8.5kw Shower on a 6.0mm cable

[Sb8389]

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.

thanks.

 

[PEG]

....Still need to clamp it,regardless of what it says on the tin...have a concrete fact,to apply the theory to...if you clamped three units of the same stated size,especially if of vintage,you would get three different values,and possibly three separate ways forward,bearing in mind the fractional amounts discussed.

...8.5Kw?...that bus had "Kelloggs" wrote on the side of it...but it wasn't full of cornflakes :crazy:

 

[Gavin A]

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 40A 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.

thanks.

It would be a risk to swap it to 40A MCB without changing the cable or moving it out of the insulation as the MCB would then be rated higher than the cable's current carrying capacity in insulation.

The MCB already looks to be higher rated than the cable rating when completely enclosed in insulation. It's probably not been an issue because showers only tend to be used for 10-15 minutes at a time, but leave it on for long enough and that cable could well be in trouble.

 

[Paul.M]

Lets not forget that an MCB can handle 1 & 1/2 times it rating for up to an hour!

Clamp it, do the cable calcs for the installation method and if the the load is too much for the cable, rewire it or reduce the load/change the shower unit.

As for coding, code 3 before investigation and then code 2 once you can see the install is incorrect (going by the info in this thread).

 

[overskilled]

Tbh if I was your boss OP I'd punt you for gross insubordination. Seems like you can't be bothered with the hassle of rewiring the circuit...
Your boss is in the right here. ..

 

[Lazlo]

Basic cable calcs, check the regs book to see if it complies, if it doesn't which in this case it appears not to, it does not meet BS7671 which are the regulations you as a registered electrician are employed by the customer/client etc to work to and ensure all that you do meets these standards, it is the whole point of the game. If you choose to ignore the regs then why bother being registered at all, just connect it all up, if it doesn't go bang, job done? Or let the customer know what the issue is, quote for rectifying the issue, if they choose not to take your advice and don't proceed with the works, issue a dangerous situation report and walk away.

 

[HappyHippyDad]

The obvious here is that the mcb is too small for the load, the only soloution to that is a bigger mcb.

The rating of the mcb will have no bearing on the load current which flows when the shower is operating. It is an 8.5kW load, whatever the mcb is rated at it will draw 35.4A at 240V
Of you fit a lower rated mcb it will just operate faster!

When designing or using calculations we should be using 230V.

It will operate faster, but a 32A MCB with 37A flowing through it will not operate.

 

[telectrix]

in far less time than it's taken to read this thread, i'd have had the cable out of the insulation, with said insulation being consigned to the bin. no cost, installation compliant, down the pub.

 

[davesparks]

When designing or using calculations we should be using 230V.

It will operate faster, but a 32A MCB with 37A flowing through it will not operate.

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.

 

[davesparks]

It would be a risk to swap it to 40A MCB without changing the cable or moving it out of the insulation as the MCB would then be rated higher than the cable's current carrying capacity in insulation.

The MCB already looks to be higher rated than the cable rating when completely enclosed in insulation. It's probably not been an issue because showers only tend to be used for 10-15 minutes at a time, but leave it on for long enough and that cable could well be in trouble.

What is wrong with the mcb being rated higher than the cables ccc? It only needs to be equal to or greater than the load current for a fixed load.

 

[telectrix]

What is wrong with the mcb being rated higher than the cables ccc? It only needs to be equal to or greater than the load current for a fixed load.

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.

 

[Marvo]

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.

So the In of the MCB being < the load current isn't an issue?

 

[Marvo]

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.

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?

 

[davesparks]

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?

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.

 

[marconi]

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?

 

[HappyHippyDad]

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.

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?

 

[Sb8389]

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?

Its about 15 metres max pal

 

[ipf]

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?

Bleedin' 'ell........we've all gone hydro.....dams all over the place

 

[telectrix]

soon fix them dams:

 

[GMES]

Put the Damn handbags away Godammit.

 

[Dave OCD]

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.
I worked with a bloke years ago who had the shower at home through a B16- to stop his teenage kids being in there too long and costing too much with the leccy. This fella is also a Yorkshireman which is purely coincidental. No damn damns from me.

 

[Marvo]

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.

I won't be passing a B&Q anytime soon but it sounds like the manufacturers have cottoned on that people were misunderstanding the kilowatt rating and attempted to clarify by stating the heat output at different voltages.

 

[Simonslimline]

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?

Is it though!!!

The voltage in my flat is around 248. Rarely see the mythical 230 figure anywhere to be honest.

 

[Spoon]

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.

That would help the cable out in the loft, but the OP stated in his original post #1 that "the cable it is going through cavity full of insulation and then through the loft covered in insulation."

 

[overskilled]

Think the Op might be mistaken on the walls unless he thinks the cable is run through cavity wall insulation? . Don't know of many houses built with insulated stud walls pre 1990 build...

 

[Spoon]

I'm presuming the OP is saying the cable through the wall cavity. Will have to wait for the OP to confirm.

 

[marconi]

Its about 15 metres max pal

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.

 

[Midwest]

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?

Can't add to the argument, but I have to say that's quite.....I mean damn funny

 

[Sb8389]

Hi guys,
Just to let you know my boss has convinced the customer to upgrade to 10mm.
Bit of scare tactics going on i think as i overheard him saying that because the MCB is overloaded it is a potential
fire hazard.
so im back there on monday to rewire it.
Cheers for your thoughts and info fellas
Much appreciated.

 

[marconi]

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.

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?

 

[Hellmooth]

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?

Yeah be interesting to find out!

 

[Sb8389]

I did marconi,
It came in at 33.2 amps.

 

[Sb8389]

Voltage reading was 238v incase anyone wanted to know aswell.

 

[soulman]

I cannot see anything wrong with a 8.5kw shower on a 32A cb. I've seen plenty of 8.5kw showers on 32A circuit breakers. A small overload for a short period of time.

 

[Rpa07]

Nearly the best thread ever - time for bed - night Johnboy!