A mate of mine has just bought an early 70's house. This has an 8.5 kW
shower heater which has just failed completely, he was wondering whether
he might be able to upgrade to 9.5 with the existing wiring.

I have three questions.

He still has the original CU. Later (but maybe not much later) a 6 mm^2
circuit has been added for a shower heater (probably a predecessor of
the present one). The layout of the present board is as follows.

Incomer - Supply Company Fuse - New Smart Meter - Old Wylex current
operated ELCB (100 ma trip) - Henley Block.

From this the supply splits to the original CU which has plug-in MCBs,
and what I can only describe as a one-way CU for the shower, this has a
switch (havn't checked if it is one or two pole) with an integral 30A
plug-in MCB. This suggests to me that the original shower had a lower
rating than 8.5kw, I'd have expected to see a 40A trip to protect an
8.5kW heater circuit but maybe you can get away with 30A for short showers.

From there is a simple short run of 6 mm^2 t&e. It's surface mounted on
the "board", disappears into the ceiling, and runs up about two metres
of wall to the shower unit. It runs behind ceramic tiles on an outside
wall which (I guess from the age) might be normal rather than
lightweight concrete blocks for the inner skin. The outer skin is a
single layer of brickwork. Exterior walls are plastered inside, not
plasterboard.

So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Clipped direct is OK to 47A. What does the team think about the wire run
behind the tiles? I'm inclined to think that while it *might* run
slightly hotter than clipped direct, the insulation is not particularly
good and it can't set fire to anything sandwiched between blockwork and
tiles (presumably chased into the original plaster). Do you think he
could go up to 9.5kW?

Second question, the overall RCD protection with 100mA trip is a bit out
of date, should he get this changed to 30mA? I know, in an ideal world
he'd swap the lot for a metal split CU compliant with 17th edition, but
he's also renovating his original flat prior to sale, so he's not
rolling in cash. The rest of the electrics doesn't look too bad and
doesn't appear to have been hacked about significantly.

Third question, the overcurrent protection for the shower circuit is
this unit with a combined MCB and switch. I guess he needs to go up to
40A especially if he changes to 9.5kW. But does he need a switch here?
After all, he can isolate the shower circuit by tripping the RCD device
(at the price of turning off the rest of the house). He has, of course,
the normal double pole ceiling mounted pull switch outside the shower
cubicle. If he does need a switch, can anyone suggest a reasonably
compact switch and overcurrent protection unit? I guess it doesn't have
to have an MCB, could he use an industrial type 40A cartridge fuse?

On Saturday, 14 July 2018 13:55:08 UTC+1, newshound wrote:

A mate of mine has just bought an early 70's house. This has an 8.5 kW
shower heater which has just failed completely, he was wondering whether
he might be able to upgrade to 9.5 with the existing wiring.

I have three questions.

He still has the original CU. Later (but maybe not much later) a 6 mm^2
circuit has been added for a shower heater (probably a predecessor of
the present one). The layout of the present board is as follows.

Incomer - Supply Company Fuse - New Smart Meter - Old Wylex current
operated ELCB (100 ma trip) - Henley Block.

From this the supply splits to the original CU which has plug-in MCBs,
and what I can only describe as a one-way CU for the shower, this has a
switch (havn't checked if it is one or two pole) with an integral 30A
plug-in MCB. This suggests to me that the original shower had a lower
rating than 8.5kw, I'd have expected to see a 40A trip to protect an
8.5kW heater circuit but maybe you can get away with 30A for short showers.

From there is a simple short run of 6 mm^2 t&e. It's surface mounted on
the "board", disappears into the ceiling, and runs up about two metres
of wall to the shower unit. It runs behind ceramic tiles on an outside
wall which (I guess from the age) might be normal rather than
lightweight concrete blocks for the inner skin. The outer skin is a
single layer of brickwork. Exterior walls are plastered inside, not
plasterboard.

So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Clipped direct is OK to 47A. What does the team think about the wire run
behind the tiles? I'm inclined to think that while it *might* run
slightly hotter than clipped direct, the insulation is not particularly
good and it can't set fire to anything sandwiched between blockwork and
tiles (presumably chased into the original plaster). Do you think he
could go up to 9.5kW?

Second question, the overall RCD protection with 100mA trip is a bit out
of date, should he get this changed to 30mA? I know, in an ideal world
he'd swap the lot for a metal split CU compliant with 17th edition, but
he's also renovating his original flat prior to sale, so he's not
rolling in cash. The rest of the electrics doesn't look too bad and
doesn't appear to have been hacked about significantly.

