Taking SWA out to the garden...

[Grabble]

Ok, before I get beset by lots of posts saying it's external work, it's regulated by Part P... you can't do any of this yourself... etc... let me first comment that a) I am very aware of Part P, and b) I have no intention of doing the final connections at each end - I'll be getting a qualified sparky in to do this - I am aware however that I can for example dig the trench as it's fairly pointless to pay for this!

Ok with that said here's the situation:

My wife would life to have electricity taken out to the front garden to provide sufficient power to drive a water feature (small pump circa 80 watts - she doesn't want a solar powered one) and some low wattage lights (circa 100 Watts) - total of about 10 meters from the house.

I would also like to provide a 13Amp external socket (on the side of the house) for connecting the lawn mower / strimmer.

Ideally I would like to be able to pull cable through conduit (so that should it ever be damaged I don't have to pull up the whole of the front garden again!).


My proposal would be to:

Dig a 'suitably deep' trench to bury the cable - the cable will go partly under a path and then under a raised flower bed. I expect to go about 500mm down under the path and 600mm under the flower bed. I can obviously do this work myself...

I plan to lay straight (trying to bend cable is always a nightmare) PVC 32mm conduit with swept bends through which I can run 4mm SWA (3 core). I (by use of a sparky) would then look to terminate one end of the cable to a suitable ground mounted (min height above ground?) IP66 switch box in the flower bed from which I can switch on the lights / water feature. The other end would run up the side of the house into a junction box that provides an external socket and passes through the wall to a spur off the main house ring main.

Does all of that sound sensible? Is there anything I have overlooked / that you professional sparkys don't like the sound of etc?

Also should I drill out the base of the conduit to allow for drainage?

 

[securespark]

Why 4milli?

 

[JohnW2]

Ok, before I get beset by lots of posts saying it's external work, it's regulated by Part P... you can't do any of this yourself... etc... let me first comment that a) I am very aware of Part P, and b) I have no intention of doing the final connections at each end - I'll be getting a qualified sparky in to do this - I am aware however that I can for example dig the trench as it's fairly pointless to pay for this!

You may be very aware of "Part P", but you don't appear to completely understand it (at least, the requirements for 'notification'). Assuming that you are in England, 'outside work' is no longer notifiable, and nor is extension of an existing circuit, which is what you appear to be proposing. The work you describe is therefore probably not notifiable. As with any electrical work, if you are suitably competent and equipped to do it, in compliance with part P (i.e. safely), then you could do it all yourself. However, even though not notifiable, an Electrical Installation Certificate should be issued, certifying that the design, construction and testing of the work is compliant with the regulations (BS7671) - although you could theoretically issue such a certificate yourself, in practice you would probably need an electrician for that (particularly in relation to testing) - as well as wanting the electrician to 'connect up' the circuit etc..

It is therefore important that you involve the electrician at the start, so that (s)he can sign the declaration on the certificate relating to design, construction and testing.

As for your proposal, it sounds conceptually reasonable. The electrician will be able to advise you as regards requirements for the trench etc. You ought to use ducting, rather than conduit, otherwise pulling cables through could be difficult/impossible. The spur from your ring would have to go through a 13A fused connection unit, in which case the electrician might decide that 4mm² SWA is unnecessarily large for the short run.

Kind Regards, John

 

[PrenticeBoyofDerry]

If you are extending the circuit from an existing one then as John has posted, you do not require to notify the work. That being said you never actually mentioned notifying the work in your post.
So assuming compliance to part p regarding design and safety, that is what you must be stating you are fully aware of.

As touched on by others 4.00mm SWA, would seem a little excessive for distance and load, and possible the use 1.5mm SWA would be adequate.

I would personally want switching from the inside rather than externally, to prevent mischievous use. Plus the use of a 13A double pole isolator at the intersection of the internal ring final would be advantageous, for both switching and prevention of complete power-loss of internal ring final.

Other than that, the depths proposed are acceptable and the of installing the cable with a duct is also advised.

I don't believe a electrical installation certificate is the only method of proving safe, as a minor works should be sufficient if extending an existing circuit,
The other consideration would be 30ma RCD protection for the socket.

 

[JohnW2]

If you are extending the circuit from an existing one then as John has posted, you do not require to notify the work. That being said you never actually mentioned notifying the work in your post. So assuming compliance to part p regarding design and safety, that is what you must be stating you are fully aware of.

Maybe. I was suspecting that, like many people (include one of the electrician 'regulars' here ), when the OP spoke of Part P he was probably thinking mainly of notification ("... it's regulated by Part P... you can't do any of this yourself.").

I would personally want switching from the inside rather than externally, to prevent mischievous use. Plus the use of a 13A double pole isolator at the intersection of the internal ring final would be advantageous, ....

I would say not just advantageous but, as I said, a 13A FCU would be required, since this spur would be supplying more than just one socket. As you say, if that FCU was DP switched (I think all SFCUs actually have DP switches), that switch would serve multipe desirable purposes.

I don't believe a electrical installation certificate is the only method of proving safe, as a minor works should be sufficient if extending an existing circuit,

Sure. Although I wrote EIC, a MWC may well be sufficient. However, that still involves the same declaration (as regards 'design, construction and testing') - and, of course underlines the need to test.

