shower suppl and fuse

[russellsmiler]

I have searched and read many threads relating to this topic but thought its worth a new post.

currently overseeing and helping a freind fit an 8KW triton shower, have 6mm twin and earth cable approx 6m from fuseboard clipped to surface generally and lyin flat accross joists (insulation removed from aside cable) , we have and propose to use a 32 amp breaker. The reason i have posted this is because i am calculating 33.3 amps.. should i use a 40amp fuse, wish not to change the cable.

Thanks
Russell

 

[Taylortwocities]

If it is a FUSE, as apposed to an MCB, then 32A will be OK, unless your friend tends to have very long showers!
Strictly speaking, the CPD should be 40A, but you would need to measure the EFLI to check that the circuit will be within limits if the CPD were 40Amp.

You have not mentioned the presence of an RCD for the circuit. This will be needed. IS there one.?

So two questions. Fuse or MCB and do you have an RCD protecting this circuit?

 

[russellsmiler]

Thank you Taylor

answer to your queries, MCB, and yes circuit protected by RCD

 

[riveralt]

Thank you Taylor
answer to your queries, MCB, and yes circuit protected by RCD

So you will be fitting a 40A MCB then? Which is what Triton will be recommending I'll bet.
What was your Zs by the way?

 

[russellsmiler]

well this is why I post! do i need to fit a 40A MCB?? other historical posts have differing advice.

 

[PrenticeBoyofDerry]

32A MCB will be out of range, so 40A MCB required, 6.00 T&E can carry a maximum of 47A, so providing no derating factors that part should be fine. You would expect to find the recommended cable size and protective devices ratings, within the manufacturers instructions. Also there will be a requirement to inspect and test your work to prove it is safe and compliant to part p/BS7671. I would expect the MI to also mention this, as stating in some way that it must comply to BS7671/wiring regs. So I hope you have a fully calibrated MFT or a selection of appropriate calibrated test equipment.
It is a legal requirement to prove what you have done is safe!

 

[riveralt]

well this is why I post! do i need to fit a 40A MCB?? other historical posts have differing advice.

For my sins I am Triton installer and the chart they give us says for an 8kw shower shows they expect 40A MCB or 35A cartridge fuse.
I don't know what your 'friends' shower model is, but all of the Triton manuals that come with the shower have the same chart in.
Check the manual and don't forget to inform your friends LABC - wouldn't want to put his house insurance under pressure.

 

[PrenticeBoyofDerry]

Check the manual and don't forget to inform your friends LABC - wouldn't want to put his house insurance under pressure.

Oh so this could be a new installation?
Well if that is the case, as rightly stated, requires notification and I would forget about 6mm and put in 10mm.

 

[PrenticeBoyofDerry]

I have searched and read many threads relating to this topic but thought its worth a new post.

currently overseeing and helping a freind fit an 8KW triton shower, have 6mm twin and earth cable approx 6m from fuseboard clipped to surface generally and lyin flat accross joists (insulation removed from aside cable) , we have and propose to use a 32 amp breaker. The reason i have posted this is because i am calculating 33.3 amps.. should i use a 40amp fuse, wish not to change the cable.

On the evidence so far shown, I think your overseeing is best supervised by an electrically skilled person

 

[JohnW2]

32A MCB will be out of range, so 40A MCB required ...

I would agree that it's very marginal. Assuming that the 8kW is (as usual) quoted at 240V, that equates to 31.94A at 230V - so (if my assumption is correct), strictly speaking, a 32A MCB would be compliant, albeit sailing close to the wind (although, as EFLI is always reminding us, "just compliant" is "compliant"!).

Kind Regards, John

 

[PrenticeBoyofDerry]

32A MCB will be out of range, so 40A MCB required ...

I would agree that it's very marginal. Assuming that the 8kW is (as usual) quoted at 240V, that equates to 31.94A at 230V - so (if my assumption is correct), strictly speaking, a 32A MCB would be compliant, albeit sailing close to the wind (although, as EFLI is always reminding us, "just compliant" is "compliant"!).

8000/240=?
8000/230=?

I don't get less than 32 for either calc!

and if we did transpose 230V/240V 8000W, it would be rated at 7666.5w Surely?
Still making it above 32A or am I missing something?

 

[riveralt]

8000/240=?
8000/230=?
I don't get less than 32 for either calc!

I think JohnW2 is working on the basis that an 8kw shower is advertised as 8kw but has a 230v equivalent of around 7.4kw.
Thing is I am not sure Triton actually make an 8kw shower any more - I have seen both 7.5 and 8.5 but not an 8 for a while anyway.

 

[PrenticeBoyofDerry]

8000/240=?
8000/230=?
I don't get less than 32 for either calc!

I think JohnW2 is working on the basis that an 8kw shower is advertised as 8kw but has a 230v equivalent of around 7.4kw.
Thing is I am not sure Triton actually make an 8kw shower any more - I have seen both 7.5 and 8.5 but not an 8 for a while anyway.

My maths must need brushing up then.

 

[JohnW2]

8000/240=? 8000/230=? I don't get less than 32 for either calc!

8000/240 = 33.33A
Assuming it's a simple constant resistance load, that equates to 33.33A x (230/240) = 31.94A at 230V

or, if you would prefer ...
8000/240 = 33.33A Assuming it's a simple constant resistance load, that means that the load resistance is 240/33.33 = 7.20Ω
hence current at 230V = 230v/7.20Ω = 31.94A

and 230V it would be rated at 7666.5w Surely?

Not quite - actually a bit lower than that. As above, if it drew 8000W at 240V (240V x 33.33A = 8000W) it would draw about 7347W (31.94A x 230V) at 230V

or, again, if you would prefer ...
if (as above) resistance of load is 7.20Ω
then power at 240V = 240² / 7.20 = 8000W
and power at 230V = 230² / 7.20 = 7347W

Manufacturers of showers, ovens etc., almost invariably continue to quote what the power would be if the supply were 240V (even though the nominal supply voltage has been 230V for years), presumably to make their products sound "more powerful"!

Kind Regards, John

 

[russellsmiler]

ok, looks like its a 40 Amp MCB, will get the work certified, this is a requirement by the housing association.