Derating cable with insulation?

[civhback]

Hi - hoping someone can help put my mind at rest! I'm about to insulate my suspended timber floor for my lounge from below and wanted to check whether I'd have an issue with the cables. I have limited knowledge on the subject from what I've researched but can't quite figure out whether my existing cables will be an issue or whether I should get them replaced before I insulate.

Essentially the sockets are positioned around the room with the cables going through the floor down into the ceiling of the basement below. Looks to be 2.5mm cables in a ringmain and they've been clipped direct to the wall of the basement. I'll be placing insulation with a depth of 140mm on the underside of my flooring - so wherever a socket is the wiring will run against one side of this for 140mm and in a couple of places the insulation will be above cables that are running through the joists (joists are deeper than insulation so there is room to fit insulation above cables).

From what I gather an Electrician would need to de-rate the circuit to allow for the insulation - would this cause me a problem? and therefore would it be more appropriate to get an electrician in to upgrade the cables before I do the insulation?

Thanks for any help

Dan

 

[JohnW2]

Looks to be 2.5mm cables in a ringmain and they've been clipped direct to the wall of the basement. I'll be placing insulation with a depth of 140mm on the underside of my flooring - so wherever a socket is the wiring will run against one side of this for 140mm and in a couple of places the insulation will be above cables that are running through the joists (joists are deeper than insulation so there is room to fit insulation above cables). .... From what I gather an Electrician would need to de-rate the circuit to allow for the insulation - would this cause me a problem? and therefore would it be more appropriate to get an electrician in to upgrade the cables before I do the insulation?

If I am understanding you correctly then, IF it is a ring circuit and IF it is 2.5mm cable, then the de-rating of cable consequent upon insulation such as you describe should not be a problem which requires any action. However, you probably should get an electrician to confirm those two 'IFs' (and also to confirm that he agrees with my conclusion).

Kind Regards, John

 

[oldbutnotdead]

I hit the same issue a couple of years ago. In one of the tables for current carrying capacity it requires the CCC of any given cable to be halved if running through insulation greater than 100mm thickness (IIRC). In your case, rather like mine, the cable is not running through the insulation for its entire length, just short sections. So short version is you probably have nothing to worry about, especially if this is a living room RFC (so no heavy loads like washing machines, tumble dryers, ovens etc).

 

[JohnW2]

I hit the same issue a couple of years ago. In one of the tables for current carrying capacity it requires the CCC of any given cable to be halved if running through insulation greater than 100mm thickness (IIRC). In your case, rather like mine, the cable is not running through the insulation for its entire length, just short sections.

For a start, what you are talking about is a situation in which a cable is totally surrounded by insulation (i.e. on all sides). My understanding is that the OP's situation will involve insulation only on one side of the cable (which will be clipped directly to wall), in which case the de-rating requirements are much less harsh.

When cable IS completely surrounded by insulation the 50% de-rating to which you refer only applies when the length of cable so surrounded is 500mm or more. As you say, the situation when only 'short sections' are totally surrounded by insulation is not so bad, and the regs specify the following:

Kind Regards, John

 

[civhback]

Thanks both appreciate the help, John - that's how I'd interpreted it - cable is clipped to brick basement walls so it would only touch the insulation on one side for the 140mm depth of the insulation. Therefore I believed it wouldn't be derated as per your table as not surrounded by insulation but instead I should refer to another table with reference methods but I wasn't sure which it would be! (Ref 102# or C) (from what I gather I need at least 20 amps before considering cable size or fuse)

I've got to bring an electrician in at some point for a load of stuff anyway but I was hoping to avoid doing it prior to sorting the floor out as wanted to crack on this weekend, but at the same time I'm a bit of a stickler for making sure it's done right!

Cheers

Daniel

 

[JohnW2]

Thanks both appreciate the help, John - that's how I'd interpreted it - cable is clipped to brick basement walls so it would only touch the insulation on one side for the 140mm depth of the insulation. Therefore I believed it wouldn't be derated as per your table as not surrounded by insulation but instead I should refer to another table with reference methods but I wasn't sure which it would be! (Ref 102# or C) (from what I gather I need at least 20 amps before considering cable size or fuse).

Indeed. As you've undoubtedly discovered, there is no installation method in the regs which is exactly as per your situation - but, I would also have though that Method 102 was about the closest (not Method C, since that would require no insulation at all) - and, as you have presumably discovered, that would give a CCC (assuming no other de-rating factors) of 21A - which is above the minimum for a ring final circuit (which, as you say, is 20A). That's why I originally said that I didn't think there was a problem.

Kind Regards, John

 

[AndyPRK]

in any case, for a living room with small loads, you could not be changing cables.

If you really wanting to do things, you could.

a) clip the cables better to conform with the regs.
b) fit a B20 MCB. Would this comply with the derating factor?

 

[JohnW2]

in any case, for a living room with small loads, you could not be changing cables. If you really wanting to do things, you could.
a) clip the cables better to conform with the regs.

What do you mean by that?

b) fit a B20 MCB. Would this comply with the derating factor?

As I've said, if we assume Installation Method 102 (which is the closest the regs has), then a B32 would be compliant with regs for a 2.5mm² ring final - so no need to change it

Kind Regards, John

 

[civhback]

Indeed. As you've undoubtedly discovered, there is no installation method in the regs which is exactly as per your situation - but, I would also have though that Method 102 was about the closest (not Method C, since that would require no insulation at all) - and, as you have presumably discovered, that would give a CCC (assuming no other de-rating factors) of 21A - which is above the minimum for a ring final circuit (which, as you say, is 20A). That's why I originally said that I didn't think there was a problem.

Kind Regards, John

Thanks John appreciated, been really useful and learnt something new which always helps! Couldn't quite figure out if I was missing something so it's helped clarify.

 

[JohnW2]

Thanks John appreciated, been really useful and learnt something new which always helps! Couldn't quite figure out if I was missing something so it's helped clarify.

You're welcome - but it sounds as if you had more-or-less worked it out for yourself!

Kind Regards, John