On Sun, 24 Aug 2003 21:21:38 +0100, "Laurie"
wrote:

My builder states he will be using 8 inches of "rockwool" type insulation -
four inches running with the beams four running over.

I had thought it was good practice to

a: Cleat the cable to the ceiling joists for support, and
b. Ensure the cable runs over the insulation to prevent overheating of the
cable (related to fault current calculations).

In my situation above the two (a and b) seem to be contradictory as four
inches of insulation will be over all the joists. What do you guys do in
this situation???????

Many thanks for all your help

Laurie



You can actually achieve both scenarios quite easily.

It is reasonable to clip the cables to the ceiling joists, the
important thing is to size the cable adequately for the load.

For the most part, cabling in a roof space will be for a lighting
circuit; however watch out in case power ring circuit, immersion
heater or electric shower circuits run through the space.

A good book for working this all out is the Electrician's Guide to the
16th Edition of the Wiring Regulations by Whitfield.

It's worth buying, although conveniently almost all of it is
reproduced on TLC's web site in the technical section.

www.tlc-direct.co.uk

If you refer to Chapter 4, all is revealed. There are tables of
current carring capacity for various scenarios, including insulation
around the cable.

The worked example 4.1 covers the deratings for insulation and high
ambient temperature in a loft for an immersion heater cable. You can
easily adapt it for other types of circuit.

I suspect that most electricians would just ignore all of this and
just go ahead and run 1.0 or 1.5 sqmm cable for lighting anyway, but
it is a good idea to check.




..andy

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