Is there a good book explaining all the niggly things regarding 17th edition wiring, like a wiring "bible"? This is so I know the "standard" ways of doing things, which always helps, and stops me asking a million and one small questions on the group. I have on onsite guide already. I will give the wiki a good read.
What I was wondering presently whilst considering the wiring requirements:
What is the usual way to run cables around a room with a solid floor ? In this case it is a kitchen and most of the sockets will be just above work surface height, plus a couple of armoured cable lengths (freezer, exterior wiring). Running inside the ceiling void and down behind cabinets to everything would not seem like a good solution. Note: between the CU and kitchen will be a small downstairs toilet, so running all the wiring at socket level would be a problem at this area, as would running everything visible, such as inside kitchen units.
If a cable is run though the centre of 9" floor joists, is it considered "buried", i.e. RCD not required ? Obviously at some point it will leave this "buried" area, but you could junction to a different type of cable at this point.
If an armoured cable is used (non-RCD freezer or cable for external wiring), can it be hidden anywhere, or does it still have to be in a safe zone (corners, in line with visible fittings etc) ?
Is it true that all visible wiring must be in trunking, even right next to the CU ? What about the need to retro-fit such trunking to all existing cables for circuits that are not being touched ?
I would imagine any such wiring "bible" would include these sort of issues. Cheers, Simon.
sm_jamieson wibbled on Saturday 17 July 2010 10:10
The OnSite Guide would be a good starting point. There are many other explanatory guides including various NICEIC publications.
Horizontally or ceiling drops depending on what's most convenient.
It's the way I've done it. One drop at each end of a run of sockets, horizontally between.
Yes. 50mm is the rule where applicable.
You can do what you like with SWA, MICC or foil shielded. BTW have you considered the XL-Shield class of cables as an alternative? Another approach would be to drop T+E in some surface microtrunking - then it is deemed visble and does not require RCD protection.
BTW - are you really paranoid about not having an RCD on the freezer circuit? The number of times I've had a spurious trip since 1995 is zero, though I realise some people may have been unlucky.
No. Where did you get that idea? Visible cables are quite common, especially under the stairs on older houses.
type a search term in at the top should help with some of your questions.
Horizontal between the sockets is fine. Also clipping to the skirting or wall behind the cupboards is fine. I often do this where the cable run goes past the kitchen window and it also then supplies the sockets in the cupboards for the washer/dishwasher which are usually under the window by the sink.
A cable through the centre of the joists would not need RCD protection.
Pg 60 of your OSG is your friend. It is not something I would consider doing. A nail will penetrate armoured easliy enough and then you have to start repairing it.
It might be as easy and as cheap to give the fridge it's own RCBO and just use T&E. That way there is no messing about with glands etc.
Not true. If you wanted to, you could surface mount all your cables (subject to impact assesment) and not use any trunking. The wife would love that:-)
150mm ceiling zone with vertical drops from it to sockets.
150mm corner zone to get the cables up there in the first place.
Sloping ceiling, espec. wet verge with sharp trowels every 10yrs: a) Horizontal ceiling zone created by a 1G or 2G box with blankplate on wall. Vertical drops to sockets from that zone. b) Horizontal skirting zone created by a 1G or 2G box with blankplate on wall. Vertical rises to sockets from that zone.
Sit down and draw out several solutions, then choose between them. Nothing worse than regrets.
As a point of note, SWA requires glands to be accessible for inspection & testing. You would now use BS8436 which is far simpler, neater & quicker, no glands required.
BS8436 is hard to source (TLC will not carry it after contacting NICEIC) because it imposes *critical* restrictions on the type of circuit breaker (type-b) & rating (32A max for 2.5mm ring, 20A max for
2.5mm radial) that it may be used with. This is because the cable's earthed foil surround can only withstand a certain amount of let- through current (I^2t) before it gets vapourised before a fault can be disconnected. You can buy BS8436 in cut lengths from
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(not on their website) under Prysmian Earthshield or Guardian brands or Nexans NXS. It occasionally crops up on Ebay. Prysmian have a new version for the revised BS8436 due in 2011/2012. Guardian is commonly available in white and black which can be useful.
