Garden Office Electrics

[siross85]

Hi all,

I'm currently in the process of building an outside office in the garden. Due to the distance from the consumer unit I'll be spurring off a socket inside the house with 6mm SWA. I chose 6mm just in case we decide to go to the consumer unit in the future.

Because it's only a spur and not a new circuit I'd like to do as much of it myself as possible, I've made a diagram below which I believe works well. The sockets are straight forward, the SWA will be terminated in a 13A FCU and then into the first socket continuing as a radial, all in 2.5mm T&E.

The lighting will be a case of taking the 2.5mm from the 13A FCU in to a 3A FCU and out of that will be 1.5mm T&E to the light switch (it's only a small room so doesn't need 2 switches). From the switch to the first downlight and then daisy chained (radial) from there.

The additional wires in the diagram are just for speaker wires, HDMI and Cat6. The Cat6 will be protected as much as possible from the power cables. All wires will be run behind the celotex insulation in the walls.

Is there anything I've done incorrectly or anything that could be done better?

Diagram below with a more detailed diagram of the lighting circuit.

 

[ericmark]

I see nothing really wrong with it. It is no real difference to supplying a caravan in your garden.

 

[winston1]

The 13amp fused spur would be better near the socket in the house in my opinion.

1.5mm2 cable is oversized for the lighting, use 1.0mm2.

 

[JohnW2]

... The sockets are straight forward, the SWA will be terminated in a 13A FCU ...

That might be a bit of a challenge.

... and then into the first socket continuing as a radial, all in 2.5mm T&E. The lighting will be a case of taking the 2.5mm from the 13A FCU in to a 3A FCU and out of that will be 1.5mm T&E to the light switch (it's only a small room so doesn't need 2 switches). From the switch to the first downlight and then daisy chained (radial) from there. ...

In reality, I don't think that the 3A FCU would achieve very much. Even 1mm² cable (which, as winston has said, would be adequate - albeit nothing wrong with using 1.5mm²) would be adequately protected by the 13A fuse in the other FCU. Some may argue that light fittings/switches are not 'rated' for 13A, but I personally think that is a silly argument (others may disagree)! Lighting circuits up to 16A are permitted by the regulations.

The most obvious observation is that, doing it as you propose, your entire office will be limited to a total of 13A total - so I take it that you feel that will be adequate for your immediate needs (i.e. no heating etc.)?

Kind Regards, John

 

[siross85]

The 13amp fused spur would be better near the socket in the house in my opinion.

1.5mm2 cable is oversized for the lighting, use 1.0mm2.

Thanks for that, you're right it probably would look neater.

That might be a bit of a challenge.
In reality, I don't think that the 3A FCU would achieve very much. Even 1mm² cable (which, as winston has said, would be adequate - albeit nothing wrong with using 1.5mm²) would be adequately protected by the 13A fuse in the other FCU. Some may argue that light fittings/switches are not 'rated' for 13A, but I personally think that is a silly argument (others may disagree)! Lighting circuits up to 16A are permitted by the regulations.

The most obvious observation is that, doing it as you propose, your entire office will be limited to a total of 13A total - so I take it that you feel that will be adequate for your immediate needs (i.e. no heating etc.)?

Kind Regards, John

Thanks for the advice. Would it be that difficult to terminate 6mm cable into a FCU? I've never done it but the terminals should be able to take it? Or am I in for a shock ( no pun intended). Cable is already buried now but I've not bought the FCU. Any alternatives?

In terms of the lighting, I was under the impression that 13a wouldn't protect a 1.5mm cable sufficiently? If that isn't the case, would it just be a case of having 2 cables connected to the FCU, a 2.5mm for the sockets and 1.5mm or 1mm for the lighting?

In terms of power requirements, this was a concern of mine too, yes it will have heating in the form of a 2kw heater. There will then be computer equipment and a TV. I had a chat with an electrician who came round and he believed it would be sufficient. A 2kw heater would draw around 8.69-9 amps at full blast and the computer equipment would be minimal. The room will be around 7m2 and insulated all round with 50mm celotex so hopefully won't need the heater on full blast for long periods of time.

 

[JohnW2]

Thanks for the advice. Would it be that difficult to terminate 6mm cable into a FCU? I've never done it but the terminals should be able to take it? ...

It's not the terminating of the 6mm² conductors in the FCU terminals that is the main potential issue but, rather, installing an SWA cable gland.

