Hi everyone – I’m new to the forum, but have been an avid reader for months and found lots of good advice. I must have read nearly every post on outside wiring topics but nothing exactly answers my question! I am a DIYer, but I like to think competent with electrics, and my priority is making sure what I do is safe. I am trying to learn more as a hobby interest rather than to avoid paying a pro.
We bought a new (old) house last year, the place had a full re-wire in 2004. All seems to be generally in order, NICEIC certificate etc. However I’ve come to realise that there have been a few electrical “mods” done after the original install, in particular garden lighting and a socket in a summerhouse. Whilst done by a “professional”
as part of a garden landscaping scheme by the previous owners, there would seem to be more than a few discrepancies from best practice…
I am planning to add additional wiring & a light in the summerhouse, and then add some additional PIR controlled external lights around the summerhouse. One thing at a time though…(and Part P of course, will either go down the LA cert route or get a pro)
My first plan is to evaluate what I’ve got, and then add additional sockets in the summerhouse, and an internal light & switch.
There is a 4mm2 SWA running from the main board (16A MCB, on the non-RCD side of a split board – although the non-RCD side is actually protected by a 100mA RCD, due to high earth impedance on mains supply – I assume to ensure adequate disconnection time). This runs into a smaller consumer unit in the utility room, with a 25A 30mA RCD. From here a 2.5mm2 SWA runs from a 16A MCB out to the summerhouse, where the SWA terminates, and then runs in 2.5mm2 T&E to a double socket (in effect a single, radial circuit). There are two other 6A garden lighting circuits run from the small consumer unit in the utility room.
(a quick question, I understand about RCD discrimination, which is okay here, but should the same theory be applied to MCBs/ Main switches, or is it just ‘convenience’ to have the mcb trip nearest to the circuit protected?)
Now, I wouldn’t have done it this way….for a start and based on the installed design max current it is already pushing past voltage drop limits on the run (c. 3.52v on the 4mm2 (20m) and c. 7.2v on the 2.5mm2 (25m) – although I’ve been conservative on length and this is at max temp 70C – surely never reached in buried SWA?)
I see a few things to consider (sadly re-cabling the 2.5mm2 SWA is not an option).
Downrate the 16A MCB in the smaller consumer unit to 10A as a start – risk nuisance trips but would only run small tools etc in the summerhouse. This gets under the 9.2v drop limit. But I understand there are other ways to calculate voltage drop in the on-site guide (haven’t got a copy – yet) for circumstances where the usual voltage drop factors are too conservative?
At the summerhouse end, I could use an MCB enclosure (or mini enclosure) to hold a 10A MCB (or even RCBO, although discrimination issue). From here I could add a small 2.5mm T&E ring circuit to provide 3 double sockets. From this a 5A FCU could supply the lighting.
The other (simpler) approach could be to simply extend the existing radial circuit, by adding an FCU (13A, or 10A) in place of the current double socket and extend the radial from there to the new sockets and in time take a further FCU off the radial (5A) to power a ceiling light inside the summerhouse and then extend that one further for outside lights (perhaps by adding a second 5A FCU). The second option would also I think not be Part P (extending existing radial circuit) until I add the outside lights?
I have assumed that I don’t need to be overly concerned about EFLI – it’s a TN-S supply, and disconnection times will be met by the 30mA RCD in the smaller consumer unit?
As a side, but important issue - although correctly terminated using glands the protective sheath on the SWA is not earthed anywhere although it is 3 core with one used as earth. The first section (4mm2) is above ground its whole length so it is as important to earth? The second section (2.5mm2) is below ground and clearly should be earthed. Power goes off at the consumer unit now whenever the spade comes out until I can fix!
Sorry for the long post, but any advice or thoughts would be very much appreciated,
Cheers
Philip
We bought a new (old) house last year, the place had a full re-wire in 2004. All seems to be generally in order, NICEIC certificate etc. However I’ve come to realise that there have been a few electrical “mods” done after the original install, in particular garden lighting and a socket in a summerhouse. Whilst done by a “professional”
as part of a garden landscaping scheme by the previous owners, there would seem to be more than a few discrepancies from best practice…I am planning to add additional wiring & a light in the summerhouse, and then add some additional PIR controlled external lights around the summerhouse. One thing at a time though…(and Part P of course, will either go down the LA cert route or get a pro)
My first plan is to evaluate what I’ve got, and then add additional sockets in the summerhouse, and an internal light & switch.
There is a 4mm2 SWA running from the main board (16A MCB, on the non-RCD side of a split board – although the non-RCD side is actually protected by a 100mA RCD, due to high earth impedance on mains supply – I assume to ensure adequate disconnection time). This runs into a smaller consumer unit in the utility room, with a 25A 30mA RCD. From here a 2.5mm2 SWA runs from a 16A MCB out to the summerhouse, where the SWA terminates, and then runs in 2.5mm2 T&E to a double socket (in effect a single, radial circuit). There are two other 6A garden lighting circuits run from the small consumer unit in the utility room.
(a quick question, I understand about RCD discrimination, which is okay here, but should the same theory be applied to MCBs/ Main switches, or is it just ‘convenience’ to have the mcb trip nearest to the circuit protected?)
Now, I wouldn’t have done it this way….for a start and based on the installed design max current it is already pushing past voltage drop limits on the run (c. 3.52v on the 4mm2 (20m) and c. 7.2v on the 2.5mm2 (25m) – although I’ve been conservative on length and this is at max temp 70C – surely never reached in buried SWA?)
I see a few things to consider (sadly re-cabling the 2.5mm2 SWA is not an option).
Downrate the 16A MCB in the smaller consumer unit to 10A as a start – risk nuisance trips but would only run small tools etc in the summerhouse. This gets under the 9.2v drop limit. But I understand there are other ways to calculate voltage drop in the on-site guide (haven’t got a copy – yet) for circumstances where the usual voltage drop factors are too conservative?
At the summerhouse end, I could use an MCB enclosure (or mini enclosure) to hold a 10A MCB (or even RCBO, although discrimination issue). From here I could add a small 2.5mm T&E ring circuit to provide 3 double sockets. From this a 5A FCU could supply the lighting.
The other (simpler) approach could be to simply extend the existing radial circuit, by adding an FCU (13A, or 10A) in place of the current double socket and extend the radial from there to the new sockets and in time take a further FCU off the radial (5A) to power a ceiling light inside the summerhouse and then extend that one further for outside lights (perhaps by adding a second 5A FCU). The second option would also I think not be Part P (extending existing radial circuit) until I add the outside lights?
I have assumed that I don’t need to be overly concerned about EFLI – it’s a TN-S supply, and disconnection times will be met by the 30mA RCD in the smaller consumer unit?
As a side, but important issue - although correctly terminated using glands the protective sheath on the SWA is not earthed anywhere
although it is 3 core with one used as earth. The first section (4mm2) is above ground its whole length so it is as important to earth? The second section (2.5mm2) is below ground and clearly should be earthed. Power goes off at the consumer unit now whenever the spade comes out until I can fix!Sorry for the long post, but any advice or thoughts would be very much appreciated,
Cheers
Philip
