Shower cable

My aunty had a 8kw shower fitted a few years ago 6mm cable on 40a mcb, couple years later she got a new 10.5kw shower and it appears the cabling or mcb sizes were not touched just the shower replaced, this worked fine for another couple of years until it died recently, and so they bought pretty much the same 10.5kw model (only slightly newer version).

They found now that altering the temperature dial, would only give them hot water at top position and any other setting would be cold. So I had a look and apart from the cable being too small couldnt see annything else that was wrong.

They got a engineer from Gainsbourough out to look at it and he confirmed that the shower was ok but as I had mentioned the cable was too small.

So can any one help me to understand how the cable size would cause this as although the cable is only 6mm it is still capable of handling around 47a (max without factors considererd) so my assumption was that the shower would work as should but the cable would be under strain and overheat or the breaker would pop on overload.

Im assuming as the cable heats up resistence rises and so effectively not enough current to heat the water as required but would appreciate the input.

The shower is broken. Probably has 2 heating elements and one of them has failed.

The cable is probably too small, the MCB certainly is, however neither of these could cause the problem you describe.

what do the manufacturers instructions, recommend?
Did the breaker size get increased?
10.5kw if nominal voltage is 230v would require about 45amps to operate somewhere near optimum performance

well thats pretty much as I thought, but with the Gainsbourough engineer testing the shower and then saying that the cable was the problem then I could only assume he was right.

My other trail of thought is with it being winter the water is sure to be a lot colder in the underground pipes at this time of year.

Prob now is wether to take him at his word and for her to get the cables uprated or downrate the shower but that means a brand new shower just bought wasted..

never saw the mf info but the mcb remained at 40a and the cable at 6mm
however the cable is pretty much a straight run approx 5 -6 metres and clipped most of the way

ezekiel said:

cable is pretty much a straight run approx 5 -6 metres and clipped most of the way

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Granted the cable/MCB combination isn't large enough to withstand prolonged higher current above 40A, but even so the volt drop in the cable will only be

V.d. = 7.3 x 45 x 6 / 1000 = 2.0V

... and a 2V (<1%) volt drop won't cause the problem described.

It does sound as though the shower is at fault, but is there sufficient water pressure? The only other explanation I can come up with is that as you approach maximum temperature on the dial, the water flow to the shower head is restricted (as the temperature is usually controlled by limiting water flow through the heating element), and this in turn is increasing the dynamic water pressure enough to make the pressure switch. It's a longshot, mind.

Is it running direct off of the mains? Some power showers must not be connected to mains (water) pressure systems.

not sure where the shower is fed from however the previous shower ran fine for a coupl years so would assume water pressure was ok. I did check stop cock was fully open.
engineer did say he was getting a larger shower head sent out?

Johnmelad502 said:

Is it running direct off of the mains? Some power showers must not be connected to mains (water) pressure systems.

Click to expand...

Sounds more like a standard electrically heated shower rather than a pumped 'power shower'.