In terms of the lighting, I was under the impression that 13a wouldn't protect a 1.5mm cable sufficiently?

As far as cable protection is concerned, using the most likely installation method ('Method C' = 'clipped direct'), 1.0mm² cable has a current-carrying capacity of 16A and 1.5mm² a current-carrying-capacity of 20A - so both would be adequately protected by a 13A fuse.

If that isn't the case, would it just be a case of having 2 cables connected to the FCU, a 2.5mm for the sockets and 1.5mm or 1mm for the lighting?

That is what I was suggesting - although I did indicate what some people then might (to my mind unnecessarily) 'fuss' about

In terms of power requirements, this was a concern of mine too, yes it will have heating in the form of a 2kw heater. There will then be computer equipment and a TV. I had a chat with an electrician who came round and he believed it would be sufficient. A 2kw heater would draw around 8.69-9 amps at full blast and the computer equipment would be minimal.

Even if the maths just about works, that's really cutting things a bit fine - as I implied, it would have been much more 'reasonable' if no heating had been involved. Since you have 6mm² cable installed already, and given that you seem to have an electrician 'involved', have you not considered having something meatier than a "13A feed" to the office?

Kind Regards, John

 

[siross85]

It's not the terminating of the 6mm² conductors in the FCU terminals that is the main potential issue but, rather, installing an SWA cable gland.
As far as cable protection is concerned, using the most likely installation method ('Method C' = 'clipped direct'), 1.0mm² cable has a current-carrying capacity of 16A and 1.5mm² a current-carrying-capacity of 20A - so both would be adequately protected by a 13A fuse.
That is what I was suggesting - although I did indicate what some people then might (to my mind unnecessarily) 'fuss' about
Even if the maths just about works, that's really cutting things a bit fine - as I implied, it would have been much more 'reasonable' if no heating had been involved. Since you have 6mm² cable installed already, and given that you seem to have an electrician 'involved', have you not considered having something meatier than a "13A feed" to the office?

Kind Regards, John

Ahh I forgot about the gland . Can this be done with a 20mm hole saw through the unit assuming it doesn't have a ready made knock out for a gland?

Thanks for the info on the cable. So really the 3a FCU is superfluous as you suggest. Even if there's nothing necessarily wrong with it.

To be honest with you. My original plan was always to have a 6mm SWA direct from the consumer unit to the office done by the electrician. This is why I asked him round to have a look. It would mean digging up the front garden and taking up some flags and then there's the cost of the 6mm cable. This is why the conversation led to a 13a FCU supply being sufficient. I agree it's close with the heater on max.
When I say the 6mm cable is installed, it's only buried about 4m between the house and office ready to be terminated at both ends. There's a 10m run currently with 3m slack at either end.

A full run to the consumer unit would be around 25m of cable.

 

[EFLImpudence]

Why are you using an FCU - with 13A fuse - for 6mm² SWA?

 

[JohnW2]

Ahh I forgot about the gland . Can this be done with a 20mm hole saw through the unit assuming it doesn't have a ready made knock out for a gland?

Making the hole (one way or another) if necessary, wouldn't be the problem. However, the glands protrude a bit into whatever they have attached to, and it might well hit 'the works' of the FCU - or, at least, make things very 'cramped'.

Thanks for the info on the cable. So really the 3a FCU is superfluous as you suggest. Even if there's nothing necessarily wrong with it.

As I've said, that's my personal view.

... My original plan was always to have a 6mm SWA direct from the consumer unit to the office done by the electrician. This is why I asked him round to have a look. It would mean digging up the front garden and taking up some flags and then there's the cost of the 6mm cable. This is why the conversation led to a 13a FCU supply being sufficient. I agree it's close with the heater on max.
When I say the 6mm cable is installed, it's only buried about 4m between the house and office ready to be terminated at both ends. There's a 10m run currently with 3m slack at either end. A full run to the consumer unit would be around 25m of cable.

Once the SWA is inside your house, it could convert to standard (presumably 6mm²) T+E cable (at some sort of junction box) for the rest of the journey to your consumer unit. I would think that that would be by far the best approach.

Kind Regards, John

 

[JohnW2]

Why are you using an FCU - with 13A fuse - for 6mm² SWA?

Because the whole setup is being run as a 'fused spur' from a (I presume ring) sockets circuit. However, as I'm discussing with the OP, that's far from ideal, particularly since he wants to be able to use a 2kW heater on the circuit (amongst other things). [the 6mm² SWA is 'future proofing]

Kind Regards, John

 

[siross85]

Why are you using an FCU - with 13A fuse - for 6mm² SWA?

