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wrote:
In uk.d-i-y, Ross wrote:

................................................ so I was wondering
how I can safely connect some new cable to the old cable in the loft
above the bathroom bearing in mind that it is going to be pulling
approx 40 amps and if connected incorrectly would get pretty hot.

"Normal" junction boxes are available in ratings up to only 30A, so
(as you've already realised) you don't want to be using one of they.
Rather than using the DP switch, a "widely available" way to get a
40/45A junction box would be to use a cooker final connection unit -
not the gert big thing with a DP switch and optional 13A socket, but
the accessory plate which goes on the wall, mounts on a deep galvanised
box or (in your application) deep surface-mount box, and expects 6mm or
10mm sq cable both in (in original application, fixed wiring embedded
in plaster) and out (in original application, final cable to the cooker
which once a year after the marmalade-making season is over gets pulled
out from the wall ;-). Mounted in the loft counts as an "accessible"
location for fault-finding/repair purposes, assuming you don't board
over it later!

A 'Henley box' as used for making a junction to two CUs would also be
suitable.

You should also be able to get junction boxes for 10sq mm cable, try an
electrical wholesaler rather than a DIY place.

--
Chris Green