Third question, the overcurrent protection for the shower circuit is
this unit with a combined MCB and switch. I guess he needs to go up to
40A especially if he changes to 9.5kW. But does he need a switch here?
After all, he can isolate the shower circuit by tripping the RCD device
(at the price of turning off the rest of the house). He has, of course,
the normal double pole ceiling mounted pull switch outside the shower
cubicle. If he does need a switch, can anyone suggest a reasonably
compact switch and overcurrent protection unit? I guess it doesn't have
to have an MCB, could he use an industrial type 40A cartridge fuse?

If money is really tight, I'd replace the small shower CU with a new one that includes a modern 30mA RCD. There are lots on ebay, but it must be all metal cased amendment 3 compliant. Putting the whole house on a single 30mA RCD is not a great plan & not compliant.

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.


NT

On 14/07/2018 13:55, newshound wrote:
A mate of mine has just bought an early 70's house. This has an 8.5 kW
shower heater which has just failed completely, he was wondering whether
he might be able to upgrade to 9.5 with the existing wiring.

Possibly...

Probably worth a read of:

http://wiki.diyfaq.org.uk/index.php/...lectric_shower

I have three questions.

He still has the original CU. Later (but maybe not much later) a 6 mm^2
circuit has been added for a shower heater (probably a predecessor of
the present one). The layout of the present board is as follows.

Incomer - Supply Company Fuse - New Smart Meter - Old Wylex current
operated ELCB (100 ma trip) - Henley Block.

From this the supply splits to the original CU which has plug-in MCBs,
and what I can only describe as a one-way CU for the shower, this has a
switch (havn't checked if it is one or two pole) with an integral 30A
plug-in MCB. This suggests to me that the original shower had a lower
rating than 8.5kw, I'd have expected to see a 40A trip to protect an
8.5kW heater circuit but maybe you can get away with 30A for short showers.

Yup, if you look at the time vs current trip curve for a 30/32A MCB, the
likely load is going to be below the trip threshold near enough
indefinitely.

From there is a simple short run of 6 mm^2 t&e. It's surface mounted on
the "board", disappears into the ceiling, and runs up about two metres
of wall to the shower unit. It runs behind ceramic tiles on an outside
wall which (I guess from the age) might be normal rather than
lightweight concrete blocks for the inner skin. The outer skin is a
single layer of brickwork. Exterior walls are plastered inside, not
plasterboard.

So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Yup, from a design point of view that is correct - although with showers
that 8.5kW may well be quoted at 240V...

Clipped direct is OK to 47A. What does the team think about the wire run
behind the tiles? I'm inclined to think that while it *might* run
slightly hotter than clipped direct, the insulation is not particularly
good and it can't set fire to anything sandwiched between blockwork and
tiles (presumably chased into the original plaster).

"In masonry" is generally treated as if clipped direct.

Do you think he
could go up to 9.5kW?

He would be pushing it from a cable life point of view - its likely to
run hot. That in turn can cause more problems with overheating at the
terminations.

10mm^2 would be a better choice.

Second question, the overall RCD protection with 100mA trip is a bit out
of date, should he get this changed to 30mA?

The 100mA trip will offer no protection from electrocution.

However if the existing installation meets proper design criteria, and
the equipotential bonding is adequate, then it could still be considered
"safe".

(without more details of the installation it would be hard to say for
sure). The fact that the shower has probably already been uprated does
case some doubt however as that will invalidate any previous
calculations and testing.

Any new installation though really aught to have a RCD (even if that
just means swapping the MCB for RCBO)

I know, in an ideal world
he'd swap the lot for a metal split CU compliant with 17th edition, but
he's also renovating his original flat prior to sale, so he's not
rolling in cash. The rest of the electrics doesn't look too bad and
doesn't appear to have been hacked about significantly.

Third question, the overcurrent protection for the shower circuit is
this unit with a combined MCB and switch. I guess he needs to go up to
40A especially if he changes to 9.5kW. But does he need a switch here?
After all, he can isolate the shower circuit by tripping the RCD device
(at the price of turning off the rest of the house). He has, of course,
the normal double pole ceiling mounted pull switch outside the shower
cubicle. If he does need a switch, can anyone suggest a reasonably
compact switch and overcurrent protection unit? I guess it doesn't have
to have an MCB, could he use an industrial type 40A cartridge fuse?

There is plenty of choice of small one and two way CUs to choose from.
You will probably even find some described as a "shower CU".



--
Cheers,

John.

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

On 14/07/2018 14:50, wrote:

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?


--
Adam

On Saturday, 14 July 2018 15:03:05 UTC+1, ARW wrote:
On 14/07/2018 14:50, tabbypurr wrote:

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?

your logic escapes me


NT

On 14/07/2018 15:03, ARW wrote:
On 14/07/2018 14:50, wrote:

Fuses are still permitted, but the cable must then be derated. I can
see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?



