Shower Electrics Help

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Hi all.

I have a question about shower fuses and cable sizes etc.

I believe (i admit this is just from seeing the external part of the wire and not the internal core size) that i have a 10mm cable - i will obviously check this first.

I also believe (again i will check this first) that my fuse on my consumer unit is only 32A.

My questions are, if i buy a shower which is 9.5kw,

a) will my cable (if it is 10mm core) be ok?
b) would it be as simple as changing out the fuse for a bigger one?
c) is there a table anywhere which will tell me recommended cable size and fuse size for each kw shower rating?

Thanks in advance.

Ned
 
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few things ned, do you have a fuse or MCB. a shower should have RCD protection does it ? the current carrying capacity of the cable is affected by a number of things ,length of the cable , how it is installed ,if it is surrounded by insulation to name a few. the fuse /MCB protects the cable and the size ( amperage ) would be determined by the circuit designer / electrician installing it after taking various things into account . other things to take into account when swapping showers. especially when the cable and water pipes are behind tiled wall ,is their positions relative to your new shower. you very often have to choose a shower to accommodate them .
 
9.5KW is 41A @ a nominal 230Volts. So some rethinking may be necessary!

A GUIDE to cable sizes in in the WIKI at
https://www.diynot.com/wiki/Electrics:flatpvccables

If your MCB is 32A then it is likely that your cable is only 6mm², which may limit the size of the shower that you can put on that circuit.

And as terry has said, you MUST have RCD protection on that circuit too.
 
Hi terryplumb,

Yes it is an MCB with RCD protection, with all the above in mind is there an easy way for me to determine the maximum KW shower i can have? i am competent and comfortable enough switching the shower around and switching the fuse in the MCB, but what i dont want to do is muck around with the wiring.

Ned
 
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i am competent and comfortable enough switching the shower around and switching the fuse in the MCB,
In that case, you will know that you cannot just UP the MCB size.
If you have 6mm² cable, what size MCB will you select. The Ib for the circuit is 41amps. next MCB size up is 45amps.
Can 6mm² cable hack 45amps under fault conditions?
There is a table to calculate all of this. Its in here
BK1717SLASH3.JPG


And of course, as you say you are competent, you will know that you will need to test the EFLI of the circuit before swapping out a CPD. Do you have the necessary, calibrated, test equipment for this.??
 
Ha ok maybe not that comfortable and competent! what i meant was that i would be able to do the job if i was told what i was supposed to do! sounds like i may just have to wait until the shower completely packs up and then i can justify getting someone to come and fix it completely
 
you do not change the MCB to suit the shower. the MCB protects the cable.as Taylor said above ,32 amp breaker most likely 6mm t/e . so you are limited to around 8.5 kw max. In terms of showering experience there is not a massive difference .
 
Several inaccuracies in the above answers.

4mm² is adequate with a 32A MCB, method C.

6mm² is 47A, method C. So, yes it can handle 45A fault current. How will that happen?
Showers cannot overload so a 50A MCB would be compliant.

9.5kW @ 240V (which is what will be quoted) is 39.6A which is just right for a 40A MCB - coincidence?
32A @ 240V is 7.7kW.
 
is there a table anywhere which will tell me recommended cable size and fuse size for each kw shower rating?

Where

  1. Ib is the current the shower draws
  2. In is the rating of the circuit protective device
  3. Iz is the current carrying capacity of the cable

IbInIz


This article has more of an explanation, and a graphical illustration of the rule: https://www.diynot.com/wiki/Electrics:MCBtoCable


Here's a table showing the current carrying capacity of different sized cables for different installation methods:

screenshot_1299.jpg
 
Thanks everyone - it seems like i will have to wait, the whole idea of working out equations is a little too much for me! I was hoping i may be able to just change the MCB fuse (as i thought my existing wiring could cope with a higher KW shower).

How much time do you pros estimate a job like that would take? or is that too vague of a question to answer? All my floorboards are already exposed (and that is where the cable runs from consumer unit to shower) and i would suggest there is approx 8m of cabling to change (if i wanted the 10mm2 cable).

Thanks again

Ned
 
i am competent and comfortable enough switching the shower around and switching the fuse in the MCB
vs​
the whole idea of working out equations is a little too much for me
Ah....

I'm not having a go, but it's a good job you posted, and hopefully other people might take this on board in the future - you nearly fell victim to the old "unknown unknowns" problem.
 

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