8.5kw Shower on a 6.0mm cable

telectrix said:

agreed. overload protection is not required for a fixed load. the only problem is that the ccc of the cable is < the actual current drawn.

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davesparks said:

Utter rot! The shower will be rated 8.5kW at 240V not 230V so damn well calculate it at 240V
The power rating will be less at 230V.
This is basic physics you should have learned at school.

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Marvo said:

I don't think HHD's question is as stupid as you made it out and I damn well took needlework instead of physics so I have to ask the question

The KW rating on the appliance is a nominal value, it refers to the fact the element will give 8.5 kW of heat output at 240v. The heat output will coincide with this nominal 8.5kW figure only at this voltage, so if you drop the voltage then it's no longer a 8.5kW heater because the load current will vary proportionally to the supply voltage hence so will the heat output.

If the nominal supply voltage is 230v why would would you blindly use the nominal heat output figure given over the nominal 230v voltage figure? Why would one nominal figure be preferable to use over another nominal reference figure?

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Sb8389 said:

Hi guys,
want your opinion on a issue I have found while changing a consumer unit today.
I came across the shower wired in 6.0mm on a 32amp MCB (8.5KW). I know this will draw just over 35amps and could do with being upgraded to a 40amp MCB, but checking the install of the cable it is going through cavity full of insulation and then through the loft covered in insulation.
i know that the protective device will be overloaded but would you say that this would be a risk to leave on a 32amp MCB and would you be able to tell me how long it would take for the MCB to trip at 35amps as I don't have a regs book to hand to check the time current curve graph.
if you need any more info let us know.

How long is the run of 6mm2 between mcb and shower?

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davesparks said:

If the supply voltage genuinely is 230V then use the 230V power rating, it will be written on the damn data plate for both voltages.
Go in to B&Q and look at the showers, they have power input at both 240V and 230V these days.

Older appliances will only have a 240V rating so you should use 240V in your calculation otherwise you are introducing an error into your calculation to begin with.

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marconi said:

Sb8389 said:

Hi guys,
want your opinion on a issue I have found while changing a consumer unit today.
I came across the shower wired in 6.0mm on a 32amp MCB (8.5KW). I know this will draw just over 35amps and could do with being upgraded to a 40amp MCB, but checking the install of the cable it is going through cavity full of insulation and then through the loft covered in insulation.
i know that the protective device will be overloaded but would you say that this would be a risk to leave on a 32amp MCB and would you be able to tell me how long it would take for the MCB to trip at 35amps as I don't have a regs book to hand to check the time current curve graph.
if you need any more info let us know.

How long is the run of 6mm2 between mcb and shower?

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Its about 15 metres max pal

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happyhippydad said:

So if they are rated at 8.5kW at 240V and the damn supply voltage in the UK is 230V are you saying that you would still base your damn current on 240V?

Also, I'm a little concerned that you have suggested to the damn OP to increase the damn MCB to 40A given the damn reference method (covered in insulation). Would you still suggest he increases the damn MCB to 40A?

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davesparks said:

If the supply voltage genuinely is 230V then use the 230V power rating, it will be written on the damn data plate for both voltages.
Go in to B&Q and look at the showers, they have power input at both 240V and 230V these days.

Older appliances will only have a 240V rating so you should use 240V in your calculation otherwise you are introducing an error into your calculation to begin with.

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happyhippydad said:

So if they are rated at 8.5kW at 240V and the damn supply voltage in the UK is 230V are you saying that you would still base your damn current on 240V?

Also, I'm a little concerned that you have suggested to the damn OP to increase the damn MCB to 40A given the damn reference method (covered in insulation). Would you still suggest he increases the damn MCB to 40A?

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Dave OCD said:

Sorry if this has already been posted up but I'm tired and only skim read the 4 pages worth on this - wouldn't the most practical and easy solution be to see if there's a bit of slack on the 6mm in the loft and if so just lift the cable up and clip it /otherwise secure it above the insulation ? This would greatly reduce the risk of overheating - and I'd leave it on a 32a MCB as well, in the real world there will be no issues.

