Kitchen Extension Electrics

[jamesmjf]

I'm having an extension built which will house a new kitchen and the old kitchen will become a study.

In the existing kitchen is a radial circuit that was for the original cooker, although this has been changed to supply a double socket as a newer oven and gas hob could be plugged in. The MCB was reduced in the CU.

My main question is as this radial circuit is already half way to the new extension, can I extend this cable (4mm I think) to run double sockets and appliances or should I run a new 2.5mm ring main from the CU.

Thanks

James

 

[PrenticeBoyofDerry]

I would personally have dedicated circuits for the new kitchen.
But there is nothing to say that you cannot extend it in to the new kitchen, but you must consider the load this circuit will take.

 

[jamesmjf]

The radial will only be used in the kitchen so it will be dedicated. How much wattage can a radial circuit deal with? Or is that a how long is a piece of string question?

 

[ban-all-sheds]

My main question is as this radial circuit is already half way to the new extension, can I extend this cable (4mm I think) to run double sockets and appliances or should I run a new 2.5mm ring main from the CU.

Before you go down a path which is one way to a little spot between Rock and Hard Place, what did you tell Building Control would be the way that you would ensure the electrical work would comply with Part P when you applied for Building Regulations approval?

 

[ban-all-sheds]

The radial will only be used in the kitchen so it will be dedicated. How much wattage can a radial circuit deal with? Or is that a how long is a piece of string question?

It's a question to which anybody who is competent to be designing circuits would know the answer.

Why are you doing circuit design work?

 

[jamesmjf]

Once again I get the usual answer "part P...building regs..why are you looking at this, step away, leave it to a pros" etc etc.

What's the point!

 

[ericmark]

You get the Part P because you ask questions that if you were doing the job you should be able to answer.

I take the attitude you will likely do the job Part P or not so telling you the safest method is good policy. However without test equipment it will always be likely to have some dangerous errors.

4mm sq cable is good for between 17.5A and 45A but the method 100 will give it a 27A rating. So supplying from a 25A MCB will likely comply. However the 2.5mm sq with same method is 21A so a 20A MCB would be used so to fit a mini consumer unit just to gain 5A seems a little OTT.

433.2.1 Except where Regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current-carrying capacity of the conductors of the installation.

This means joining 2.5mm to 4mm needs a mini consumer unit. I have myself questioned this many times as it would seem a good idea to run a single cable then split when at point of use but because people will often look at the cable at the supply end and install a MCB to suit cable any joints could mean overload in the future.

To supply a kitchen with a 10mm or even 16mm cable and have a local consumer unit makes a lot of sense. But not really worth it with a 4mm cable.

The kitchen is likely one of the largest loaded rooms. As well as hob and oven both likely over 13A often today over the 32A, there are often washing machines, tumble driers, and dish washers all which draw over 2kW so really should have dedicated circuits.

It is common with kitchens to instead of using a 4mm or 6mm cable to feed the distribution unit which is often a grid switch to use two x 2.5mm cables purely as larger cables will not fit the terminals. Although often referred to as a kitchen ring technically its conductors in parallel as they feed just one item. Here 433.2.2 raises it head with 3 meters from the point of splitting to where the fuse is likely the fuse being in the plug.

So it would seem using a grid switch you could use the 4mm cable to supply a load of radials as long as less than 3 meters long. This assumes that the loop impedance is within limits.

As to the loop impedance it is the volt drop with a modern insulation which limits the loop impedance rather than the earth connections as all sockets under 20A now need RCD protection.

Although there are cables which don't need RCD protection these do not include twin and earth.

To be frank today there are so many rules even electricians make errors from time to time. I have watched those builders from hell programs and often the so called experts get it wrong saying how the "Cowboy" has broken rules when in fact he hasn't. It goes both ways and on here I am very wary of saying you should not do things as there are so many exceptions.

To my mind the loop impedance tester is together with the RCD tester is a minimum requirement before attempting to do any work.

 

[Mikejf]

Many thanks for your reply.

 

[ban-all-sheds]

Once again I get the usual answer "part P...building regs..why are you looking at this, step away, leave it to a pros" etc etc.

What's the point!

The point is that you are building an extension. Unless you are a complete idiot you must have applied for Building Regulations approval.

Therefore you must have had to describe to the council how you would ensure that the electrical work would comply with Part P.

Assuming that when the extension is finished you'd like it to have a Building Regulations completion certificate then you must do what you told the council.

If explicitly, or by default or tacit acceptance of an assumption which they told you about, you said you'd be using an electrician who could self-certify, or an electrician who was qualified to issue BS 7671 certificates, then you must do that. If you do not, then your LABC will refuse to give you a completion certificate.

So, before you go down a path which is one way to a little spot between Rock and Hard Place, what did you tell Building Control would be the way that you would ensure the electrical work would comply with Part P when you applied for Building Regulations approval?

 

[ban-all-sheds]

You get the Part P because you ask questions that if you were doing the job you should be able to answer.

And the corollary of that, James, is that as you can't answer the questions you should not be doing the job.

Or, more precisely, should not be doing it yet. Nobody is saying that you're incapable of becoming able to do it, but you are not there yet, and the distinct impression you give is that the project is underway, and you are asking fundamental knowledge type questions as and when they occur to you.

That is a very bad plan.

The answers to the specific questions you've asked are here:

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm (all subsections)
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• //www.diynot.com/wiki/electrics:mcb2
  
  
• http://www.tlc-direct.co.uk/Book/7.13.3.htm
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm[/list]
  and the only acceptable way for you to assume the role of designer, and/or installer and/or tester is for you to become knowledgeable and competent enough to be able to confidently and honestly make the following declaration:
  
  I being the person responsible for the design, construction, inspection & testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design, construction, inspection & testing hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to ............. (date) except for the departures, if any, detailed as follows:
  
  No lower standard of ability is acceptable. Runny paintwork, crooked wallpaper, wonky shelves, doors that don't quite fit, plumbing which leaks a bit - none of these matter greatly. But flaky electrics do.
  
  There is nothing wrong with DIY. There is a great deal wrong with BIY (Bodge It Yourself).
  
  If you cannot work to the same standards of safety as a qualified electrician then you must not DIY.
  
  The electrical work you want to do is not rocket science, but neither is it trivial job, and I can assure you that it involves knowing more than you think it does.
  
  Asking questions here can be a useful part of a learning process, but they are not a substitute for proper structured studying. The key term there is "learning process" - you cannot learn all the things you need to know just by asking questions here. It isn't structured enough - it won't provide you with a way to progress where each step builds on what you learned before.
  
  You can't carry out a job like this by asking whatever random questions happen to occur to you. What if you get something wrong because you have no idea your knowledge is wrong? What if you miss something because you simply have no idea it even exists, and just don't realise you don't know it?
  • //www.diynot.com/wiki/electrics
    
    
  • //www.diynot.com/wiki/electrics:books
    
    
  • http://web.archive.org/web/20080213151445/http://www.kevinboone.com/electricity.html
    
    
  • http://web.archive.org/web/20080213151445/http://www.kevinboone.com/domesticinstallations.html
    
    
  • http://www.tlc-direct.co.uk/Book/1.1.htm[/list]I suggest you get stuck into the last link right away - it won't give you design ideas, and unfortunately it doesn't refer to the current edition of the Wiring Regulations, but it's free, and will still give you a good grounding which you can augment with more up to date publications.
    
    Do you have the time, given the schedule for the rest of the building work?
    
    And will it match what you told the council before you started?