A little guidance please

[TH3 M0L3]

Hi people

I am new to this forum and hope to have a fruitful experience.

I am in the process of buying a 3 bed 1930's semi that requires a full rewire.
I have had a few quotes to do the work and am astounded at the astronomical cost.

I am however a fully qualified refrigeration and air-conditioning engineer, so have a practical knowledge and a healthy respect for electricity, as the equipment I work on ranges from small 1kw units up to and exceeding 415v 200kw systems.

In a bid to save money (£3k after buying materials - not to be sniffed at!!) I have elected to do the job myself.

I have more than enough confidence to do the first fix and second fixes, and fairly confident to do the final fix myself.

What I need clarification on is the mcb ratings for the cookers and hob.
I have not selected the appliances as of yet as the kitchen is going to be replaced within 6 months.
Our initial thoughts are to have an induction hob and two seperate ovens.

For this configuration, am I correct in having a dedicated 32A mcb and 40A local DP switch for each oven, each on their own radial circuit with 6mm twin and earth cable? For the hob, a 20A mcb on a radial circuit with 2.5mm twin and earth and a local 20A DP switch?

Is it possible to run the three appliances in an easier configuration? If so, could you please make some suggestions.

I am almost positive that I will get abuse for what I am asking as I am a member of a refrigeration forum
and we are always weary of the "diy"-ers and cowboys posting for help.

I do know that the work will need to be certified, and I have friend who is a certified electrician who will be inspecting during the works, testing the final installation and making the final connections before signing it off.
I know what you're thinking, "why not ask your sparky mate to do the calcs". Am I right? Thought so.
I will confirm it with him as he will be signing it off, but as he is doing me a huge favour already I dont want to be a massive burden on him with 20questions.

Thanks to those of you kind enough to share some of your knowledge with me.

 

[bugsy]

First things first your friend even if part p registered cannot sign off your work, you can however contact your local LABC who will, at a cost of between £ 150 / £300.00 providing everything complies with BS7671 , 2008.

secondly depending on the rating of the ovens 2no. 20a should be fine
The induction hobb could be between 32a / 50a depending on size / make etc etc

Hope this helps

 

[TH3 M0L3]

First things first your friend even if part p registered cannot sign off your work, you can however contact your local LABC who will, at a cost of between £ 150 / £300.00 providing everything complies with BS7671 , 2008.

secondly depending on the rating of the ovens 2no. 20a should be fine
The induction hobb could be between 32a / 50a depending on size / make etc etc

Hope this helps

It does help, thanks.

The mcb will be easy to upgrade once a hob is selected. It is the cable size i am concerned about. It seems unreasonable to think that 2.5mm suffice for a load of up to 50A?
Would I not be better off using 6mm as well?

 

[bugsy]

No , 2.5 is not rated @ 32a.

As a very , very basic rule for you project

S/O ring 2.5mm @ 32a subject to area
Lighting radial 1.5mm 6a/10a
Load up 16a radial 2.5mm
Load up to 32a radial 6mm
Load up to 50a radil 10mm

This is really rough , and as i say only a guide. all are subject to calc's for voltage drop / thermal effect / grouping etc

 

[TH3 M0L3]

No , 2.5 is not rated @ 32a.

As a very , very basic rule for you project

S/O ring 2.5mm @ 32a subject to area
Lighting radial 1.5mm 6a/10a
Load up 16a radial 2.5mm
Load up to 32a radial 6mm
Load up to 50a radil 10mm

This is really rough , and as i say only a guide. all are subject to calc's for voltage drop / thermal effect / grouping etc

Thanks again.
I have this table saved somewhere on my laptop.

My basic plans are to have the following:

Fed from the 80A 30mA DP RCD section of the split load cu

4 ring final circuits: 2.5mm tw+ e 32A mcb
4 lighting radial circs 1.5mm tw+ e 6A mcb (max 7 lights per radial)
2 mains smoke detector on one radial 1.5mm tw + e 6amcb

fed from the 100a mcb part of the split load cu

2 radials for oven 6mm tw + e 20A mcb (thanks)
1 radial for hob 6mm tw +e 32A mcb (for now)

How does that sound?

 

[bugsy]

The reg's have all changed , your quoting 16th edition, you now need dual rcd or all rcbo plus isolator

 

[TH3 M0L3]

The reg's have all changed , your quoting 16th edition, you now need dual rcd or all rcbo plus isolator

oh i see.

I am just going on the cu i was going to buy from screwfix.

So the 100Amp mcb should act as the main isolator and I should have one 63A 30mA rcd for ring final and radial, and another one of the same rating for ovens/hobs etc?

 

[Phil]

dual board (high integrity) 2 rcds, or as post before rcbos, if the board is already in i would go for the rcbos.
do you know the new regs regarding cable in the 17th ? protection by rcd etc, not being sarcastic just asking so someone can advise

 

[bugsy]

100a isolator, 2x 80a RCD, split the loads over the RCD's ground floor s/o first floor lighting etc and first floor s/o ground floor s/o on the other -

You need to get your installation right , otherwise that 3k your gonna save is going to be spent putting things right

 

[TH3 M0L3]

I know it seems as though i am quoting it parrot fashion, but i assure you i am taking high regard of what i have researched thus far.
I have not even finished the process of buying the house yet, so am a while off doing any work.
I appreciate that rcbo's are prob the best option, but I am running 11 circuits and at roughly 30 quid a pop, I think I will go for the dual rcd option...

this is what i have researched re the regs safeguarding the cable...

