sheds]

it's MICC at present and I intend to replace it but I was scared at the time of writing that an authority such as your good self would mentally hurt me over cable sizing questions!

Have you no confidence in them?

I had thought 4mm/7.5kW max. load seemed adequate to me,

But it's not - over that distance the capacity of the cable is 25A. And is 25m the distance between the house and the garage, or the total length of the cable run from the house CU to the garage one? It's the latter that matters.

So I don't need, say, a separate 16A MCB protection for the socket then?

No.

You don't have it on your house sockets, do you?

I had thought it was going to be needed to protect the shed related wiring; so my described 30A fuse from the meter will make the MCB redundant?

You're only having one socket, you said...

Would there be any justification to fit a CU if one wanted to maintain the supply to the garage in the event of a relevant fault in the shed?

Possibly, but it's unlikely - what could go wrong with SWA looped into and out of a socket and an FCU?

I was considering the CU box likely to rust in a simple garden shed over the medium term rather than any serious affect on the electrical components. What typical application is there for a IP56 CU box by the way? And are they intended only where electrical accessories are not fitted locally?

My comment was slightly tongue-in-cheek. A metal one will eventually rust, but then so will metalclad accessories. However see below re local TT'ing....

So in summary for the shed, the junction box is out, the cables can come in thru' the floor & terminate directly in the 2-way metalclad RCD socket using earth nuts on the glands. Use min 6mm SWA;

Make sure your accessories will accept 6mm² cables. You'll go into the socket, out to the FCU for the lights, and then out of that onwards to the garage. If you do decide to use 10mm² you will need other arrangements in the shed.

remove the existing house RCD;

Well you'll have to keep the 100mA protection

keep the existing house 30A FCU, and add separate RCD protection to each outbuilding (I think I will need to change the garage fused CU in order to facilitate a RCD there).

A lot of this is notifiable.

PLUS: It would be worth canvassing opinions on the pros/cons of exporting your TT earth vs isolating the armour at the outbuildings and installing local earth rods.

PS: It might seem silly to ask, but should the SWA glands (now to be located in the shed) be an internal or external type?

Might as well be external.

 

[chapeau]

Morning Flinstone.

You posted recently and said the existing cable was damaged and close to the surface so was going to be replaced. Would make a lot more sense, then just run it into the shed and out again back down the garden. Standard procedures would then apply, with due consideration given to earthing arrangements.

 

[Flinstone]

Thanks b-a-s and Chapeau,

b-a-s,

Well you'll have to keep the 100mA protection

In case this has a bearing on your earlier comments, to clarify, the RCD in question is a 30mA RCBO dedicated to the garage circuit and is located in the house, adjacent to the main CU. There is no 100mA RCD present; the main CU is a fusebox with another independent 30mA RCBO dedicated to the house wiring.

With this in mind, and I note your point about emergency lighting, should I keep the aforementioned 'garage' RCD within the house - feeding both the garage & new shed install - or continue to remove it & fit two new RCDs locally to each outbuilding as you originally advised? I'm double-checking particularly because of the additional work & cost involved. If you still advise I fit new RCDs, I'm thinking I may have bitten off more than I can chew and perhaps I should return to my original plan to solely replacing the cable between house and garage (ie no shed electrics, RCD remains inside house, not in garage). I think I wouldn't have to notify if replacing existing cable only. From little acorns...


And a couple of follow-up Q's if you or anyone else would be kind enough..

Is it possible to explain how the 25A limit for the 4mm² cable @ 25m cable length is derived? I'm not doubting it but just keen to learn how the derating is obtained.

And cable pricing: is Screwfix a reasonable place to purchase SWA - I see a lot of links to them for electrical accessories in this forum - or do you think it's worth asking at an electrical wholesaler? Screwfix 3-core 4mm is £44.91 compared to £65.79 for 6mm which is a substantial enough cost difference to me.

PLUS: It would be worth canvassing opinions on the pros/cons of exporting your TT earth vs isolating the armour at the outbuildings and installing local earth rods.

Any suggestions? Would it be better to further this with a new topic and a few details?

Thanks again as always.

Flinstone

 

[bongos]

And cable pricing: is Screwfix a reasonable place to purchase SWA - I see a lot of links to them for electrical accessories in this forum - or do you think it's worth asking at an electrical wholesaler?

TLC is a good bet for cable also.

25m of 6mm² 3 core SWA = £49

25m of 4mm² 3 core SWA = £40

At only £9 difference (before VAT admittedly) then you're paying very little extra for a considerable increase in capacity.