Mixing Cable Sizes

[Melv]

My abode is a 1930's-built bog-standard small/medium 3-bed terrace house, which has been rewired sometime in white cable. The upstairs lighting circuit is 1mm with 6amp breaker and I estimate the total circuit length as 30-35m max supplying 5 ordinary domestic ceiling lights, plus two low-watt bathroom fans which I added last year via a spur. Much of the cable is beside fibreglass wool insulation.

I am now decking-out part of my loft and want to add a couple of extra lights there by once again connecting into the upstairs light circuit. While I do the work I intend to protect as much of the entire ring as possible with loose-fit ducting to provide an air-break from the insulation.

I would like to replace the entire existing 1mm circuit with 1.5mm, but part of it is too embedded in the wall to easily do that so it's a non-starter! Therefore can anyone advise me whether I am getting too ambitious by adding to the circuit?? And, if not, would it help if I use 1.5mm cable for the 3 extra lights to lessen voltage drop? And will it be best to break into the existing circuit and extend the loop for the new lights, or merely add another 'spur'?

Advice would be much appreciated.

 

[steptoe]

honest opinion,

if you are going to board out your loft, unless you are VERY confident in the condition of your existing wiring then I'd get the upstairs lights rewired,

I'm personally in this position my self in my own house,

I know the wiring is OK,

but, I'm rewiring them anyway as then I know I shouldnt have any issues for the forseeable future, 30+ years I hope.

oh, and check you have the recommended depth of insulation, currently 270mm iirc

this will mean suspending your loft floor in some manner, proprietary kits are available

 

[Lurch]

Advice would be much appreciated.

My advice - don't touch it.

A lot of what you say doesn't make much sense. What relevance is the colour of the cable? What is the current volt drop on the circuit, what is the VD on the proposed circuit? How did you determine that it would all need replacing, including switch drops? What are you doing about the CPD? Why did you decide that 1mm² was too small, and how have you determined that 1.5mm² is what is required across every last centimetre of the circuit?

 

[Essex1]

My advice - don't touch it.A lot of what you say doesn't make much sense. What relevance is the colour of the cable? What is the current volt drop on the circuit, what is the VD on the proposed circuit? How did you determine that it would all need replacing, including switch drops? What are you doing about the CPD? Why did you decide that 1mm² was too small, and how have you determined that 1.5mm² is what is required across every last centimetre of the circuit?

You heading for a ban. All this 'call in an electrician ' advice. Hehe

 

[Sidewinder]

Lurch is not as close to a ban as you are Essex.

 

[Melv]

Thanks for reply Steptoe.

The existing ring looks fine, and serves only 5 lights and switch drops, but my concern was the fact I had already spurred 2 low-watt bathroom fans, and now want to add 2 lights in the area of loft I am decking-out (about 1/3 of the total).

A full re-wire would be difficult as the existing vertical cable run from the CPD is too embedded in wall, staircase etc. to be replaced without major work, and thanks to 'improvements' by the previous owners I have enough to do already. However if really necessary I could maybe renew from where the vertical cable emerges in the loft, and then renew the vertical cable later when redecorating is due. But would increasing the cable size from 1mm to 1.5mm for only part of the ring actually be adviseable??

 

[apprentice87]

Hi Melv,

The problem you have, is that whatever alterations you make, whilst they might be perfect from a design point of view, and beautifully executed, is that you have not got the equipment required to test the finished install and ensure that it is safe.. It is up to you what you do, but once it is all boarded in you will be stuck with it..  and then maybe spend the next 30 years worrying about it..

You say the lights are wired in a ring.. are you sure??? How do you know??

Could you not maybe run new cables [if this is what your decide to do] in areas where you could get to them without too much trouble in future??

You do know that if you have the "ordinary" type of junction boxes they HAVE to be accessible. [Hopefully you know what i mean!]

Could you do a circuit drawing of what you have there now and what you want to do instead. We might all be able to help you more then..

john..

 

[Melv]

Thanks for your reply, Lurch.

The reason I mentioned 'white' cable is I thought someone might know it's rough age and therefore if it is still ok provided the new load and voltage drop are not excessive?

 

[Lurch]

Thanks for your reply, Lurch.

The reason I mentioned 'white' cable is I thought someone might know it's rough age and therefore if it is still ok provided the new load and voltage drop are not excessive?

And the answers to the rest of the questions?

 

[Doc Hudson]

Thanks for your reply, Lurch.

The reason I mentioned 'white' cable is I thought someone might know it's rough age and therefore if it is still ok provided the new load and voltage drop are not excessive?

The colour of the outer sheath bears no relevance to identifying its condition. The only way to check the integrity of a cable is to undertake some tests of the conductor continuity and the insulation resistance, and to visually check any accessible terminations to look for obvious signs of damage, overheating, perishing etc. If available to check previous test results on the circuit schedule.  Much the same as buying a secondhand car. If you only look at the exterior appearance you wont get much idea about the condition of the essential parts.  Things such as MOT test's AA / RAC vehicle check, test drive, etc will give this better detail. You need to have the existing circuit properly tested to have any reasonable idea of its condition and how much expected life is left in it.  A quick search will show that white cable is still available and has been available for a long time. So colour tells us nothing. http://www.screwfix.com/c/electrical-lighting/cable/cat8960001#category=cat8960001&cableproducttype=twin___earth&conductorarea=1_5_mm_&page_size=100

Doc H.