On 21/11/10 14:17, Stephen wrote:
On Sun, 14 Nov 2010 06:10:00 -0800 (PST),
wrote:

Some sockets require 35mm backboxes.
Indeed, if your wires enter from the rear then it is a good idea to
actually use 42mm backboxes due to bend-radius, risk of wire crushing
and space.

Another reply said not to shorten the cables so I suppose there must
be an art to getting the right length: not too short that it is
difficult to insert and screw them up but not so long that they get
crushed and squashed behind the socket. What's the problem the
overheating? Is there a rule of thumb for the correct length?

I follow a rough rule of thumb that says the cable should be about a
diagonal of the box in length.

I then form the conductors into some rough approximation of a semicircle
between the cable entry and the location of their terminal.

This gives me enough slack to work on the plate, but as it is replaced,
the conductors fold back aka the coils in a spring which avoids
unnecessary pressure and wire strain.

I agree with the sentiments re rear entry. I have 35mm boxes, plus
5-10mm of plaster and two have rear entry and they were getting towards
being a little tight with normal plates (GET Ultimate plastic). If the
box was hard level with the surface of the plaster and I'd tried to use
metal plates, it probably would have been tighter than I would like.

The main problem with crushing is damage to the insulation and the
possibility of fracturing the conductor.

There is a secondary problem with T+E depending on how it is bent to do
with the conductors cutting through the insulation over time. This would
be more of a problem with a bend "on the edge" (ie the hard way with
T+E) where the outer conductor may be under tension. Not seen that
myself, but I've seen it mentioned somewhere).
--
Tim Watts