Isn't everything he posts to **** someone off?
Like woders and TNP.
..

newshound wrote:

in an ideal world
he'd swap the lot for a metal split CU compliant with 17th edition
I know the 18th ed is available now, does it have a grace period before
it comes into effect?

The BSI website slipped-up for a few days last week while they updated
the editions and allowed library subscriptions online access to the 17th ed

On 14/07/2018 14:50, wrote:


If money is really tight, I'd replace the small shower CU with a new one that includes a modern 30mA RCD. There are lots on ebay, but it must be all metal cased amendment 3 compliant. Putting the whole house on a single 30mA RCD is not a great plan & not compliant.

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.


NT


Thanks, hadn't thought of eBay (had a quick look at TLC and SF sites
without spotting anything obvious). Most seem to be 32A or 50A, neither
ideal, but there's at least one 40A.

Space is a bit tight, but some of these look as though they will be OK.

He'll be getting a Sparks in, but I wanted to get the requirements clear
before people start confusing him.

Didn't realise it would need to be metal cased.

On 14/07/2018 15:11, wrote:
On Saturday, 14 July 2018 15:03:05 UTC+1, ARW wrote:
On 14/07/2018 14:50, tabbypurr wrote:

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?

your logic escapes me



Why would you derate a cable for fixed known load just because you are
using a fuse?


--
Adam

On 14/07/2018 16:19, dennis@home wrote:
On 14/07/2018 15:03, ARW wrote:
On 14/07/2018 14:50, wrote:

Fuses are still permitted, but the cable must then be derated. I can
see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?



Isn't everything he posts to **** someone off?
Like woders and TNP.
.


Have you had a sip out of the **** goblet again?

--
Adam

On 14/07/2018 14:53, John Rumm wrote:

Probably worth a read of:

http://wiki.diyfaq.org.uk/index.php/...lectric_shower


Done!



Â*From this the supply splits to the original CU which has plug-in
MCBs, and what I can only describe as a one-way CU for the shower,
this has a switch (havn't checked if it is one or two pole) with an
integral 30A plug-in MCB. This suggests to me that the original shower
had a lower rating than 8.5kw, I'd have expected to see a 40A trip to
protect an 8.5kW heater circuit but maybe you can get away with 30A
for short showers.

Yup, if you look at the time vs current trip curve for a 30/32A MCB, the
likely load is going to be below the trip threshold near enough
indefinitely.

Hadn't had time to hunt up the curves, sorry!




So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Yup, from a design point of view that is correct - although with showers
that 8.5kW may well be quoted at 240V...

Didn't realise that


Clipped direct is OK to 47A. What does the team think about the wire
run behind the tiles? I'm inclined to think that while it *might* run
slightly hotter than clipped direct, the insulation is not
particularly good and it can't set fire to anything sandwiched between
blockwork and tiles (presumably chased into the original plaster).

"In masonry" is generally treated as if clipped direct.

That's really useful to know. Makes sense to me (but from finding quotes
from some of the Sparks' forums it looks as though some people take 34A
for 6mm^2 (except clipped) which seems to be Method 4).


Do you think he could go up to 9.5kW?

He would be pushing it from a cable life point of view - its likely to
run hot. That in turn can cause more problems with overheating at the
terminations.

Good point. Although there is probably a foot of cable "in air" at the
bottom, and several inches at the top.


10mm^2 would be a better choice.

Changing the cable is not a straightforward option, that would mean
replacing tiles, although I did wonder whether a sparks might suggest a
new run of 10mm^2 in trunking outside the cabinet, and then over the top
of the door into the unit. This would not look *too* untidy if done nicely.


Second question, the overall RCD protection with 100mA trip is a bit
out of date, should he get this changed to 30mA?

The 100mA trip will offer no protection from electrocution.

I recognise that

However if the existing installation meets proper design criteria, and
the equipotential bonding is adequate, then it could still be considered
"safe".

I havn't tried to check this, only had a chance for a *very* quick look
and I was concentrating on confirming the supply cable size.

(without more details of the installation it would be hard to say for
sure). The fact that the shower has probably already been uprated does
case some doubt however as that will invalidate any previous
calculations and testing.

Any new installation though really aught to have a RCD (even if that
just means swapping the MCB for RCBO)

Trouble is the present "shower MCB" is plug in, not DIN rail, so
(presumably) direct replacements not available.


There is plenty of choice of small one and two way CUs to choose from.
You will probably even find some described as a "shower CU".


Found some now thanks to NT.

We'll see if we can find an electrician who is prepared to do a
relatively small job.

