45A mcb too hot?

[Skip_2000]

Hello all,
I have recently installed a 10.8kw shower and it worked fine for 7 days.

Now the 45A mcb trips when I run it at the 'High' setting. It still works fine on 'Medium' and 'Low'.

There was an existing run of 8 Meters of 6mm cable from the mcb to a 40A ceiling power switch which I changed to a 50A and then added 2 meters of 10mm cale from the switch to the shower.


When I reset the mcb I notice that it is almost too hot to touch after tripping.

Is this temperature normal and what are the main things to look for to correct the tripping fault?

Thanks in advance.

 

[IanDB]

Big slap on wrist. You haven't worked this one out have you? 10.8kW will draw 47 Amps at 230V let alone allowing for cable run and connection resistance.

You need a bigger mcb to handle the larger current. No wonder it gets hot!

There is no tripping fault, the mcb is doing what it should.

 

[Skip_2000]

Thanks for the reply.

I did think that 45A was too small but that is the stated mcb to use acording to the shower instructions...

Do you think I can use a larger rated mcb with the existing cable sizes and lengths? If so what size do you recommend?

 

[GrantD]

You also need 10mm2 cable from the MCB to the shower

 

[Skip_2000]

Yeah - I considered that, but I telephoned the shower manufacturers and they said 6mm was fine for 10m runs and 10mm would be ok upto 20m runs. My run is less than 10m so I hoped that 8m of 6mm would be ok.

Could the mcb be faulty?

Could the shower be faulty?

 

[plugwash]

don't listen to the manufacturers

6mm is NOT suitable in any installation method you are likely to see in a domestic situation

you should use 10mm cable and a 50A breaker
this should solve your problem

 

[IanDB]

..................and just to throw a bit more in if you have a look here http://www.diydoctor.org.uk/projects/electricshower.htm you'll see they recommend a 50A mcb and 16mm2 cable.

I'm surprised at the manufacturer's recommendation - it's worth going back on that one as 45A isn't man enough for continuous operation.

 

[AdamW]

Also I'm not sure it is good practice to be running 10mm cable from a switch that is fed with 6mm.

Hopefully the 6mm will be run in conduit and thus easy to replace.

 

[GrantD]

A 45A mcb should not trip on 47A load unless it is on for a long time, so the mcb may be faulty. It certainly shouldn't be getting hot.

I haven't got the regs and tables to hand but as a quick check go to

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

It shows that anything over 8.74kW needs 10mm2 cable for a 10m length
(assuming surface clipped cable i.e. best case)

 

[IanDB]

A 45A mcb should not trip on 47A load

. Agree, having gone back over the characteristics of the breaker it should actually run up to 50A before tripping on continuous load.

 

[Skip_2000]

Thank you for all your replys - I will change the cable to 10mm and upgrade the mcb to 50A - hopefully this will cure the tripping.

Again thank you all for your help.

 

[ban-all-sheds]

Should do.

What do shower manufacturers hope to gain by lying about the size of cable needed?

 

[securespark]

It's just that some instructions are written by monkeys. They're not fit to wipe your @rse on..

Who's shower was it anyway? Give me the details and I'll ring em up & give em some stick...I like that sort of challenge!

 

[masona]

How the hell do thet get away with it ? Doesn't the BS kitemark come into it or whatever ?

 

[StirlingScott]

i just picked up a 50 amp crabtree pullswitch for a job im doing, as the customer in his wisdom went for one of these 10.8kw showers as well - really why do people get so excited about that extra couple of kw or so. keep showers about 9-9.5 kw and we will all get along fine -
but no,manufacturers keep on upping the rating causing all these problems