Shower related: watts and amps.

[amfisted]

We had the electrical installation checked, a new CU fitted and a certificate issued when we moved to this bungalow 3 years ago.

Our shower is an 8.5kw Redring that the previous owner fitted and which is about 5 years old now. Redring stipulate a 40a MCB for their 8.5kw shower, but the new CU has a 32a MCB on the shower circuit. Despite the shower being in daily use, this 32a MCB has never tripped. Why would this be?

Secondly, I’ve looked at the Wiki section on measuring cable (because I’d like to know the size of the cable supplying the shower) and it's stated that measuring isn’t simply a matter of putting a ruler across from one side of the cable to another. What, then, is the correct method for measuring cable, and are we looking for cross section, circumference or diameter when working out what cable can be used for?

Thanks in advance.

 

[winston1]

MCBs don't trip at precisely their rated current. 8500 watts at 240 volts is 35.4 amps. A 32 amp MCB will not trip at 35.4 amps during the duration of an average shower.

For cable you need to measure the diameter of a copper conductor with a micrometer or similar and calculate the cross sectional area, pi*dia squared upon 4, and multiply by the number of strands.

 

[Taylortwocities]

There is a rough size guide in the WIKI that may help
https://www.diynot.com/wiki/Electrics:flatpvccables

 

[ban-all-sheds]

and are we looking for cross section, circumference or diameter when working out what cable can be used for?

The first, which is why cable sizes are specified in square mm.

And you also need to consider how the cable is installed to determine what current it can safely carry.

 

[EFLImpudence]

pi*dia squared upon 4,

I presume you mean pi*dia squared divided by 4.

That's a strange way to do it and strage terminology - to me at least - more geographical differences?
Where did you learn that?

pi*radius squared.

 

[Lectrician]

Typically though, if the earth is a solid single conductor, you have 6mm² cable, if it’s a stranded earth, you most likely have 10mm² cable, but confirmation would be needed.

 

[Taylortwocities]

I presume you mean pi*dia squared divided by 4.

That's a strange way to do it

pi*radius squared.

I too looked at that and thought that ain’t right. But then I tried it on a calculator and it works.

In Winston’s defence (see it’s the season of goodwill!) he was starting out with telling the OP to measure the diameter as the starting point.
So it’s logical to have a calculation that uses d as the starting point for the formula.
But for me I was always told to start with radius and use
πr²

 

[amfisted]

Thanks very much for all that information.

Geometry was never my strong point, but (putting it crudely) 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?

I did locate the measuring guide in Wiki but to be frank, couldn't make head or tail of it. Maybe it's more simple than it looks.

The reason I ask about the MCB not tripping is because the original spark who did the CU clearly regarded a 32a as adequate for a shower which, according to the installation instructions, requires 40a, and thus far he's been proven right in that regard. I want to replace the shower because its developed an internal fault, and I intend to get the installation checked by an electrician first. If this electrician insists that the 32a MCB is adequate, I'd like to be able to argue for a change of MCB to 40a from a position of knowledge, not ignorance, and not merely point to the manufacturer's recommendations.

Thanks again.

 

[Taylortwocities]

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. A ruler? Haha. That will not work. It’s an accurate measurement. I’ll give you an example.
6mm² cable, installed in ideal conditions, is OK for a 40amp MCB. The next size cable down is 4mm² and the max MCB you can have on that cable is 32amp.

The visual difference between the two cables is minuscule and even experienced sparkies have problems distinguishing between the two. You need a digital micrometer and a calculator to get it right. (There are a couple of other wrinkles, but won’t go into it here and now).
The other way is to examine the outer sheath of the cable, more recent cables often have the size embossed in the sheath.

But this is academic. Unless you know what you are doing you shouldn’t be changing cables and MCB sizes. This would only be needed if you opt for a larger power shower. That’s the time to get a proper electrician in to advise you.
Your existing shower, on a 32A MCB is just fine, as outlined in the posts above.
Leave it alone.

 

[amfisted]

Yes, I guess you're right. Unless one had eyes like an outside toilet rodent, it would be very hard to accurately measure without the proper equipment, even assuming you could find a section of cable where the corners were absolutely 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.

Shower manufacturers and their recommendations? Pfft.

 

[ban-all-sheds]

MCBs don't trip at precisely their rated current. 8500 watts at 240 volts is 35.4 amps. A 32 amp MCB will not trip at 35.4 amps during the duration of an average shower.

At that low an overload there's no guarantee it would trip during the duration of the average lifetime.

For cable you need to measure the diameter of a copper conductor with a micrometer or similar and calculate the cross sectional area, pi*dia squared upon 4, and multiply by the number of strands.

Given the limited numbers of sizes, it's enough just to measure the entire bundle of strands with a simple caliper gauge.

 

[winston1]

I presume you mean pi*dia squared divided by 4.

That's a strange way to do it and strage terminology - to me at least - more geographical differences?
Where did you learn that?

pi*radius squared.

Back in my school days, my physics master mentioned it as an alternative when you measure the diameter.

 

[EFLImpudence]

I too looked at that and thought that ain’t right. But then I tried it on a calculator and it works.

Me, too. Just have never come across it before.

Also the 'upon 4' - 'over 4' is surely the usual.

In Winston’s defence (see it’s the season of goodwill!) he was starting out with telling the OP to measure the diameter as the starting point.
So it’s logical to have a calculation that uses d as the starting point for the formula.

Back in my school days, my physics master mentioned it as an alternative when you measure the diameter.

Yes, thanks. I see that but - as above - and

surely it's easier (just) to divide the diameter by 2 than the result by 4.

 

[DetlefSchmitz]

I've come across 'upon'. (Many years ago in the Reading area.)

 

[Taylortwocities]

I've come across 'upon'. (Many years ago in the Reading area.)

Me also, she was quite good looking too!