On 14/07/2018 17:14, ARW wrote:
On 14/07/2018 15:11, wrote:
On Saturday, 14 July 2018 15:03:05 UTC+1, ARWÂ* wrote:
On 14/07/2018 14:50, tabbypurr wrote:

Fuses are still permitted, but the cable must then be derated. I can
see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?

your logic escapes me



Why would you derate a cable for fixed known load just because you are
using a fuse?


Correction factor 0.725 for semi enclosed fuses to BS 3036 (according to
my old On Site Guide).

On 14/07/2018 17:38, newshound wrote:
On 14/07/2018 17:14, ARW wrote:
On 14/07/2018 15:11, wrote:
On Saturday, 14 July 2018 15:03:05 UTC+1, ARWÂ* wrote:
On 14/07/2018 14:50, tabbypurr wrote:

Fuses are still permitted, but the cable must then be derated. I
can see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?

your logic escapes me



Why would you derate a cable for fixed known load just because you are
using a fuse?


Correction factor 0.725 for semi enclosed fuses to BS 3036 (according to
my old On Site Guide).


Correct but irrelevant in this case, you have a known fixed load.



--
Adam

On 14/07/2018 17:22, newshound wrote:
On 14/07/2018 14:53, John Rumm wrote:

Probably worth a read of:

http://wiki.diyfaq.org.uk/index.php/...lectric_shower


Done!



Â*From this the supply splits to the original CU which has plug-in
MCBs, and what I can only describe as a one-way CU for the shower,
this has a switch (havn't checked if it is one or two pole) with an
integral 30A plug-in MCB. This suggests to me that the original
shower had a lower rating than 8.5kw, I'd have expected to see a 40A
trip to protect an 8.5kW heater circuit but maybe you can get away
with 30A for short showers.

Yup, if you look at the time vs current trip curve for a 30/32A MCB,
the likely load is going to be below the trip threshold near enough
indefinitely.

Hadn't had time to hunt up the curves, sorry!

sorry =, I intended to post the link:

http://wiki.diyfaq.org.uk/index.php/...e-MCBTypeB.png

So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Yup, from a design point of view that is correct - although with
showers that 8.5kW may well be quoted at 240V...

Didn't realise that


Clipped direct is OK to 47A. What does the team think about the wire
run behind the tiles? I'm inclined to think that while it *might* run
slightly hotter than clipped direct, the insulation is not
particularly good and it can't set fire to anything sandwiched
between blockwork and tiles (presumably chased into the original
plaster).

"In masonry" is generally treated as if clipped direct.

That's really useful to know. Makes sense to me (but from finding quotes
from some of the Sparks' forums it looks as though some people take 34A
for 6mm^2 (except clipped) which seems to be Method 4).

Method 4 would be instillation type B (38A). 34A sounds like method 100.

http://wiki.diyfaq.org.uk/index.php/...lation_Methods


Do you think he could go up to 9.5kW?

He would be pushing it from a cable life point of view - its likely to
run hot. That in turn can cause more problems with overheating at the
terminations.

Good point. Although there is probably a foot of cable "in air" at the
bottom, and several inches at the top.


10mm^2 would be a better choice.

Changing the cable is not a straightforward option, that would mean
replacing tiles, although I did wonder whether a sparks might suggest a
new run of 10mm^2 in trunking outside the cabinet, and then over the top
of the door into the unit. This would not look *too* untidy if done nicely.

Yup, no reason to follow the existing path in every case. Any chance the
old cable could pull through a new one?

Second question, the overall RCD protection with 100mA trip is a bit
out of date, should he get this changed to 30mA?

The 100mA trip will offer no protection from electrocution.

I recognise that

However if the existing installation meets proper design criteria, and
the equipotential bonding is adequate, then it could still be
considered "safe".

I havn't tried to check this, only had a chance for a *very* quick look
and I was concentrating on confirming the supply cable size.

(without more details of the installation it would be hard to say for
sure). The fact that the shower has probably already been uprated does
case some doubt however as that will invalidate any previous
calculations and testing.

Any new installation though really aught to have a RCD (even if that
just means swapping the MCB for RCBO)

Trouble is the present "shower MCB" is plug in, not DIN rail, so
(presumably) direct replacements not available.

That will need a new enclosure then... but for a one or two way one,
they are cheap.


There is plenty of choice of small one and two way CUs to choose
from. You will probably even find some described as a "shower CU".


Found some now thanks to NT.

We'll see if we can find an electrician who is prepared to do a
relatively small job.



--
Cheers,

John.

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

On 14/07/2018 16:52, Andy Burns wrote:
newshound wrote:

in an ideal world he'd swap the lot for a metal split CU compliant
with 17th edition
I know the 18th ed is available now, does it have a grace period before
it comes into effect?

