Ring circuit cable size for first fix

[converse]

We are completely gutting and refitting an old property, all electrics have been ripped out as were completely suspect.

We want to do the first to second fix of electrics ourselves (running cables etc), then let our electrician do second to final fix, testing and connections.

Upstairs is 4 bedrooms with 3 to 4 doubles in each plus a double on the landing, so 14 double sockets in total on our design.

For the upper ring is 2.5mm ok to run for this?

We haven't finished lighting design yet, but will be a lot of 7W LED downlighters and some pendants, separate circuits for up and downstairs.

Downstairs we are knocking it about a bit, going to have a separate ring for the kitchen and utility, then the remaining rooms on a main downstairs ring.

Plus obviously separates for cookers, heating etc.

Thoughts?

 

[ericmark]

You will be told how the design needs a signature and the point is although there are forms which allow different people to design install and test these mean the notifying process has to be done by the local authority building control which will likely cost more than having a scheme member electrician do the whole lot.

As to your design fixed items over 2kW should according to BS7671:2008 have a dedicated supply. We have always done this with immersion heater and cooker but in theroy it should also be done with washing machine, tumble drier, and dishwasher. In the main we consider the kitchen ring will cater for this load but that is a personal decision of the designer and where there is a utility room then the load in the kitchen is not that great.

Most of us wire as we have always done and when we find we are close to the limit we mentally note the near miss and when we do the house of similar size next take measures to reduce ring or radial sizes.

However when you don't have the experience then there is nothing for it but to dust off the old maths books and work out all cable lengths and volt drops. Plus loop impedance.

In my time yes I have got it wrong. Run 5 singles down trunking for a socket taking an after noon to then find too small is bad enough. Do the same with a ring main in a house is very different. We with the calculation allow 20A in centre and 12A spread around circuit for a 32A MCB. But with a 20A MCB it is still 20A in the centre so if you make an error it's not just reduce the MCB size a bit it's reduce it a lot so can't afford to make mistakes.

Cost I would think is a major concern with all RCBO's with a TN-C-S supply then unlikely to have a problem with division of circuits. But reduce that to 2 RCD's feeding MCB's and again it's down to the person testing and inspecting to if he feels it complies there is no magic formula.

As to cables sizes unless using mineral insulated cables 2.5mm² is minimum for a ring over that size is used where there is insulation or longer than normal run but there is a problem getting the larger size cables in the socket terminals with some makes so although in theroy you could wire in 4 or 6mm² this would be rare.

 

[JohnW2]

Do the same with a ring main in a house is very different. We with the calculation allow 20A in centre and 12A spread around circuit for a 32A MCB. But with a 20A MCB it is still 20A in the centre so if you make an error it's not just reduce the MCB size a bit it's reduce it a lot so can't afford to make mistakes.

I don't quite understand what you're saying here. If the CCC of the cable were at least 20A, there would be no point in having a ring protected by a 20A MCB. On the other hand, if the CCC was less than 20A, then the regs would not allow a ring (in which In of MCB was greater than CCC of cable). Can you explain what was your point?

Kind Regards, John

 

[ban-all-sheds]

Thoughts?

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20070327014113/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  The references above which are in italics are out of date. The principles are still valid, and you can absolutely learn valid things from them, but you will need to refer to the latest edition of the Wiring Regulations to get the correct data.
  

Plus, as Eric said, you need to think about whether you have genuine grounds for believing you could legitimately sign these declarations:

I being the person responsible for the design 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 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 2011 except for the departures, if any, detailed as follows:

I being the person responsible for the construction 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 construction 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 2011 except for the departures, if any, detailed as follows:

Because someone has to, and it can't be an electrician who didn't do those things.

Also before you start you'll need to apply for Building Regulations approval and explain to their satisfaction how you will ensure that the electrical work will comply with Part P. Note, BTW, that the 3rd-party certifier regime described in Approved Document P does not exist.

 

[ericmark]

Do the same with a ring main in a house is very different. We with the calculation allow 20A in centre and 12A spread around circuit for a 32A MCB. But with a 20A MCB it is still 20A in the centre so if you make an error it's not just reduce the MCB size a bit it's reduce it a lot so can't afford to make mistakes.

I don't quite understand what you're saying here. If the CCC of the cable were at least 20A, there would be no point in having a ring protected by a 20A MCB. On the other hand, if the CCC was less than 20A, then the regs would not allow a ring (in which In of MCB was greater than CCC of cable). Can you explain what was your point?

Kind Regards, John

It's not the CCC but the volt drop and loop impedance which would mean you could not use a 32A MCB.

As you know it takes 5 time the rated current to open a type B MCB within the time allowed so the loop impedance must be such that with a direct short that current will flow. It is only simple maths be it a B, C, or D MCB but in the main we don't do the maths as we have a good idea what will pass and fail and just take the final reading.

However on the odd time we will get it wrong but much more likely for a DIY guy to get it wrong as he does not have the experience. So more than us he needs to do his maths.

 

[JohnW2]

I don't quite understand what you're saying here. If the CCC of the cable were at least 20A, there would be no point in having a ring protected by a 20A MCB. On the other hand, if the CCC was less than 20A, then the regs would not allow a ring (in which In of MCB was greater than CCC of cable). Can you explain what was your point?

It's not the CCC but the volt drop and loop impedance which would mean you could not use a 32A MCB.

Fair enough - but it would surely be an exceptionally/unusually long (domestic) 2.5mm² ring final circuit that would not satisfy VD and EFLI requirements, wouldn't it?

Kind Regards, John

 

[converse]

Thanks for everyone's input. I understand the wariness.

All done with me working with the sparky, saving a lot of time and money, all signed off and all extremely well done according to spark - but I am extremely careful and diligent.

Next up house alarms, new thread I think!