A few rewiring queries

[Lobster]

Am currently doing a rewire and have a few questions please...

1. (the main question!). I'm proposing the following circuits - does this plan sound sensible? BTW total floor area is 70m2; CU is a Volex split-load item: (Screwfix 82204). So:

Non-RCD protected: a) upstairs lights, 6A MCB, 1.5mm T&E cable b) downstairs lights, 6A MCB, 1.5mm T&E c) linked smoke alarms, 6A MCB, 1.5mm T&E d) downstairs ringmain feeding specifically the oven, fridge-freezer and gas combi boiler, 32A MCD, 2.5mm T&E cable

RCD protected: e) upstairs ringmain, 32A MCB, 2.5mm T&E f) downstairs ringmain - all sockets other than those in circuit (d) 32A MCB, 2.5mm T&E g) electric shower, 40A MCB, 10mm T&E

I expect circuit (d) to cause most comment?! I don't see the need for a dedicated cooker circuit because I'll be fitting a built-in single oven with 13A plug (plus gas hob), which could be plugged into an ordinary ringmain. OK?

1. I've got some spare 1.0mm red/black cable which I'd like to use up - would there be any issue with me mixing-and-matching 1.0mm and 1.5mm cable within one installation (I don't have enough 1.0mm do do the whole house)?

1. CU is under the stairs, which means that the route for all cables will be running under the stairs, in a straight line, at ~45deg, in the space created behind plasterboard attached to battens on the bottom of the stairs (clear?!.) Now, if the plasterboard were 'ceiling', that arrangement would unequivocably be perfectly OK; however if it were 'wall', it would definitely be unacceptable because the cables would be well within 50mm of the surface. What I'm unclear about is what does a

45-deg-mounted surface count as - ceiling or wall? I think it's ceiling, but I'd like reassurance that some future inspector isn't going to throw a wobbler at it! Not sure what to do if it's unacceptable as it is.

Thanks a lot David

[Dave Plowman (News)]

I'd certainly have a separate ring for the kitchen - and also look carefully at the load on that before running the oven from that. Because it's all too easy to exceed the loading. A washing machine can take 13 amps when heating. Also a dishwasher. Drier? A kettle perhaps 10 amps. Already, you're over the nominal 30 amps. Add in a toaster and say grill - let alone your oven, and it's easy to see a possible problem.

None. I'd use 1.5mm for the feeds and the spare 1.0mm for the switch drops. But for 6 amp circuits, 1.0mm is fine anyway.

[Andrew Gabriel]

You haven't said what type of earthing your system is.

It's fine. Shower probably doesn't need to be RCD protected. Smoke alarm circuits are often 3A MCBs if they're available for your CU.

No problem mixing them providing all your cable is red and black. (Whilst an installation can be mixed, you can't mix old and new colours in the same job.)

I don't have the regs on me to confirm, but I don't think it makes any difference from the point of view of depth of burying.

[Lobster]

Thanks for the replies.

At the risk of provoking a "what-the-hell-are-doing-rewiring-for-if-you-don't-know-this-for-chrissakes-get-a-pro-in"-type response, what significance does the earthing system have on the circuit design? In mine, it's provided by the armoured sheath of the main supply cable; have to say I don't know any more than that. Do I need to find out?

Thanks David

[Christian McArdle]

OK (although 1mm would be fine and easier to squeeze cables into the terminals).

OK.

OK.

• See below.

OK.

• See below.

OK.

The mains issue here is the dispostion and type of the socket circuits. Firstly, I would ensure that the kitchen has at least one general purpose socket circuit entirely to itself in addition to any fixed appliance circuits. Secondly, fixed appliance circuits might be better implemented as radial circuits, due to the unusual loading patterns.

For example, when designing my own setup, I had:

1. 32A 2.5mm RCBO (RCD protected) socket ring circuit (includes small oven, but no other fixed appliances)
2. 32A 6.0mm MCB (not RCD protected) radial circuit for washing machine, tumble dryer and dishwasher.
3. 16A 2.5mm MCB (not RCD protected) radial circuit for fridge/freezer.
4. 16A 2.5mm MCB (not RCD protected) radial circuit for central heating (including immersion heater).
5. 32A 2.5mm RCBO (RCD protected) socket ring circuit for all non-kitchen sockets in house (intend to split into 2 when can be bothered). This preexisting circuit used to cover all sockets (including kitchen) in entire house.

Christian.

[RichardS]

Thought these had to be triple & earth? (though it would be T&E from the CU to the first one).

[Christian McArdle]

Correct. The linking cable is usually 3&E, although some systems using T&E and either a radio link or inject a high frequency comms channel onto the main mains signal.

Christian.

[Andrew Gabriel]

"what-the-hell-are-doing-rewiring-for-if-you-don't-know-this-for-chrissakes-get-a-pro-in"-type

If the system was a TT system with your own earth rod, the use of RCD's would have to be different.

[Andrew McKay]

Lots.

If you don't understand the difference between TT, TN-C, TN-C-S, etc then you really should not be doing the job.

If you intend to do the re-cabling yourself then your follow-up certificate which will be required may prove to be very expensive. If you've hot-wired the earthing within the installation wrong then it will need to be ripped out and started over. It will be cheaper and easier to do the wiring correctly the first time.

Andrew

[Lobster]

OK, I'm listening...