The BSI website slipped-up for a few days last week while they updated
the editions and allowed library subscriptions online access to the 17th ed

Shame I missed that... an electronic searchable version would have been
handy.

--
Cheers,

John.

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

On Saturday, 14 July 2018 17:14:42 UTC+1, ARW wrote:
On 14/07/2018 15:11, tabbypurr wrote:
On Saturday, 14 July 2018 15:03:05 UTC+1, ARW wrote:
On 14/07/2018 14:50, tabbypurr wrote:

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?

your logic escapes me



Why would you derate a cable for fixed known load just because you are
using a fuse?

Fuses allow more overload than MCBs, they're not type B. Hence the regs permit less cable current when protected by fuses.


NT

On Saturday, 14 July 2018 17:22:32 UTC+1, newshound wrote:
On 14/07/2018 14:53, John Rumm wrote:

Probably worth a read of:

http://wiki.diyfaq.org.uk/index.php/...lectric_shower


Done!



Â*From this the supply splits to the original CU which has plug-in
MCBs, and what I can only describe as a one-way CU for the shower,
this has a switch (havn't checked if it is one or two pole) with an
integral 30A plug-in MCB. This suggests to me that the original shower
had a lower rating than 8.5kw, I'd have expected to see a 40A trip to
protect an 8.5kW heater circuit but maybe you can get away with 30A
for short showers.

Yup, if you look at the time vs current trip curve for a 30/32A MCB, the
likely load is going to be below the trip threshold near enough
indefinitely.

Hadn't had time to hunt up the curves, sorry!




So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Yup, from a design point of view that is correct - although with showers
that 8.5kW may well be quoted at 240V...

Didn't realise that


Clipped direct is OK to 47A. What does the team think about the wire
run behind the tiles? I'm inclined to think that while it *might* run
slightly hotter than clipped direct, the insulation is not
particularly good and it can't set fire to anything sandwiched between
blockwork and tiles (presumably chased into the original plaster).

"In masonry" is generally treated as if clipped direct.

That's really useful to know. Makes sense to me (but from finding quotes
from some of the Sparks' forums it looks as though some people take 34A
for 6mm^2 (except clipped) which seems to be Method 4).


Do you think he could go up to 9.5kW?

He would be pushing it from a cable life point of view - its likely to
run hot. That in turn can cause more problems with overheating at the
terminations.

Good point. Although there is probably a foot of cable "in air" at the
bottom, and several inches at the top.


10mm^2 would be a better choice.

Changing the cable is not a straightforward option, that would mean
replacing tiles, although I did wonder whether a sparks might suggest a
new run of 10mm^2 in trunking outside the cabinet, and then over the top
of the door into the unit. This would not look *too* untidy if done nicely.


Second question, the overall RCD protection with 100mA trip is a bit
out of date, should he get this changed to 30mA?

The 100mA trip will offer no protection from electrocution.

I recognise that

However if the existing installation meets proper design criteria, and
the equipotential bonding is adequate, then it could still be considered
"safe".

I havn't tried to check this, only had a chance for a *very* quick look
and I was concentrating on confirming the supply cable size.

(without more details of the installation it would be hard to say for
sure). The fact that the shower has probably already been uprated does
case some doubt however as that will invalidate any previous
calculations and testing.

Any new installation though really aught to have a RCD (even if that
just means swapping the MCB for RCBO)

Trouble is the present "shower MCB" is plug in, not DIN rail, so
(presumably) direct replacements not available.


There is plenty of choice of small one and two way CUs to choose from.
You will probably even find some described as a "shower CU".


Found some now thanks to NT.

We'll see if we can find an electrician who is prepared to do a
relatively small job.

Wylex plug-in MCBs, like anything else, can be found on Ebay. Those old CUs can get hot on 30A loads but they claim to handle them. But you need a new mini-CU with RCD anyway.

FWIW 100mA RCDs do give partial shock protection, but only partial.


NT

On Saturday, 14 July 2018 17:50:06 UTC+1, ARW wrote:
On 14/07/2018 17:38, newshound wrote:
On 14/07/2018 17:14, ARW wrote:
On 14/07/2018 15:11, tabbypurr wrote:
On Saturday, 14 July 2018 15:03:05 UTC+1, ARWÂ* wrote:
On 14/07/2018 14:50, tabbypurr wrote:

Fuses are still permitted, but the cable must then be derated. I
can see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?

your logic escapes me



Why would you derate a cable for fixed known load just because you are
using a fuse?

Correction factor 0.725 for semi enclosed fuses to BS 3036 (according to
my old On Site Guide).

Correct but irrelevant in this case, you have a known fixed load.