Frankly I would RCD it, if you want no nuisance tripping use a 16-20A RCBO with dedicated radial.
Where does this trunking view come from?
Under 16th 522-06-02 (17th has similar): "in a fixed installation where an impact of Medium Severity (AG2) or Higher Severity (AG3) can occur protection shall be afforded by: i) The mechanical characteristics of the wiring system, or ii) The location Selected, or iii) The provision of additional local or general mechanical protection or by any combination of the above.
Domestic environment is defined as Low severity from impact (AG1). Flat Twin & Earth has Medium impact severity (AG2). Therefore no trunking is required.
You may want to add trunking if you run a cable where it becomes target #1 from impact with appliances (surface run on the face of skirting would be dumb where furniture can hit it, but not if run above skirting). Likewise trunking comes in many flavours, in some situations D-line looks ok, the larger sizes can look fugly though - like 1.2m sockets every 6ft around a room above appliances, tables, TV :-)
Wiring regulations are not retrospective incidentally except.
1 - MEB must be correct, ESC website has interesting comments re 6mm incidentally.
2 - if you extend a f.c. that lacks RCD protection you need to add RCD protection - most use a RCD fused spur, personally I would use a 20A RCD at the CU end in the cupboard with surface or BS8436 w/20A MCB breaking the cable so the whole circuit has it.
If you have a dedicated CU cupboard you certainly do not need to run trunking all around it, more often than not you end up breaking bend radius limits and grouping factors. It is a myth because trunking is the most effective way of working up the hours known to man short of digging a hole and staring down it for half a day, ok that and fitting a fish-switch at a central location for every single EmLight in an entire building.
On Site Guide, Electrician's guide to BR, John Whitfield guide although he likes bonding kitchen sinks although there is no reason - the whole bonding nonsense got out of control with even an IEE article (or two) on it. 17th makes things more realistic and frankly "Class-II everything" in a bathroom someday would be a good idea. A lot of loop- in supplies out there, shared water mains, and lots of peculiar things can happen with faults far away from you. Undo the MEB at one end of a terrace (without bridging cable in place) and you become the conductor if there is a fault at the other end; it happens.
BS8436 is hard to source (TLC will not carry it after contacting NICEIC) because it imposes *critical* restrictions on the type of circuit breaker (type-b) & rating (32A max for 2.5mm ring, 20A max for
2.5mm radial) that it may be used with. This is because the cable's earthed foil surround can only withstand a certain amount of let- through current (I^2t) before it gets vapourised before a fault can be disconnected. You can buy BS8436 in cut lengths from
formatting link
(not on their website) under Prysmian Earthshield or Guardian brands or Nexans NXS. It occasionally crops up on Ebay. Prysmian have a new version for the revised BS8436 due in 2011/2012. Guardian is commonly available in white and black which can be useful.
What have you paid for the 2.5mm BS8436 cable? I have not used it yet and I am interested in what it costs (google is not my friend on this)
An FP cable can not comply with BS8436 because an FP cable requires FP sheath whereas the actual BS8436 standard specifies the sheath be GP8. Therefore marketing may say their FP cable complies with the BS8436 nail test, but the sheath material will not comply with BS8436. Someone might want to telephone Prysmian engineering who have just submitted an updated BS8436 standard to see if it still specifies GP8 sheath (I guess it might :-)
What is it like to handle. Prysmian Earthshield BS8436 2.5mm - chunky, stiff (holds shape re dressing), large bend radius or it crushes, XLPE insulation. Guardian BS8436 1.5mm - same, wood reel weight similar to 4mm FTE. Almost certain it is XLPE insulation.
I have not used any, just "on the shelf" if needed. The 1.5mm black will get used for an outside light with RCD protection in place of
1-1.5mm H07RNF flex (for added safety) and for a tortuous route to a ceiling rose with RCD protection that would otherwise require removal of a wall that was added.
