Shower related: watts and amps.

[ban-all-sheds]

I did locate the measuring guide in Wiki but to be frank, couldn't make head or tail of it.

So a table which for each cable size tells you what its overall dimensions typically are, and what the diameter of each conductor is, makes no sense to you?

would it be correct to say that the size of a cable can be roughly assessed by looking at it in cross section and measuring width times height to find the size in square millimetres?

No.

Cable sizes refer to the csa of the L & N conductors, not the overall cable.

For cable you need to measure the diameter of a copper conductor...

You can use a ruler to measure the overall size, and then look up in the table what a cable with an overall size of x x y would be

.... even assuming you could find a section of cable where the corners were absolutely square.

You still seem to think that if a cable is stated to be, for example, 6mm², that that refers to any overall dimension. It doesn't - it refers to the csa of the L & N conductors, and they are not even slightly square.

So I'll go with my original thought and change this faulty shower for one of equal rating, in the sure and certain knowledge that the circuit will cope.

It's up to you.

IMO it would make sense to find out the size of the cable you actually have, just in case the electrician who fitted a 32A breaker was an idiot.

Shower manufacturers and their recommendations? Pfft.

Their recommendation is spot on. The breaker should have a rating ≥ the load. In this case the load of the shower (assuming they are quoting the performance at 240V) is 35.4A. You cannot get 35.4A breakers - the next size up after 32A is 40A.

 

[amfisted]

A difference of opinion seems to have emerged.

One expert says that I should leave the installation alone because the 32a MCB has been fine with my old shower. This would indicate to me, as a layman, that it'll be fine with my new shower also, which is the same rating. You, however, say I should stick with the manufacturer's recommendation and change to 40a, which was my original thought.

No, I didn't understand the size guide, as I admitted earlier. Best admit to lack of comprehension rather than pretend to know what something means and risk what could be serious consequences. Having read further posts I'm nearer to an understanding of cable sizes, but that is now academic because it seems clear to me, judging by the tone of your last post, that I should get a spark to check the cable size and fit a 40a MCB.

Which is where I came into this conversation.

 

[Taylortwocities]

Mate. In your original post you did not ever say you were thinking of replacing the shower. My replies were relating to the fact that the existing one works fine with the existing arrangement.
So I have replied accordingly.

The reply may be different if you change the requirements half way through the story.

 

[EFLImpudence]

One expert says that I should leave the installation alone because the 32a MCB has been fine with my old shower. This would indicate to me, as a layman, that it'll be fine with my new shower also, which is the same rating. You, however, say I should stick with the manufacturer's recommendation and change to 40a, which was my original thought.

To confuse you even further:

Your shower will not draw any more current whatever rating of MCB you fit.

So, 8500/240=35.4A.
It may rise to 36.9A should the voltage rise to 250V, but this must be in tolerance for the cable and the current ratings given for cables.
Therefore, even a 4mm² cable clipped to surface or buried in masonry would be adequate.

A 32A MCB will never trip with a current of up to 36.16A (it just won't); it will take an hour to trip at a current of 46.4A although heating by such currents will weaken it over time.

Any rating of MCB would be satisfactory as long as the cable can cope with the earth fault current associated with that MCB and the earth fault loop impedance of the circuit. An electrician would be needed to test these values.

If the circuit is satisfactory for the earth fault with a 40A MCB, or even 50A, then you could fit one with absolutely no effect on the cable.


There is more to think about than just a yes or no answer.

 

[amfisted]

Mate. In your original post you did not ever say you were thinking of replacing the shower. My replies were relating to the fact that the existing one works fine with the existing arrangement.
So I have replied accordingly.

The reply may be different if you change the requirements half way through the story.

Fair comment.
Would a new shower with the same power rating be likely to work as well with that 32a MCB? Or is that likely to be dependent on other things which cannot be known until the circuit is checked by a competent person?

 

[ban-all-sheds]

Would a new shower with the same power rating be likely to work as well with that 32a MCB?

Your shower will not draw any more or any less current whatever rating of MCB you fit.

But you should still find out what size the cable is, and where it runs, as you might be able to have a better shower than an 8.5kW.

 

[amfisted]

To confuse you even further:

Your shower will not draw any more current whatever rating of MCB you fit.

So, 8500/240=35.4A.
It may rise to 36.9A should the voltage rise to 250V, but this must be in tolerance for the cable and the current ratings given for cables.
Therefore, even a 4mm² cable clipped to surface or buried in masonry would be adequate.

A 32A MCB will never trip with a current of up to 36.16A (it just won't); it will take an hour to trip at a current of 46.4A although heating by such currents will weaken it over time.

Any rating of MCB would be satisfactory as long as the cable can cope with the earth fault current associated with that MCB and the earth fault loop impedance of the circuit. An electrician would be needed to test these values.

If the circuit is satisfactory for the earth fault with a 40A MCB, or even 50A, then you could fit one with absolutely no effect on the cable.


There is more to think about than just a yes or no answer.

Thanks for that.

So its safe to conclude that I should can rely on a qualified electrician to test the circuit and use those findings to decide what rating of MCB I need to have. Which presumably is what the spark who fitted the new CU did in 2014.

 

[amfisted]

But you should still find out what size the cable is, and where it runs, as you might be able to have a better shower than an 8.5kW.

Thanks again. Unfortunately I've already bought a new shower of the same rating, and its been in a box in the spare bedroom since June while I've kept the old one going. Time to do the dirty deed, I think.

 

[ban-all-sheds]

Which presumably is what the spark who fitted the new CU did in 2014.

Who knows what he did.

Hard to see how he could have ended up deciding that a 32A breaker for a 35A load was the right thing to do.

 

[amfisted]

Who knows what he did.

Hard to see how he could have ended up deciding that a 32A breaker for a 35A load was the right thing to do.

I've emailed the boss of the firm he works for and mentioned that a 32a MCB is fitted when the manufacturer stipulates 40a. It'll be interesting to note his reply.

Its not something I became aware of until I realised that I'd have to replace the shower and then (as you do), started to do some research on the internet.

.

 

[Harry Bloomfield]

We had a 7.5Kw shower original install, 32amp MCB and 10mm and our supply is 240v always within a volt or two. When that needed replacing I fitted a fancy 9.5Kw model, temperature controlled, which could modulate both the water flow and Kw input, that was some 7 years ago and I fully expected the MCB might trip and I would need to uprate it. It never has tripped, so I never bothered to replace it.

One thing I would strongly suggest, is to ensure the shower is supplied by an RCD and everything is properly bonded for safety.

 

[JohnW2]

One thing I would strongly suggest, is.....

Some 13 months on, I suspect the issue has been resolved (or forgotten!).

Kind Regards, John

 

[Harry Bloomfield]

Some 13 months on, I suspect the issue has been resolved (or forgotten!).

Kind Regards, John

No doubt, but always worth adding to a thread for others who might read a thread later.

 

[ban-all-sheds]

No doubt, but always worth adding to a thread for others who might read a thread later.

Even when what you are adding is support for putting a 40A load on a 32A device?

 

[Harry Bloomfield]

I agree the MCB ought to have been swapped out in my case for a larger one, but there is nothing unsafe about an MCB which is rated for less than the load and less than the cable rating, as bourn out by the fact it has never tripped. The MCB survives without tripping, because a shower's load is not continuous one and because the shower's load is modulated. Tighter fuse or MCB protection is always better/ safer.