Electric shower

[tymalbrough]

I have an electric shower which requires changing as it is at least 15 years old (still working)The unit is a 7.2 Kw powered through a 30 amp MCB and pull switch. I am concerned that the cable to the pull switch seems to be shared with a power point and the single core cable is 1.75 m.m. The cable from the pull switch to the shower unit is also 1.75m.m.
I would like to change to a 9Kw or higher. I am aware that the MCB would have to be changed to 40 /45 amp but am concerned about the cable.
Is the cable even good enough for a 7.2Kw.
Have a nice day.

 

[GaryMo]

It's not a 1.75mm2 cable.
How are you measuring it?

 

[Taylortwocities]

How are you measuring the size of the cable. Cable sizes are the cross surface area.

You need to measure the diameter with a micrometer.
Standard cable sizes are

1mm²
1.5mm²
2.5mm²
4mm²
6mm²
10mm²
16mm²

For a 30amp breaker the cable should be 6mm² but it needs checking before anything else is done.

 

[ban-all-sheds]

I am aware that the MCB would have to be changed to 40 /45 amp but am concerned about the cable.

Are you also aware of this: //www.diynot.com/wiki/electrics:part-p ?

 

[TicklyT]

If the OP is measuring the diameter of the conductor as 1.75mm, that suggests the CSA is about 2.5 sq. mm, which is inadequate even for a 7.2KW shower.

1.75 / 2 = 0.875

0.875 ^2 = 0.766

X 22 / 7 = 2.41sq. mm.

OK, that's a little under, 2.5 sq. mm conductors should be about 1.79 mm diameter. That's 0.04 mm or just over a thousandth of an inch larger than the dimension given by the OP. If this is the case, I think we can forgive that order of error in the measurement.

Forgiving anyone who runs a shower circuit in undersized cable, connects a shower to a spur from a ring final circuit or runs a spur from a shower circuit is an entirely different matter....

 

[tymalbrough]

Hi All.
Thanks for your replies.
The (positive) single copper strand measured with a micrometer is 1.75m.m.
Measuring the red covering (again with a micrometer) I make it 3.3m.m.
Hope this makes some sense.

 

[ban-all-sheds]

I am concerned that the cable to the pull switch seems to be shared with a power point

You are right to be concerned.

and the single core cable is 1.75 m.m. The cable from the pull switch to the shower unit is also 1.75m.m.

Which we now know is 2.5mm². Rated at 27A, best case, when clipped to the surface. In practice often with a lower capacity.

When you say "single core" - where does the shower get its neutral and earth?

I would like to change to a 9Kw or higher. I am aware that the MCB would have to be changed to 40 /45 amp but am concerned about the cable.

You are right to be concerned.

Is the cable even good enough for a 7.2Kw.

No, nor for being put on a 30A MCB. See //www.diynot.com/forums/viewtopic.php?p=81696#81696


Quite honestly, the sort of tw@ who would install a shower like that can't be trusted not to have done any number of other hideous things - I strongly urge you to have the whole installation checked by a competent electrician asap.

 

[tymalbrough]

I only gauged the positive cable. I assumed that the negative and earth of the three core are the same.

 

[ban-all-sheds]

Ah - you said single core cable, not twin & earth. The neutral will be the same size, the earth smaller.

And it isn't "positive" and "negative", it's "phase" or "line" or usually (but not quite correctly) "live", and "neutral". As we have an AC supply the voltage of the line with respect to neutral varies between +325V and -325V 50 times per second...