Does my new split-load CU plan sound OK (RCD + RCBO)?

[Hibbo]

OK, OK, I think you've convinced me!

Here's a Wylex 10-way dual RCD CU for £58, with MCBs at £2.88. I think I will go with that.

That also gives me the option of having two lighting circuits, arghhh!

So, a Wylex dual RCD CU, with;

RCD-1:
32A main ring
40A shower
6A possible extra lighting circuit, or at least one extra light
32A/40A cooker (for future use)

RCD-2
6A lighting
32A kitchen ring

How's about that then?

I notice that the RCDs are called "80A", does this mean they will trip at 80A (like an RCBO) or that 80A is the maximum current they can handle through them? I ask because if an electric cooker was added that would take the total (theoretical) load on RCD-1 way above that. Is that an issue according to the regs?

 

[flameport]

As you have found, a dual RCD Wylex unit and MCBs as required can be obtained for around £80.

However for only 5 circuits, you could get a Hager 8 way unit with 5 RCBOs for about £140 - a much better solution for only £60 more, which in the context of refurbishing a property and doing a complete rewire is a tiny amount of money.
(8 ways so that you have 3 spares for future use)

 

[skenk]

Looks like a decent price. I usually budget around £100 for a CU+bits, you'll need tails, main earth, stickers, henleys? etc.

Arrangement sounds good to me, 2 lighting circuits isn't much more hassle than 1 but if you do have only one then you could have your cabinet/plinth etc. lighting running off the kitchen ring.

Cooker + Shower on full load together would be on the limit for 80A rcd but rare - use your diversity calculations (guidelines in OSG). If you're in an older flat you'll be lucky if your supply fuse is even 80A.

Note re: shower: If you're going for a quality full renovation u might be better putting in a shower off the combi boiler (or ideally unvented cylinder w/pump) in 22mm.

Smoke alarms?
Seperate circuit for boiler?
Seperate circuit for appliances?

it's page 6 you want in that document. If you are in a flat and not carrying out "repair or replacement works to a level equal to the installation (or part therof) being repaired or replaced." you should have a warrant. You may need one anyway depending on the extent of the other works in the renovation (layout and drainage changes).

Tim

 

[Hibbo]

That's another thing I was going to ask - all the CUs come with a 100A incomer, but I'm not sure what my supply is rated at (I'm away at the moment so I can't check), it may well be 80A or even 60A, although that said, they (the electricity board) replaced the supply to the flats a couple of years ago, so it might be 100A now. Is it possible to get a lower rated incomer with a new CU?

I don't think I'll need a separate circuit for appliances as the kitchen will be on it's own ring, with the combi fed from FCU from it.

Smoke detectors are battery jobbers.

I like electric showers as you can always get a shower even if your boiler is on the blink. Are modern combi boilers up to the job of supplying a shower?

I will check exactly what I need to do/inform before I start the work. It is a new kitchen, new bathroom, new boiler & plumbing, rewire and redecorate project, no I'm guessing that WOULD count as "repair or replacement works to a level equal to the installation (or part therof) being repaired or replaced." Or would it as there's no structural or draining changes?

Bah, flameport has got me considering going for an all RCBO install now... I understand what you mean by £60 not being much in the scheme of things, but it's still £60. Would I get £60 of advantage out of it? Do you think a single lighting circuit would be OK if every circuit was on its own RCBO?

Thanks for all your help here guys.

 

[davelx]

The 100A rating on the incomer and the 80A rating on the RCD are both maximum current handling. Neither of these has any overcurrent tripping functionality (surely you would know this from 2391 course?) so there is no reason to change them for a lower capacity supply.

 

[ban-all-sheds]

I understand what you mean by £60 not being much in the scheme of things, but it's still £60.

I'm in Scotland.

 

[skenk]

Thanks for all your help here guys.

Hit the button!

 

[AndyPRK]

your wylex solution sounds good to me, although as you only have 1 lighting circuit (excluding your one other light which doesn't count!) I would still be temped to put the lights on an RCBO. It's aways good to have lights working for you or your tenants.

 

[mikhailfaradayski]

Do you have any form of desk/standard/bedside lamps at all?

As you you are talking about a flat, i would say that one lighting circuit would be sufficient, especially as you say you intend to install an emergency light fitting aswell.

People are correct ot tell you about reg 314.1, but a lot of people have fixed views on how that is interpretted. It actually guides a sparky to 'take into account' the consequence of a single failure, it doesn't say how to achieve that. If you are doing a complete rewire yourself, you have saved enough money to buy a copy of the 17th, read it yourself and form your own opinion.

I thought 'Chint' was a blend of the words 'China' and 'Sh*t'

As you've suggested there may be a possibility of renting this property in the future, i would suggest that you install mains powered smoke alarms aswell. They don't cost much to install, in the long run, cheaper and more reliable than battery jobbies and might just save either yours or you tenant's life one day.

oh, and another thing about the type of shower you intend to have-

How frequently do you think an electric shower will fail compared to the frequency of your boiler failing?

