Replacing my electric shower

[jimmbo]

Hi, i'm doing the bathroom at the moment including replacing the shower. I've taken the old shower out (which does appear to be 9.5kw), and started looking at putting the new one in (definately 9.5kw). I'm not excatly an expert, but it seems that the existing wiring is only 6mm2 not 10mm2 (i think!), and the cartridge fuse in the CU is only 30A. We've been in the house 3 years and never actually had a problem with the old shower, but according to the installation instructions for the new one, 10mm2 wiring and 45A fuse should be used. Did whoever installed the old shower do it wrong and left us with a bit of a fire risk? Or am i ok just to put the new one in?!
Cheers

 

[Chri5]

I'm surprised the 30a cartridge fuse didn't fail on the old shower.

9.5kw = 9500w x 230v = 41.xxx amps

The cable needs to be checked, likely replaced and the fuse updated as necessary- this means compliance with 17th ed so will require an rcd.

Best suggestion would be a standalone rcd carrier with 10mm cable and direct feed to the henley box / dno demarcation. This will avoid issues with needing to update the whole CU.

That should be a mid term plan, cartridge fuses are a bit antiquated and if they are 30+ years old what's the rest of the wiring 'system' like?

Do you have earth cables to gas and mains water supply (if metal supply in) ?

 

[ban-all-sheds]

6mm² can carry 40A in some circumstances, but not all.

You should replace it.

 

[jimmbo]

Thanks for your quick replies.
Yeah, i couldn't really understand why the 30A fuse never blew!
The house is only 20 years old so i pressume the rest of the wiring is ok! Although i was a bit surprised to find the cartridge fuse box rather than the modern circuit breakers.
How would i know for sure that the cable is only 6mm² not 10mm²?
I'll probably have to get an electrician in then!

 

[Steve]

How would i know for sure that the cable is only 6mm² not 10mm²?

Take an offcut of it to B&Q or Homebase and compare it.

Is its earth core stranded or a single core?

 

[ColJack]

if it's 13.5mm wide it's 6mm², if it's 17mm wide it's 10mm²

 

[ban-all-sheds]

//www.diynot.com/wiki/electrics:flatpvccables

 

[Goldberg]

I've taken the old shower out (which does appear to be 9.5kw), and started looking at putting the new one in (definately 9.5kw).

Have you measured the flow rate that you're getting at the inlet to the shower, to make sure that it will support a 9.5kW shower?

BTW, 6mm² cable can carry 46A (not merely 40A) in some circumstances, and a 9.5kW shower will draw no more than 39.6A (not "41.xxx"), unless the supply goes above 240VAC.

 

[Chri5]

I've taken the old shower out (which does appear to be 9.5kw), and started looking at putting the new one in (definately 9.5kw).

Have you measured the flow rate that you're getting at the inlet to the shower, to make sure that it will support a 9.5kW shower?

BTW, 6mm² cable can carry 46A (not merely 40A) in some circumstances, and a 9.5kW shower will draw no more than 39.6A (not "41.xxx"), unless the supply goes above 240VAC.

Please don't pull my maths up. All calcs MUST be done on 230v, so to be precise (rather than my original mental calc)

9.5kw = 9500w x 230v = 41.30435 amps

or in simple terms 9.5kw = 9500w x 230v = 41.xxx amps

 

[ban-all-sheds]

BTW, 6mm² cable can carry 46A (not merely 40A) in some circumstances.

I mentioned 40A as that's the (approx) draw of the shower, not 46A.

Please don't pull my maths up. All calcs MUST be done on 230v,

Yup, and they MUST be done using the correct rating of the appliance rather than an incorrect one.

so to be precise (rather than my original mental calc)

No point being precise when you don't know if 9.5kW is the rating of the shower at 230V, (and when in fact you do know it probably isn't...)

 

[ricicle]

9.5kw = 9500 x 230v = 41.30435

9500 / 230 = 41.30435

 

[Goldberg]

Please don't pull my maths up.

So whom do you allow to point out your glaring errors?

All calcs MUST be done on 230v, so to be precise (rather than my original mental calc)

9.5kw = 9500w x 230v = 41.30435 amps

Your new calculation is also mental, on two counts:

1. Your logic is wrong.
The shower rating at 230VAC is 8725W.

2. Your equation is wrong.
"9500w x 230v =" yields a large and meaningless number.
To be precise (sic.), it's 2,185,000 Volt Watts.

If you're really bent on envisioning the current drawn when the supply is at 230V, which it never is in the UK, then it would be 38A. This would yield merely 2,006,710 of your Volt Watts.

Gordon Brown is 58.

 

[Chri5]

Calm down children-

1st post time

Mon Jul 20, 2009 7:44 am

Hadn't had my 1st cuppa

Calcs (as you well should know) should be based on 230v, not the on site variance.

So excuse the 'x' when it should have been a '/'.

Anyway you started it by questioning the math,

 

[Goldberg]

Calcs (as you well should know) should be based on 230v, not the on site variance.

Please explain why you think that an 8725W shower will draw 41.xxx Amps, without using any "on site variance".

 

[ColJack]

BS3036 30A fuse, the line on the chart goes off the top of it at about 55A, and the top of the chart is 10,000 seconds.. so that's 55A for 2:40 before it fails...