The other consideration would be 30ma RCD protection for the socket.

Indeed. I meant to mention that, but then forgot to do so!

Kind Regards, John

 

[Grabble]

Ok a fair few good points... to answer some of them with a bit more information:

I was indeed thinking of notification and the certification of the job itself for that matter. To be honest while I'm not a regular sparky I do have an electrical engineering degree and work in a design department where we design LV systems so I suspect I probably am more than competent to do the work - all the same I'll simply feel happier (my wife won't nag me) if a regular sparky has had a good look over the design as it can't hurt (I know a couple as friends of friends - this was just a starting point to ensure I wasn't suggesting anything too stupid ).

I should have indicated in the original post that I agree that it will need an internal isolation where it meets the existing ring main. I assume a regular Switch/Spur 13A Double Pole & Front Flexible Outlet would suffice for this?

Noted that ducting would be better than conduit.

I should also have mentioned the reason for using 4mm was a) if in the future it needed to be uprated the cable should suffice for a higher load and b) more importantly I already happen to have 25meters of it in the garage

Also given the house is only two years old the Ring is obviously already RCD protected back at the CU. Is there any real advantage in having a separate RCD on the outside spur? (Given that RCD's will trip at some typical point between 15ma and the 30ma rating won't it simply be a race as to which drops first?) obviously if it can have an RCD for the spur that will drop before the internal one for the ring then that would be ideal but I didn't think that could be guaranteed?

 

[Grabble]

Maybe. I was suspecting that, like many people (include one of the electrician 'regulars' here ), when the OP spoke of Part P he was probably thinking mainly of notification ("... it's regulated by Part P... you can't do any of this yourself.").

Kind Regards, John

PS: Yes I wasn't aware that you could take electricity outside without it being notifiable - I thought that was part of the 'special areas' bit of Part P like bathrooms and kitchens etc. So thank you.

In practice then can I get a sparky to have a look over the design and write up an Electrical Installation Certificate and then test it to ensure it complies? Does this not have to involve building control in any way?

 

[JohnW2]

I was indeed thinking of notification and the certification of the job itself for that matter.

Thanks for confirming - as I thought, then!

I should have indicated in the original post that I agree that it will need an internal isolation where it meets the existing ring main. I assume a regular Switch/Spur 13A Double Pole & Front Flexible Outlet would suffice for this?

I would not have thought that you would want the front flex output. You need to take a cable from that FCU to wherever (presumably some sort of box) it joins to the SWA (which could not be terminated directly in an FCU) - and I would have expected that cable to be 'buried' somewhere (out of the back of the FCU).

I should also have mentioned the reason for using 4mm was a) if in the future it needed to be uprated the cable should suffice for a higher load and b) more importantly I already happen to have 25meters of it in the garage

The latter is a fairly compelling reason! However, as for possible future upgrading, you're never going to be able to use your proposed feed method (from the ring final circuit, necessarily through a 13A FCU) to supply a current that could not be carried by 1.5mm² or 2.5mm² cable.

Also given the house is only two years old the Ring is obviously already RCD protected back at the CU. Is there any real advantage in having a separate RCD on the outside spur?

On the contrary, if the circuit is already protected, adding a second RCD really only has nuisance value (the only reasion for having a second one being if you wanted 'belt and braces' - i.e. so that if one RCD fails to work, the other should work!). If there are two non-faulty RCDs in series, there is no telling whether one, the other or both will operate in the case of a fault.

Kind Regards, John

 

[JohnW2]

PS: Yes I wasn't aware that you could take electricity outside without it being notifiable - I thought that was part of the 'special areas' bit of Part P like bathrooms and kitchens etc. So thank you.

'Outside installations' (and work in kitchens) was amongst the many things which ceased to be notifiable (in England) in April 2013. Prior to that date, you would have been right about both outside work and kitchens.

In practice then can I get a sparky to have a look over the design and write up an Electrical Installation Certificate and then test it to ensure it complies? Does this not have to involve building control in any way?

If it' not notifiable work then, no, LABC is not involved in any way. As for the rest, you need to discuss that (in advance) with the electrician involved, since (s)he will essentially have to sign a declaration on the certificate that (s)he was 'responsible for' all the 'design, construction and testing'. Interpretations vary, but being 'responsible for' probably does not preclude someone else doing much of the work, with them 'supervising' (in whatever manner they feel appropriate).

Kind Regards, John

 

[Grabble]

Amusingly having spoken to our local building control officer on a separate matter it appears they are (like me) still somewhere prior to April 2013 as he told me it would definately be notifiable and I'd be best off getting a sparky involved... Can't say I was surprised given my dealings with building control at other councils in the past mind you...

Spoke to a spark I know today and he was happy enough with what I'm proposing - just a case of getting him to give it a once over and testing it when he's free to get it all certificated. I'll be getting a 25 meter installation done in the back garden next year but as that involves a new separate RCD from the CU my involvement will be simply digging the trench... so he'll be kept busy.

Thanks for all the help guys. The internet is as ever a mine field thanks to old posts (pre-April 2013 in this case) obviously confusing matters...

Agreed btw that you wouldn't need anything bigger than 2.5mm in theory for a 10m run for future but given the 4mm is in the garage it'd be a bit thick not to use it