If its possible, ditch the electric shower and feed from a combi instead. The flow rates on combis will run a shower no probs.

 

[bongos]

I notice that the RCDs are called "80A", does this mean they will trip at 80A (like an RCBO) or that 80A is the maximum current they can handle through them? I ask because if an electric cooker was added that would take the total (theoretical) load on RCD-1 way above that. Is that an issue according to the regs?

Or put the shower one RCD and the cooker on the other RCD.

 

[Hibbo]

The 100A rating on the incomer and the 80A rating on the RCD are both maximum current handling. Neither of these has any overcurrent tripping functionality (surely you would know this from 2391 course?) so there is no reason to change them for a lower capacity supply.

The 2391 course was a while ago, and as I said I've never 'used' it. I do not consider myself a domestic spark, that's why I'm asking questions here.

I will look in to mains powered smoke alarms, although with it being a flat access to wiring above the ceiling is difficult.

I am also in the process of getting advice on combi boilers, so I way well end up ditching the electric shower. I suppose I am a bit subjective on this, my parents have had lots of boiler problems, but the electric shower as always been good!

If I did go for a boiler fed shower I suppose I could go for an all-RCBO install,
32A RCBO - Main ring
32A RCBO - Kitchen ring
6A RCBO - Lights
(maybe) 6A RCBO - Smoke detectors
I'd run cables for a shower and cooker also, so they're there for future use.

Do smoke detectors have to be RCD protected? Do cables run in the ceiling space count as being buried? Are mains powered smoke alarms powered directly at 230v, or can you mount a PELV type transformer next to the CU for them?

So much to think about, I'll get myself a copy of BS7671 17th and the onsite guide when I get home.

 

[ban-all-sheds]

ignore

 

[mikhailfaradayski]

The 100A rating on the incomer and the 80A rating on the RCD are both maximum current handling. Neither of these has any overcurrent tripping functionality (surely you would know this from 2391 course?) so there is no reason to change them for a lower capacity supply.

The 2391 course was a while ago, and as I said I've never 'used' it. I do not consider myself a domestic spark, that's why I'm asking questions here.

that doesn't stop you knowing a lot more than people who use it on a daily basis

I will look in to mains powered smoke alarms, although with it being a flat access to wiring above the ceiling is difficult.

You should be able to find a way. IMO, irrespective of requirements/laws etc, i think its close to an act of negligence to overlook mains smokes when doing a full rewire

I am also in the process of getting advice on combi boilers, so I way well end up ditching the electric shower. I suppose I am a bit subjective on this, my parents have had lots of boiler problems, but the electric shower as always been good!

You don't mention how many decades since the last time your parents boiler was serviced. Combi fed shower mean one less electrical item in a bathroom, its more efficient and the money saved will allow you to fit RCBOs throughout if you wanted.

If I did go for a boiler fed shower I suppose I could go for an all-RCBO install,
32A RCBO - Main ring
32A RCBO - Kitchen ring
6A RCBO - Lights
(maybe) 6A RCBO - Smoke detectors

swap maybe for definately and all is good here

I'd run cables for a shower and cooker also, so they're there for future use.

If you still intend to run a feed for an electric shower for future use, might i suggest that you run 16mm. By the time you come to ever use that feed, who knows what power rating showers will be available. You might find that running 10 may be useless (but IMO a waste of money, time and effort anyway)

Do smoke detectors have to be RCD protected?

No, but consider the cable feeding them, you should at least be able to find extracts from BS5839-6 for further information on smokes.

Do cables run in the ceiling space count as being buried?

No.

Are mains powered smoke alarms powered directly at 230v, or can you mount a PELV type transformer next to the CU for them?

run them straight from 230V, o, BTW, don't forget you will need 3 cores for the interconnection cable

So much to think about, I'll get myself a copy of BS7671 17th and the onsite guide when I get home.

Brilliant idea, check Amazon, usually cheaper by a couple of quid. Your local library may have a copy, and also some other sparky guides and info. I recently scanned through a copy of Scaddan's book, a gold mine of information, worth a read for you i think.

 

[Hibbo]

Thanks for your excellent detailed reply Mikhail!

I will definitely fit a mains powered smoke alarm then - I have a couple of options if I can't get access to above the ceiling.
1) Mount it on a wall, right up against the ceiling. If they have to be ceiling mounted to work properly (do they?) I could mount it on the ceiling but next to the wall.
2) There will be a box section hiding the duct for the bathroom extractor fan, so I could mount it on this, this would mean the alarm would be horizontal mounted outside the kitchen and main bedroom doors - a good place I think.
Obviously with the cables buried in the wall it would have to be RCD protected.


With regards to my parents' boiler problems, they're on their third boiler since the house was refurbished 15 years ago! Just unlucky I guess.


I remember using guidance note 3 when I did my courses, is a 17th edition of that available?

 

[Spark123]

Yes, the IET and Amazon sell it, along with most good book stores.