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Sb8389 said:

marconi said:

Its about 15 metres max pal

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By Marconi (with apology to Murdoch ) What follows is a contribution using the help of Mr Ohm and the table I found top right on page 3 of http://www.aeicables.co.uk/literature/CurrentRatings.pdf

Assumptions: Supply voltage at input to mcb is 240V. Ambient Temperature 30C. Conductor temperature 70C. Flat PVC thermoplastic 6mm2 T/E. Electrical power consumed by shower at 240V is 8.5kW. Ze is zero (but covered later). All electrical contacts have zero resistance (not true of course). Impedance between input and output of mcb is zero (not true of course). All connections have zero resistance(not true of course). Main cut out fuse has zero resistance (not true of course). Meter and tails in and out have zero resistance(not true of course). Shower resistance when operating in range 230V to 240V is constant.

So (240V supply) - Cut out - tails - meter - tails - main switch - rcd(if there is one)-(all the connections)- mcb - 6mm2 (15m off) - shower pull cord - shower

which with my assumptions simplifies to 240V supply - 6mm2(15m off) - shower

Let R1 be resistance of 6mm2 cable run and R2 be running resistance of shower at 240V.

R1 is 0.0073 x 15 = 0.1095 Volts per Amp = 0.1095 Ohms; R2 is 240 x 240/8500 = 6.77647 Ohms

So current flowing through is 240/(0.1095 + 6.77647) = 240/6.8859 = 34.85383A

Power dissipated in 15 m of 6mm2 is I x I x R1 = 34.85383 x 34.85383 x 0.1095 = 133.01945W or 133/15 = 8.86667 say 9W per metre.

Now let is assume the PSSC at the out put of the cutout is 1000A. This would mean Ze is 240/1000 = 0.24 Ohms.

Revised diagram - 240 supply - cutout - 6mm2cable (15m off) - shower

Current in circuit is 240/(0.24+ 0.1095 + 6.77647) = 240/7.1259 = 33.67996A

Now let us add in (all the connections). A punt now by me on what their total contribution is to the total resistance of the shower circuit - 0.1 Ohm

Current is now re-calculated as 240/(7.1259 + 0.1) = 240/7.2259 = 33.21386 A.

So may be when the total resistance of the shower circuit is considered the current drawn by the 8.5kW shower through a circuit wired in 15 m of 6mm2 is of the order 33 A and it will dissipate about 8 to 9 W every metre.

Over to you to consider the effects of the installation method and wisdom of my assumptions.

I am not attempting to answer whether the installation is compliant or not with BS7671 - I am just doing some electrical calculations which might add to the debate.

PS: Check my sums - I am using the calculator on my phone. I don't mind being corrected.

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happyhippydad said:

So if they are rated at 8.5kW at 240V and the damn supply voltage in the UK is 230V are you saying that you would still base your damn current on 240V?

Also, I'm a little concerned that you have suggested to the damn OP to increase the damn MCB to 40A given the damn reference method (covered in insulation). Would you still suggest he increases the damn MCB to 40A?

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Sb8389 said:

happyhippydad said:

Very true SB! But you've said in your post that its covered in insulation, so you have determined how its installed. If its got >100mm of insulation over it in the loft which is quite feasible then its CCC goes down to 27! I suppose you could clamp it and see how much it is drawing and perhaps use a 25A MCB? But you could start getting some tripping![/Q

i will clamp it tomorrow and see what its drawing and give myself a clearer idea of what we are dealing with.

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For interest, did you clamp and record the reading? - what was it? If you did not could you clamp and tell us the current reading before upgrading to 10mm2 please?

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marconi said:

Sb8389 said:

For interest, did you clamp and record the reading? - what was it? If you did not could you clamp and tell us the current reading before upgrading to 10mm2 please?

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Yeah be interesting to find out!

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