Derating lighting cables in insulation.

[nrwilliams]

Hi again,

Opinions please as I may have made a mistake.

Just had downstairs lights rewired while have the loft conversion done.

Anyway lighting circuit run in 1mm and clipped to ceilling joists. Very neat job, but the joists will have to be packed with insulation therefore all the cables will be covered in insulation.

So I suggested to the electrician that the circuit will need to be derated and should maybe be run in 2.5 (if I have read the details correctly).

Can anyone tell me if I am correct?

Everything about this job is becoming a nightmare!

Many thanks.

Neil

 

[holmslaw]

..

 

[chivers67]

Hi again,

So I suggested to the electrician that the circuit will need to be derated and should maybe be run in 2.5 (if I have read the details correctly).

Can anyone tell me if I am correct?

Neil

What equation have you used to come up with 2.5? What lights have you got? At max 1.5 mm should be more than sufficient.

 

[PrenticeBoyofDerry]

As holmslaw has posted.
A circuit that is protected by a 5A or 6A device will be okay on 1.00mm T&E, when routed through thermal insulation.
The maximum current 1.00mm T&E can carry when clipped direct is 17A.
The maximum de-rating of cable for thermal insulation would be 50% of it's maximum current.
Therefore 17A/2 would equal 8.5A, if you have either 5A or 6A device protecting the cable you should be okay.
If the device protecting the cable at 1.0mmT&E is 10A or greater, then you would need to consider re-routing cable, upgrading cable or calculating load and see if a 6A could replace the 10A.
So the questions would be, what size fuse/breaker is protecting your lighting circuit?
and what is the value of the load on that circuit?

 

[Taylortwocities]

That de-rating is if the cable is completely surrounded by insulation.
In this case, the cable is clipped to the joist so there's no issue.
Makes it Method 100 or 101 and ( IIRC ) there's no derating for those two.

 

[EFLImpudence]

That de-rating is if the cable is completely surrounded by insulation.

No, it applies if clipped to joist as well. Pg 265.

Makes it Method 100 or 101 and ( IIRC ) there's no derating for those two.

100 - 13A,
101 - 10.5A

 

[Taylortwocities]

Fine, either way, with a 6amp (or evan a 10A) CPD