On 31/01/2018 11:20, jim wrote:
John Rumm Wrote in message:
You have 6kW of load or 26A. 2.5mm^2 T&E is good for 27A in its "clipped
direct" or buried in masonry installation method. So much depends on the
detail of how its installed. (and also how warm it is in the cupboard
with the cylinder, and how much cable running at a potentially higher
than normal temperature)
You might find the worked example in this page worth looking at:
http://wiki.diyfaq.org.uk/index.php/...istics_of_load
Thanks John that's been educational :-)
I'm sure more detail is required....
From the CU the (what I think is a 4mm2 stranded) cable1 is routed
loose laid through a ceiling void for say 4m. Then through an
uninsulated masonry wall into the boiler room area. Then it is
clipped to a bare masonry wall for another 3m.
Then it goes straight through that wall into the backbox of switch A.
Switches A & B are in drywall back boxes fitted into 25mm celotex
backed plasterboard.
From switch A there's a 4mm2 "link cable2" feeding switch B
(length approx 8inches total).
From both A & B the "load cables3&4" (more 4mm2) to the two 3kw
immersion heaters go straight back through this same wall, and
then are clipped (along the same masonry wall & next to cable1
from the CU) for 5m to another pair of switched fused connection
units next to the heat bank. From these switches the flexes run
straight to the immersion heaters.
ok that all sounds fine.
Both can therefore be switched independently & isolated next to
the heat bank. When in use (fairly rare) they are generally on
together.
ok
So trying to apply the theory you linked to
I start with the Ib of 26A, & the 4mm cable at 37A.
Ambient temperature for any of the above won't exceed 30deg C, I
think the number of times it exceeds 25deg C will be small... So
let's say no derating for that?
Agreed
Grouping - Mmm well the only parts I can see are the surface
clipped parts & the general area around the CU as the cable
leaves. The ceiling void is large approx 15" deep & 16m2 open
void so no need for cables to be "bunched" or pass through joists
at all.
What derating should I factor in here?
I would suggest none (i.e. a Cg of 1) by the sounds of it...
Insulation derating - Mmm well the only obvious bit is the link
cable2 that runs under the 25mm2 kingspan against the masonry for
6inches between A & B.... and presumably will only ever handle
the 3kw for immersion B....
You could count that as installation method 102 (in reality its actually
less severe than that), so it drops its nominal capacity to 27A - so
still plenty.
Erm....help :-)
In general this was all done by a pro a few years ago & I have the
paperwork etc.
What I suppose I really want to know is - can I change the "link
cable2" between switch A & B to some smaller cable (2.5mm2?) so
that I can fit the 4mm cable1 from the CU & the link cable2 into
the terminals on the rear of a new boost timer switch at switch
position A?
Yup, 2.5mm^2 installed as per method 102 is still 21A which is fine for
a single heater.
Or am I better following Owain's idea to somehow get a junction
box in there to reduce the number if cables I'm trying to get in
the booster switch terminals?
That would also be ok.
Or something completely different!
Using a JB to have the 4mm^2 feed supply a pair of 2.5mm^2 cables that
in turn feed A & B would also be ok. The MCB would still adequately
fault protect the smaller cable. (overload protection not required at
origin in this case)
--
Cheers,
John.
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