The problem for marketing is most people can design out the need for it. It is good commercially for freezers where you may not want RCD protection (or cheaper than RCBO to individual freezers in a retail outlet). It is good commercially for labs where you may want individual RCD protected sockets in a lab, not RCD protected at the head end. Hospitals probably like it compared to SWA re making-off time. In a domestic environment it might help with the odd light drop or socket run in convoluted timber construction. I see a problem with economy of scale, although the industry desire to move from PVC to XLPE will narrow prices a little.
There are various "guide to" style books. If I were buying one now I would probably wait for 17th edition amendment 1 to come into force since that changes things a bit.
That's a good start
Note that there are some bits that are probably still only 16th edition compliant - or at least have had notes added re the 17th but outside the main flow of the text.
I usually drop down from the ceiling to the first socket in a row, and then horizontally between them and back to the ceiling to dodge doorways etc.
For a freezer feed you could consider RCBO protection. Then there is little chance of a unwanted trip. (and if it does trip it probably means the freezer has a fault!)
You could or just put it on a RCD/RCBO circuit and be done with it.
Armoured can be anywhere if you want, although its not the simplest of thing to run in a plaster chase!
Nope, no requirement for trunking.
Also the general rule is that changes are not retrospective - so if you are not changing anything to do with a circuit, then there is no need to modify it just to meet current regs. (note that things like main EQ bonding should be brought up to standard when any change is made, since it affects the performance of the whole installation.
An FP cable can not comply with BS8436 because an FP cable requires FP sheath whereas the actual BS8436 standard specifies the sheath be GP8. Therefore marketing may say their FP cable complies with the BS8436 nail test, but the sheath material will not comply with BS8436. Someone might want to telephone Prysmian engineering who have just submitted an updated BS8436 standard to see if it still specifies GP8 sheath (I guess it might :-)
What is it like to handle. Prysmian Earthshield BS8436 2.5mm - chunky, stiff (holds shape re dressing), large bend radius or it crushes, XLPE insulation. Guardian BS8436 1.5mm - same, wood reel weight similar to 4mm FTE. Almost certain it is XLPE insulation.
I have not used any, just "on the shelf" if needed. The 1.5mm black will get used for an outside light with RCD protection in place of
1-1.5mm H07RNF flex (for added safety) and for a tortuous route to a ceiling rose with RCD protection that would otherwise require removal of a wall that was added.
The problem for marketing is most people can design out the need for it. It is good commercially for freezers where you may not want RCD protection (or cheaper than RCBO to individual freezers in a retail outlet). It is good commercially for labs where you may want individual RCD protected sockets in a lab, not RCD protected at the head end. Hospitals probably like it compared to SWA re making-off time. In a domestic environment it might help with the odd light drop or socket run in convoluted timber construction. I see a problem with economy of scale, although the industry desire to move from PVC to XLPE will narrow prices a little.
Thanks for that.
I will price some up from my wholesalers when I am next there and see how it compares. It could be handy in some installs.
I've just been doing exactly that over the weekend for our downstairs ring mains - solid floor so it all goes up into the suspended floor upstairs.
The only trouble is that you use an awful lot of cable if as in our house your windows come down to below the 450mm socket height, especially if you are strict about the guidelines for drilling joist holes between 0.25 and 0.4 of the joist span.
For instance, in our 7m x 4m lounge which is 2.5m high, between each socket uses 9 or 10m of cable. This meant I ended up with 2 ring mains downstairs to fit within the cable length limits. This all adds up in terms of T+E, extra RCBOs & time required to cap the extra cable.
Just the lighting to finish off, the smoke alarms, burglar alarm, cat 6 install and central heating to go before plasterboard can be tacked...
I often do not install the traditional upstairs/downstairs rings for exactly that reason. A left/right split on a house is often cheaper, easier and better for voltage drop.
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