As John mentions below it's a fused spur from the ring inside the house.

Making the hole (one way or another) if necessary, wouldn't be the problem. However, the glands protrude a bit into whatever they have attached to, and it might well hit 'the works' of the FCU - or, at least, make things very 'cramped'.
As I've said, that's my personal view.
Once the SWA is inside your house, it could convert to standard (presumably 6mm²) T+E cable (at some sort of junction box) for the rest of the journey to your consumer unit. I would think that that would be by far the best approach.

Kind Regards, John

Ahh right I'm with you on the size of the box, I'll see what's available with a bit more room inside.

In terms of running inside the house, this would probably be more difficult, fishing new wires through the house?

I wouldn't mind doing the work with the cable to be honest. The consumer unit is in the garage so it would go straight out of the garage with one flag before the front garden. I'd just lift a flag and then dig the trench close to the rest of the flags for the cable to run. The bigger problem would be down the side of the house which is completely flagged but it could come up there and clip to the house.

Because the whole setup is being run as a 'fused spur' from a (I presume ring) sockets circuit. However, as I'm discussing with the OP, that's far from ideal, particularly since he wants to be able to use a 2kW heater on the circuit (amongst other things). [the 6mm² SWA is 'future proofing]

Kind Regards, John

Thanks John that's right.

In a worst case scenario I could see how the fused spur works out in practice and if it doesn't work well I could take the 6mm SWA out of the spur and run it round the house as I suggest above? Not ideal because in that scenario the fuse will constantly blow...

 

[siross85]

I've just done a quick measure and it would be around 30m cable to get to the consumer unit. Or if I could extend the cable that's already there in a junction box then an additional 20m would be easy

 

[JohnW2]

Ahh right I'm with you on the size of the box, I'll see what's available with a bit more room inside.

Unless you do some bodging, you'll probably find a larger box' to which you can readily attach a FCU. You could, of course, gland the SWA into a separate ('empty') box of some sort (with a front cover or 'blank plate') and take 'ordinary cable' (maybe even the 'inner' of the SWA) from there to the FCU.

In terms of running inside the house, this would probably be more difficult, fishing new wires through the house?

Only you know the answer to that

I wouldn't mind doing the work with the cable to be honest. The consumer unit is in the garage so it would go straight out of the garage with one flag before the front garden. I'd just lift a flag and then dig the trench close to the rest of the flags for the cable to run. The bigger problem would be down the side of the house which is completely flagged but it could come up there and clip to the house.

Again, only you know the practicalities of the actual situation.

In a worst case scenario I could see how the fused spur works out in practice and if it doesn't work well I could take the 6mm SWA out of the spur and run it round the house as I suggest above? Not ideal because in that scenario the fuse will constantly blow...

That's a possibility. One 'problem' is that it actually takes at least 22A to blow a 13A fuse - so you could find yourself using a lot more than 13A (which, at the least, could make the fuse get very hot, and maybe damage the FCU) 'without realising'.

Kind Regards, John

 

[JohnW2]

I've just done a quick measure and it would be around 30m cable to get to the consumer unit. Or if I could extend the cable that's already there in a junction box then an additional 20m would be easy

There's no reason why you couldn't extend the existing cable, as you suggest.

Kind Regards, John

 

[siross85]

Unless you do some bodging, you'll probably find a larger box' to which you can readily attach a FCU. You could, of course, gland the SWA into a separate ('empty') box of some sort (with a front cover or 'blank plate') and take 'ordinary cable' (maybe even the 'inner' of the SWA) from there to the FCU.
Only you know the answer to that
Again, only you know the practicalities of the actual situation.
That's a possibility. One 'problem' is that it actually takes at least 22A to blow a 13A fuse - so you could find yourself using a lot more than 13A (which, at the least, could make the fuse get very hot, and maybe damage the FCU) 'without realising'.

Kind Regards, John

There's no reason why you couldn't extend the existing cable, as you suggest.

Kind Regards, John

Thanks for all that, that's really helpful. I've just been out looking again and it would be a pain in the backside so it might be worth trying the fused spur first. I can always upgrade.

I didn't realise that the fuse would only blow with 22A. Is that continuous or a surge of 22A? Doesn't sound very safe lol

It's a shame I can't use the neighbours consumer unit, it's right next to it lol.