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
--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

jim k 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

If you've got a spare way in the CU it might be simpler all round to run
another 2.5mm^2 cable in parallel and use two 20A MCBs.

--

Roger Hayter

On Tuesday, 30 January 2018 22:48:09 UTC, jim wrote:
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

Fit a double outlet cooker plate adjacent?

https://www.tlc-direct.co.uk/Products/AA45DCOP.html

Fits on top of an existing flush back box and has 2 x cable clamps for the outgoing cable to the boost switches.

Owain

On Wednesday, 31 January 2018 08:24:18 UTC, jim wrote:
I'm going off the 2.5mm2 feed idea, wouldn't that need fusing down
to be safe?

Probably ok, the 3kW immersion heater load is fixed and won't overload, and short circuit protection should still be adequate by the circuit fuse. (2.5mm is fine as a spur on a 32A MCB ring circuit).

Owain

Wrote in message:
On Tuesday, 30 January 2018 22:48:09 UTC, jim wrote:
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

Fit a double outlet cooker plate adjacent?

https://www.tlc-direct.co.uk/Products/AA45DCOP.html

Fits on top of an existing flush back box and has 2 x cable clamps for the outgoing cable to the boost switches.

Owain



Yes I think some sort of a junction box is looking like the
solution so far.

I don't think I've got any 4mm2 in old or new colours :-( & I'd
only need a foot or so..

I'm going off the 2.5mm2 feed idea, wouldn't that need fusing down
to be safe?
--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

On 31/01/2018 08:24, jim wrote:
Wrote in message:
On Tuesday, 30 January 2018 22:48:09 UTC, jim wrote:
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

Fit a double outlet cooker plate adjacent?

https://www.tlc-direct.co.uk/Products/AA45DCOP.html

Fits on top of an existing flush back box and has 2 x cable clamps for the outgoing cable to the boost switches.

Owain



Yes I think some sort of a junction box is looking like the
solution so far.

I don't think I've got any 4mm2 in old or new colours :-( & I'd
only need a foot or so..

I'm going off the 2.5mm2 feed idea, wouldn't that need fusing down
to be safe?

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

Some two immersion setups are intended to run one heat only at a time.
(i.e. the top one to give boost of quick hot water, and the lower one
for a full reheat). Obviously that would be fine on your cable. Both at
once is a bit more marginal, although in reality is unlikely to cause
massive problems.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm Wrote in message:
On 31/01/2018 08:24, jim wrote:
Wrote in message:
On Tuesday, 30 January 2018 22:48:09 UTC, jim wrote:
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

Fit a double outlet cooker plate adjacent?

https://www.tlc-direct.co.uk/Products/AA45DCOP.html

Fits on top of an existing flush back box and has 2 x cable clamps for the outgoing cable to the boost switches.

Owain



Yes I think some sort of a junction box is looking like the
solution so far.

I don't think I've got any 4mm2 in old or new colours :-( & I'd
only need a foot or so..

I'm going off the 2.5mm2 feed idea, wouldn't that need fusing down
to be safe?

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

Some two immersion setups are intended to run one heat only at a time.
(i.e. the top one to give boost of quick hot water, and the lower one
for a full reheat). Obviously that would be fine on your cable. Both at
once is a bit more marginal, although in reality is unlikely to cause
massive problems.


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.

Both can therefore be switched independently & isolated next to
the heat bank. When in use (fairly rare) they are generally on
together.


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?

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?

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....

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?

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?

Or something completely different!

Phew!

Thanks again

--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

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.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Tuesday, 30 January 2018 22:48:09 UTC, 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....

Or you could use a Sink/Bath switch

https://www.ebay.co.uk/itm/121269923759

(more expensive ones are available if you want a recognised brand!)

This prevents both heaters being used at once,

Owain

Wrote in message:
On Tuesday, 30 January 2018 22:48:09 UTC, 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....

Or you could use a Sink/Bath switch

https://www.ebay.co.uk/itm/121269923759

(more expensive ones are available if you want a recognised brand!)

This prevents both heaters being used at once,

Ahthat wouldn't do the business in this application - heat bank.

Cheers for your help though :-)
--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

John Rumm Wrote in message:
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)




Thankyou very much John.

Cheers
--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

On 31/01/2018 00:55, Roger Hayter wrote:
jim k 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

If you've got a spare way in the CU it might be simpler all round to run
another 2.5mm^2 cable in parallel and use two 20A MCBs.


A 16A MCB is more often used than a 20A MCB for 3kW immersion heaters,
however that makes little or no difference in most cases.

The one that gets me are garage CU's that come with a RCD main switch
and 16A MCB and a 6A MCB. Why not a 20A MCB and a 6A MCB? You would
normally run a 20A radial for the sockets from such a CU.

--
Adam

On Wednesday, 31 January 2018 18:49:50 UTC, ARW wrote:
The one that gets me are garage CU's that come with a RCD main switch
and 16A MCB and a 6A MCB. Why not a 20A MCB and a 6A MCB?

European influence where they use 16A sockets?

You would
normally run a 20A radial for the sockets from such a CU.

And preferably not have the lights on the same RCD in an area where where machiney may be used

Owain

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.

jim k Wrote in message:
John Rumm Wrote in message:
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)




Thankyou very much John.

Cheers

All sorted & working :-)

Thanks again.

--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

On 31/01/2018 19:30, wrote:
On Wednesday, 31 January 2018 18:49:50 UTC, ARW wrote:
The one that gets me are garage CU's that come with a RCD main switch
and 16A MCB and a 6A MCB. Why not a 20A MCB and a 6A MCB?

European influence where they use 16A sockets?

You would
normally run a 20A radial for the sockets from such a CU.

And preferably not have the lights on the same RCD in an area where where machiney may be used


TBH two RCBOs are cheap enough to do it that way if needed.


--


Adam

On Thursday, 1 February 2018 19:06:18 UTC, ARW wrote:
TBH two RCBOs are cheap enough to do it that way if needed.

But they never come with 2 RCBOs

Owain