All socket outlets should be protected by 30mA RCD
• All circuits in rooms with a fixed bath or shower
should be protected by one or more 30mA RCDs
• All cables buried beneath the plaster surface of a
wall or partition (at less than 50mm and without
earthed mechanical protection) should be
protected by 30mA RCDs
• All cables concealed in metal stud partitions
(without earthed mechanical protection) should be
protected by 30mA RCDs
• Installations should be designed and arranged so as to
prevent unwanted tripping of RCDs

 

[bugsy]

That;s about it for OC/ELPD's

 

[TH3 M0L3]

That;s about it for OC/ELPD's

pardon my ignorance, but i am unfamiliar with the jargon of the trade....

what is OC/ ELPD?

 

[bugsy]

Over current / earth leakage protection devices

 

[TH3 M0L3]

pardon my ignorance, but i am unfamiliar with the jargon of the trade....

what is OC/ ELPD?

I am sure if i went on about nrv's, prv's, hp's, lp's, hfc's hcfc's etc that would cause confusion too ;-)

Over current / earth leakage protection devices

Thanks fella, the acronym makes perfect sense now!

I really do appreciate all your help! I will naturally consult my friend before i do any of the work as it is a major job, and i dont want to get it wrong as the consequences could be fatal!

If you ever need any help with any air-con queries then feel free to pm me!

 

[bugsy]

No worries,

 

[TH3 M0L3]

No worries,

So I have finished the work with no problems. I am having it signed off today, but the sparks reckons I need a 100A 100mA rcd as the main breaker.

My current layout is this

100A main breaker

3 Circuits not rcd protected are
radial - Hob - currenty not in use
radial - 2 oven circuits - only one in use at present

80A 30mA RCD
Upstairs ring (7 2g sockets)
Downstairs lights
Service ring (2 2g sockets)
Kitchen ring (4 2g sockets, two un-switched 1g sockets protected by 13A fcu and boiler protected by 6amp fcu)

80A 30mA RCD
Downstairs ring - front of house (5 1g sockets)
Downstairs ring - rear of house (2 3g sockets, 3 2g sockets)
Upstairs lights
smoke alarm

Do I really need the 100A 100mA rcd?

 

[mnmwill]

Whats th earthing arrangement?? If he said 100mA RCD you probably have a TT (earth rod).

 

[TH3 M0L3]

Whats th earthing arrangement?? If he said 100mA RCD you probably have a TT (earth rod).

yeah, it's TT. I installed a new earth rod and pit etc. He's now changed his tune and said that 100A 30mA RCD is ok.

Only thing that bugs me is that I dont the main rcd it to trip unnecessarily when there is a fault on one of the circuits protected by the other two rcd's.
My worries are confirmed by means of watching him do all the tests earlier today. He was testing the rcd tripping times, and the main rcd tripped out every time too.
I dont dispute the fact that having an rcd as the main incommer is beneficial, especially as it's a TT, but I would have preferred a 100mA one. Trouble is, there aren't any places open on a sunday where one can pick one up, so I had to get the 30mA one from screwfix.

I need to get the supplier out to seal the fuse and meter, so I might get the 100mA rcd and stick it in before they come, or am I just being over the top?

 

[Sintra]

100A 100mA time delayed rcd is what you require to allow for discrimination.

 

[Guest123]

Well I'm a bit confused.....

From what I gather you have a high integrity 17th ed board with dual RCD's and a couple of non protected ways.

First of all, why arent all the circuits protected by the 30mA RCD's??? are they in galv conduit or surface mounted???

I assume this is also the reason for the 100mA RCD as they have no other form of additional protection???

My take is if all the circuits have 30mA RCD protection via the dual RCD's and the CU is an insulated one then an additional 100mA RCD is not required.

If on the other hand you have installed the other circuits to negate the requiremnet for additional 30mA protection then you will need the additional 100mA

 

[JUD]

So I have finished the work with no problems. I am having it signed off today, but the sparks reckons I need a 100A 100mA rcd as the main breaker.

My current layout is this

100A main breaker

3 Circuits not rcd protected are
radial - Hob - currenty not in use
radial - 2 oven circuits - only one in use at present

80A 30mA RCD
Upstairs ring (7 2g sockets)
Downstairs lights
Service ring (2 2g sockets)
Kitchen ring (4 2g sockets, two un-switched 1g sockets protected by 13A fcu and boiler protected by 6amp fcu)

80A 30mA RCD
Downstairs ring - front of house (5 1g sockets)
Downstairs ring - rear of house (2 3g sockets, 3 2g sockets)
Upstairs lights
smoke alarm

Do I really need the 100A 100mA rcd?

Your smoke alarms (In Red) should either be on it's own dedicated circuit, not protected by an RCD that is protecting other circuits, or should be fed from a regularly used lighting circuit.

 

[TH3 M0L3]

Your smoke alarms (In Red) should either be on it's own dedicated circuit, not protected by an RCD that is protecting other circuits, or should be fed from a regularly used lighting circuit.

Thanks for the heads up Jud. I realised this after posting on here and changed it to being on a non rcd protected circuit. It was always on it's own dedicated circuit with a 6A mcb


On another note, who's responsibility are the tails between the fuse and the meter? Just noticed that a bit of the conductor is showing where it goes into the meter, and my supplier reckons it's my responsibility. I was under the impression that they were responsible for everything up to the meter?

I am with british gas btw....

 

[JUD]

The tails between the fuse and the meter are not your responsibility. Only equipment after the meter belongs to you.