On 30/01/2018 22:48, jim wrote:

I have two immersions, & what I didn't find out til I went to fit
the boost timers is that one of the exstg two immersion
switches, A, also feeds the spur? to switch B of the other
immersion....

Net result is so far I can't fit both "feed" cables into the
terminals on the rear of the boost timer that I want to
replace
switch A..

Any workarounds please?

I'll check the ratings of the immersion heaters tomorrow but I
believe they are 3kw. The cables are thick multi stranded & I
can't fit two in the terminals of the new boost switch at A.


I suppose the radial cable from the CU (supplying both immersions)
should be ?4mm? It's protected by a 32amp rcbo...

But the feed from switch A to switch B needn't be in 4mm too if
immersion B is 3kw? I'm thinking that could be 2.5mm2? which
might be more doable...

TIA


A post on an IET forum asked a similar question:

https://www.theiet.org/forums/forum/...threadid=63568

1)Is the cable short enough, and the supply impedance low enough, to
trip the breaker reliably in a short circuit very near the load end of
the cable? (check of PSSC and adiabatic equation - both LE and LN faults
to be calculated)

2) Is the heater of a design where an overload is likely ?
(i.e. are just a fixed load or a short circuit fault the possibilities
that need consideration - for example a heater bar in a silica tube, or
any design with an internal fuse.)

3) Is the 2.5mm cable routed through an environment involving so much
cable grouping and insulation that means its rating is less than 8 amps..
(i.e. is the cable unsuitable for the normal load current.)

If the answer to
1) is yes,
2) No
3) No
then there is no defect in terms of BS7671, though I agree you may not
like it.
If any one of the answers is the other way round, then could be a 2 .

For a 4mm cable, it is most unlikely to fail unless the routing question
indicates that it is overloaded, and even that would require routing
though a bundle of hundreds of fully loaded cables.

Personally I might use a fused outlet or similar with a 13A fuse with
2.5mm^2 cable to avoid any concerns.