It means the cable requires a lower fuse - and the shower is then an overcurrent on a 30A fuse, though it will run on it, but it'd run hot (the fuse).


NT

wrote in message
...
Fuses are still permitted, but the cable must then be derated. I
can see no upside though, MCBs are only a few pounds.

Can you buy MCBs which are interchangeable with the sockets for wire fuses,
so you can remove the socket and fuse and replace it with an MCB, without
needing to replace the whole fuse box?

The only MCBs I've seen are in houses which had a "fuse box" which was
designed for MCBs, maybe with an overall master RCD as well. I've never seen
box which has had its fuses replaced with MCBs.

The only time I've seen a 30 A fuse blow was when my immersion heater
element developed a short to earth. At 1 AM, when my Economy 7 timer kicked
into night mode, switching on the full-tank heater (as opposed to the
half-tank one which could be run at any time of day/night) I was woken by a
very loud bang from downstairs as the fuse blew, and a loud metallic ping
from the hot water tank. The fuse left a huge stain of sputtered metal on
the outside of the ceramic fuse holder and its surroundings, which I never
managed to scrape off. The metallic ping became clear when I unscrewed the
immersion heater: the element had blown itself to pieces and shot part of
the coils into the copper cylinder. (And, yes, I *did* remember to drain the
cylinder, in addition to just draining the header tank, before unscrewing
the element - unlike a colleague who forgot this and when the element was on
the last thread, which could no longer hold the element in place, the water
pressure shot it out, gouging a hole in the wall and depositing a couple of
hundred litres of still-hot water onto the landing, down the stairs and all
over the ground floor carpets. He described the horror of seeing a "plug" of
water several inches of diameter emerging from the cylinder, with no way to
stop it.)

NY wrote:

Can you buy MCBs which are interchangeable with the sockets for wire
fuses, so you can remove the socket and fuse and replace it with an MCB,
without needing to replace the whole fuse box?

You can, though there are disadvantages such as a lower fault current
breaking capacity, and maybe others that I haven't remembered

e.g for old Wylex fuseboxes ...

https://www.ebay.co.uk/itm/Wylex-plug-in-MCB/281856950605

On Saturday, 14 July 2018 20:33:03 UTC+1, Andy Burns wrote:
NY wrote:

Can you buy MCBs which are interchangeable with the sockets for wire
fuses, so you can remove the socket and fuse and replace it with an MCB,
without needing to replace the whole fuse box?

You can, though there are disadvantages such as a lower fault current
breaking capacity, and maybe others that I haven't remembered

e.g for old Wylex fuseboxes ...

https://www.ebay.co.uk/itm/Wylex-plug-in-MCB/281856950605

Lots of old Wylex boxes have them fitted. Lower breaking capacity but better response in the face of overload & easier reset.


NT

On 14/07/2018 19:35, John Rumm wrote:
On 14/07/2018 17:22, newshound wrote:
On 14/07/2018 14:53, John Rumm wrote:

Probably worth a read of:

http://wiki.diyfaq.org.uk/index.php/...lectric_shower


Done!



Â*From this the supply splits to the original CU which has plug-in
MCBs, and what I can only describe as a one-way CU for the shower,
this has a switch (havn't checked if it is one or two pole) with an
integral 30A plug-in MCB. This suggests to me that the original
shower had a lower rating than 8.5kw, I'd have expected to see a 40A
trip to protect an 8.5kW heater circuit but maybe you can get away
with 30A for short showers.

Yup, if you look at the time vs current trip curve for a 30/32A MCB,
the likely load is going to be below the trip threshold near enough
indefinitely.

Hadn't had time to hunt up the curves, sorry!

sorry =, I intended to post the link:

http://wiki.diyfaq.org.uk/index.php/...e-MCBTypeB.png

Thanks, I had a mental picture of the curves, but without labelled axes!

So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Yup, from a design point of view that is correct - although with
showers that 8.5kW may well be quoted at 240V...

Didn't realise that


Clipped direct is OK to 47A. What does the team think about the wire
run behind the tiles? I'm inclined to think that while it *might*
run slightly hotter than clipped direct, the insulation is not
particularly good and it can't set fire to anything sandwiched
between blockwork and tiles (presumably chased into the original
plaster).

"In masonry" is generally treated as if clipped direct.

That's really useful to know. Makes sense to me (but from finding
quotes from some of the Sparks' forums it looks as though some people
take 34A for 6mm^2 (except clipped) which seems to be Method 4).

Method 4 would be instillation type B (38A). 34A sounds like method 100.

http://wiki.diyfaq.org.uk/index.php/...lation_Methods


Now that is a *really* useful link. I hadn't realised that DIYFAQ was so
up-to-date (but I havn't actually needed to do much electrics since I
did the main rewire in the 90's, and I only have the site guide from 2004)

Do you think he could go up to 9.5kW?

