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,