What do you mean, hot-wiring the earthing? Of course I'll be doing it properly, and getting it certified afterwards, which is why I posted my circuit design here first for feedback, which I'm pleased to have received. [It's my third full rewire, and my previous checks by a pro came up (1) with very small easily remedied errors and (2)100% OK. Previously I had run my wiring diagram past my pro first, who OK'ed it; I assume that because my earthing plans were OK there was no need for him to go into all the ins and outs of different earthing systems?]

Anyway - given the presence of an earthing block next to the CU, connected to the CU, a clamp on the armoured cable, and (now) the incoming gas/water pipes, what influence does this have on the wiring of the ring mains and lighting circuits? They will all be T&E cable, with CPCs wired in to the CU - how would this be affected by different earth systems? Under what circumstances would all the wiring need to be 'ripped out and started over'?

Thanks David

[Andrew McKay]

I regret that I can't give you details - this is a DIY group after all. I just know from having got my C&G2381 that earthing (and bonding) is a complex subject. Making assumptions is not a good idea.

Andrew

[John Rumm]

From the point of view of wiring individual "standard" circuits then there (usually) won't be any difference - the basic construction and testing will be the same.

Earth loop impedance testing is good practice for any new circuit since it will establish that you can get the required fault currents to provide the required disconnection times.

The sizing of equipotential bonding and other earthing conductors can vary with different earthing schemes. Follow the tables in the On Site Guide and you should be OK.

(you get most differences with TT systems)

Slightly alarmist wording IMHO. "When wired by a complete muppet with all the wrong cable sizes or following improper cable routes" might be an appropriate answer ;-)

[John Rumm]

That seems to be a bit of a "cop out" having made the first statement IMHO...

What has this being a DIY group got to do with it either?

[Andrew McKay]

I'm sorry you feel that way. The fact is that if you followed advice given to you on this newsgroup and it resulted in casualty or fiscal loss then you might choose to pursue a claim against the person that offered the advice.

My advice was simply to suggest that you might not understand this well enough to continue. If you disagree with that then fine.

See above. We live in a litigious society these days, and messing around with electrics is an area where there could feasibly be legal challenges made against those who offer advice. Many people these days jump on passing bandwagons.

Andrew

[Andrew McKay]

::Holds out wrists:: ::SLAP:: ::Runs off to whimper quietly to himself::

:)

Andrew

[Mike]

I know some people don't like to do it but I'd put the smoke alarms on one of the lighting circuits to give a check on there being power there. This is fine by the regs if you use battery backed up alarms, which I would recommend anyway as they are only a quid or so more expensive. This gives you a spare circuit for future use.

I assume the 13A sockets in the kitchen are non RCD protected. Personally I don't like that, especially those above the worktop.

[Martin Angove]

The thing is that as far as I can tell, Part P does not specifically exclude DIY work any more than Parts A through N (generally) exclude DIY work. What it *does* exclude is *unchecked* DIY work. In the same manner that you should involve Building Control before (for example) knocking down a supporting wall and inserting a lintel, Part P requires either a notice to BC and a full inspection of subsequent electrical work by a suitably qualified person who is willing to take on responsibility for the work, or the work to be done by a self certifying company/person. It doesn't require even this for some sorts of work ("non-notifiable"), though obviously a complete rewire doesn't come under this category.

Note that this is a lot less strict than the situation for working with gas or with pressurised hot water systems.

So one has to make a judgement here. Since DIY isn't outlawed, why should we stop giving advice in how to do it correctly? There are several highly knowledgeable electricians in this ng who give consistently accurate and well-reasoned answers, even if they don't always come up with exactly the same solutions as each other. Some are (I believe) even working under self certifying schemes and so *should* know the answers! I have learned immense amounts from them. The only reason to stop giving such advice would be if we could be certain that the person asking would carry out any given advice without following the new rules.

Perhaps an update to the electrical section of the FAQ is called for, and perhaps any posts containing electrical advice should post a direct link to that section before any advice.

Hwyl!

M.

[Lobster]

Yes, I hummed and ha'd a bit over that one. And I will be using battery-backup alarms. Maybe I'll change my mind!

No, under the above scheme, the kitchen sockets come under circuit (f), ie RCD-protected: it's just the kitchen items specified in circuit (e) which are not RCD'ed.

David

[Lobster]

Thanks to you (and Christian) for this advice. I take your point, but do you not think that by pulling the oven out of the downstairs circuit, I've pretty well done the same thing? I mean, all the high-load items you describe above will be on the kitchen circuit, just that I'm calling it a 'downstairs' circuit(!) - in addition to the (dining) kitchen, it will only cover the living room and hall, and I can't see any high-load appliances being used there. Just TV/table lights/hifi/vacuum cleaner, that sort of thing (there will be a gas fire plus gas CH, so really see no likelihood of any electric heaters.) When I looked at having another kitchen circuit, it seemed like overkill having 3 downstairs ringmains for such a small house.

Or maybe I should incorporate the living-room and hall circuits into the upstairs ringmain, releasing the 'downstairs' ring just for the kitchen. What do you think?

Thanks David

[Bob Eager]

Or on a separate circuit, linked to a visible non-maintained emergency light (say, on the stairs). If the power fails, you have light where needed. If the circuit fails, the light comes on and you get a visual indication.