He would be pushing it from a cable life point of view - its likely
to run hot. That in turn can cause more problems with overheating at
the terminations.

Good point. Although there is probably a foot of cable "in air" at the
bottom, and several inches at the top.


10mm^2 would be a better choice.

Changing the cable is not a straightforward option, that would mean
replacing tiles, although I did wonder whether a sparks might suggest
a new run of 10mm^2 in trunking outside the cabinet, and then over the
top of the door into the unit. This would not look *too* untidy if
done nicely.

Yup, no reason to follow the existing path in every case. Any chance the
old cable could pull through a new one?

Don't know, but I *think* it is unlikely. In an industrial installation
you might have proper round conduit with access boxes, I suspect that
here the cable might just be plastered into a chased groove, at best
under a plastic capping beneath the tiles. The "distribution board" has
been boxed in with a simple box with doors immediately below the
ceiling, you would not be able to see the cable route without ripping
out the box and pulling down some ceiling plasterboard. The shower tray
is boxed in above the distribution box so you can't examine the cable
route from above either. The existing cable seems to run behind the
tiles about 6 inches into the shower enclosure.


Second question, the overall RCD protection with 100mA trip is a bit
out of date, should he get this changed to 30mA?

The 100mA trip will offer no protection from electrocution.

I recognise that

However if the existing installation meets proper design criteria,
and the equipotential bonding is adequate, then it could still be
considered "safe".

I havn't tried to check this, only had a chance for a *very* quick
look and I was concentrating on confirming the supply cable size.

(without more details of the installation it would be hard to say for
sure). The fact that the shower has probably already been uprated
does case some doubt however as that will invalidate any previous
calculations and testing.

Any new installation though really aught to have a RCD (even if that
just means swapping the MCB for RCBO)

Trouble is the present "shower MCB" is plug in, not DIN rail, so
(presumably) direct replacements not available.

That will need a new enclosure then... but for a one or two way one,
they are cheap.

Agreed.



There is plenty of choice of small one and two way CUs to choose
from. You will probably even find some described as a "shower CU".


Found some now thanks to NT.

We'll see if we can find an electrician who is prepared to do a
relatively small job.

On 14/07/2018 22:08, newshound wrote:
On 14/07/2018 19:35, John Rumm wrote:
On 14/07/2018 17:22, newshound wrote:
On 14/07/2018 14:53, John Rumm wrote:

Probably worth a read of:

http://wiki.diyfaq.org.uk/index.php/...lectric_shower


Done!



Â*From this the supply splits to the original CU which has plug-in
MCBs, and what I can only describe as a one-way CU for the shower,
this has a switch (havn't checked if it is one or two pole) with an
integral 30A plug-in MCB. This suggests to me that the original
shower had a lower rating than 8.5kw, I'd have expected to see a
40A trip to protect an 8.5kW heater circuit but maybe you can get
away with 30A for short showers.

Yup, if you look at the time vs current trip curve for a 30/32A MCB,
the likely load is going to be below the trip threshold near enough
indefinitely.

Hadn't had time to hunt up the curves, sorry!

sorry =, I intended to post the link:

http://wiki.diyfaq.org.uk/index.php/...e-MCBTypeB.png

Thanks, I had a mental picture of the curves, but without labelled axes!

Yup, one of those cases where the devil is in the detail - not much use
without the labels.


So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Yup, from a design point of view that is correct - although with
showers that 8.5kW may well be quoted at 240V...

Didn't realise that


Clipped direct is OK to 47A. What does the team think about the
wire run behind the tiles? I'm inclined to think that while it
*might* run slightly hotter than clipped direct, the insulation is
not particularly good and it can't set fire to anything sandwiched
between blockwork and tiles (presumably chased into the original
plaster).

"In masonry" is generally treated as if clipped direct.

That's really useful to know. Makes sense to me (but from finding
quotes from some of the Sparks' forums it looks as though some people
take 34A for 6mm^2 (except clipped) which seems to be Method 4).

Method 4 would be instillation type B (38A). 34A sounds like method 100.

http://wiki.diyfaq.org.uk/index.php/...lation_Methods


Now that is a *really* useful link. I hadn't realised that DIYFAQ was so
up-to-date (but I havn't actually needed to do much electrics since I

Its not *that* up to date - I think Adam and I added those bits in 2010...

did the main rewire in the 90's, and I only have the site guide from 2004)

Well I just had my nice shiny new light blue version turn up, so I spose
some more updates will be required shortly.

Do you think he could go up to 9.5kW?

