Hi All

i know this is probably in old threads somewhere but i can't find what
i'm looking for so appologies if this is boring basics for most.
i was under the impression that all showers circuits should be 10mm
cable and 45 amp MCB (or 50 amp if over 10.3 kw)
I am now questioning this because our new/first house is supposed to
have been rewired professionally about 10 years ago and the circuit is
6mm cable with 32 amp MCB (shower is either 7.5 or 8.5 KW. not sure
which as model came in 2 powers and label no longer on shower unit).
This came to light because i want to replace the battered old wall
mounted shower switch located in hallway outside bathroom. Questions
a

1) are cable and mcb ok / comply as they stand?

2) if existing set up is ok but i replaced the shower in future, what
would be the maximum kw rating i could fit while retaining 6mm cable?

3) can i double check that wall mounted shower switches are still ok
provided they are located outside the bathroom, or will i have to
replace with a pull cord switch?

4) am i correct in saying that the amp rating of the new switch i'm
fitting should match that of the mcb? (in this case 32 amp, again
assuming existing set up is ok)

Thanks in advance for any help and taking the time to read this lot!

Mike wrote:

i was under the impression that all showers circuits should be 10mm
cable and 45 amp MCB (or 50 amp if over 10.3 kw)

Not necessarily...

I am now questioning this because our new/first house is supposed to
have been rewired professionally about 10 years ago and the circuit is
6mm cable with 32 amp MCB (shower is either 7.5 or 8.5 KW. not sure
which as model came in 2 powers and label no longer on shower unit).
This came to light because i want to replace the battered old wall
mounted shower switch located in hallway outside bathroom. Questions
a

1) are cable and mcb ok / comply as they stand?

Without having full details like the length of the cable run and whether
there are any factors that would de-rate its current carrying capacity
(like runs through insulation etc), one can't answer for certain.

However if you took the 7.5kW shower, that would have a current rating
of 7500/240 = 31.25A which is fine on both cable and MCB in most
circumstances. Being realistic, even the 8.5kW would be unlikely to trip
a 32A MCB, and the cable is unlikely to be overloaded in most
circumstances.

2) if existing set up is ok but i replaced the shower in future, what
would be the maximum kw rating i could fit while retaining 6mm cable?

The best case current rating for 6mm^2 is about 47A

See column C he

http://wiki.diyfaq.org.uk/index.php?title=Cables#T.26E

Assuming the total length is short enough to limit the voltage drop to
reasonable limits, that would allow a maximum rating of over 11kW.

3) can i double check that wall mounted shower switches are still ok
provided they are located outside the bathroom, or will i have to
replace with a pull cord switch?

Outside the bathroom, a wall mounted switch is fine. Inside a switch
needs to be appropriate for the location and zone. There are cases where
a wall mounted switch in the room would be ok.

4) am i correct in saying that the amp rating of the new switch i'm
fitting should match that of the mcb? (in this case 32 amp, again
assuming existing set up is ok)

The switch needs to be capable of switching the full load current - it
does not have to match exactly in that it can be larger. A 45A switch
would be fine for example.


--
Cheers,

John.

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