He would be pushing it from a cable life point of view - its likely
to run hot. That in turn can cause more problems with overheating at
the terminations.

Good point. Although there is probably a foot of cable "in air" at
the bottom, and several inches at the top.


10mm^2 would be a better choice.

Changing the cable is not a straightforward option, that would mean
replacing tiles, although I did wonder whether a sparks might suggest
a new run of 10mm^2 in trunking outside the cabinet, and then over
the top of the door into the unit. This would not look *too* untidy
if done nicely.

Yup, no reason to follow the existing path in every case. Any chance
the old cable could pull through a new one?

Don't know, but I *think* it is unlikely. In an industrial installation
you might have proper round conduit with access boxes, I suspect that
here the cable might just be plastered into a chased groove, at best
under a plastic capping beneath the tiles. The "distribution board" has
been boxed in with a simple box with doors immediately below the
ceiling, you would not be able to see the cable route without ripping
out the box and pulling down some ceiling plasterboard. The shower tray
is boxed in above the distribution box so you can't examine the cable
route from above either. The existing cable seems to run behind the
tiles about 6 inches into the shower enclosure.

Thinking laterally - how about a mixer shower ;-)




--
Cheers,

John.

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

On Sunday, 15 July 2018 00:19:55 UTC+1, John Rumm wrote:
On 14/07/2018 22:08, newshound wrote:
On 14/07/2018 19:35, John Rumm wrote:
On 14/07/2018 17:22, newshound wrote:
On 14/07/2018 14:53, John Rumm wrote:

Probably worth a read of:

http://wiki.diyfaq.org.uk/index.php/...lectric_shower


Done!



Â*From this the supply splits to the original CU which has plug-in
MCBs, and what I can only describe as a one-way CU for the shower,
this has a switch (havn't checked if it is one or two pole) with an
integral 30A plug-in MCB. This suggests to me that the original
shower had a lower rating than 8.5kw, I'd have expected to see a
40A trip to protect an 8.5kW heater circuit but maybe you can get
away with 30A for short showers.

Yup, if you look at the time vs current trip curve for a 30/32A MCB,
the likely load is going to be below the trip threshold near enough
indefinitely.

Hadn't had time to hunt up the curves, sorry!

sorry =, I intended to post the link:

http://wiki.diyfaq.org.uk/index.php/...e-MCBTypeB.png

Thanks, I had a mental picture of the curves, but without labelled axes!

Yup, one of those cases where the devil is in the detail - not much use
without the labels.


So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Yup, from a design point of view that is correct - although with
showers that 8.5kW may well be quoted at 240V...

Didn't realise that


Clipped direct is OK to 47A. What does the team think about the
wire run behind the tiles? I'm inclined to think that while it
*might* run slightly hotter than clipped direct, the insulation is
not particularly good and it can't set fire to anything sandwiched
between blockwork and tiles (presumably chased into the original
plaster).

"In masonry" is generally treated as if clipped direct.

That's really useful to know. Makes sense to me (but from finding
quotes from some of the Sparks' forums it looks as though some people
take 34A for 6mm^2 (except clipped) which seems to be Method 4).

Method 4 would be instillation type B (38A). 34A sounds like method 100.

On 14/07/2018 20:33, Andy Burns wrote:
NY wrote:

Can you buy MCBs which are interchangeable with the sockets for wire
fuses, so you can remove the socket and fuse and replace it with an
MCB, without needing to replace the whole fuse box?

You can, though there are disadvantages such as a lower fault current
breaking capacity, and maybe others that I haven't remembered

Lower compared to DIN rail MCBs, but quite often equal or better than
most 3036 rewireables (which can normally only break 1 or 2 kA depending
on type).


e.g for old Wylex fuseboxes ...

https://www.ebay.co.uk/itm/Wylex-plug-in-MCB/281856950605

Those will normally interrupt up to 3kA. The older push button type cope
with less:

http://wiki.diyfaq.org.uk/index.php/...MCB_1360-3.jpg


--
Cheers,

John.

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

On 14/07/2018 20:00, wrote:
On Saturday, 14 July 2018 17:14:42 UTC+1, ARW wrote:
On 14/07/2018 15:11, tabbypurr wrote:
On Saturday, 14 July 2018 15:03:05 UTC+1, ARW wrote:
On 14/07/2018 14:50, tabbypurr wrote:

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.

Are you posting this sort of stuff to **** me off?

your logic escapes me



Why would you derate a cable for fixed known load just because you are
using a fuse?

Fuses allow more overload than MCBs, they're not type B. Hence the regs permit less cable current when protected by fuses.

There can be no overload. You know the current rating of the shower.
Different for sockets when you can plug in and alter the load


--
Adam