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.

 

[Sb8389]

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.

 

[Lucien Nunes]

How on earth would a lower rated mcb help the situation?

It converts a 'possible cable overheating' risk into a 'shower user jumps out of cold shower and goes to reset MCB with wet hands' risk. I've lost track, was this thread about solving a problem or creating one?

 

[HappyHippyDad]

How did you get to 37A from an 8.5kW shower?
A 40A mcb would be correct for the load

I think you may be trying to push me towards saying the obvious here :smile5:

How on earth would a lower rated mcb help the situation?
The mcb does not determine the load current!

The OP has stated the reference method. i.e 'covered in insulation', so the CCC of the cable is reduced, would you suggest increasing the MCB?

 

[Marvo]

Operating current was a poor choice of words in this case.
I don't think it will be rated to operate at 35A at all.

According to the published curves all MCB's are designed to be operated above their In and perform to the predictable disconnect times which in this case, given it's a fixed load and a disconnect time far in excess of the duration of use, will not be of consequence. I think it's a stretch to say this particular circuit is overloaded just because the load is 1.16 times the In and assuming the cable size is appropriate.

 

[micknew]

Seems like the op is looking for reasons to ignore the regs.
Surely they must be adhered to?
If not throw the rule book away lol.

 

[Lucien Nunes]

Agreed - we know it's non-compliant - but it raises the question: is it dangerous to have In<Ib when in practice we know it 'never' trips? Especially if In<Iz so the cable is theoretically protected.

 

[HappyHippyDad]

Actually a 25A MCB will probably not trip at 37A or at least take a couple of hours (looking at the tables), so this could be your answer to the lower CCC of the cable due to reference method!

 

[micknew]

Yeah, why bother with regulations if you know better?

 

[davesparks]

I think you may be trying to push me towards saying the obvious here :smile5:



The OP has stated the reference method. i.e 'covered in insulation', so the CCC of the cable is reduced, would you suggest increasing the MCB?

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!

 

[davesparks]

Actually a 25A MCB will probably not trip at 37A or at least take a couple of hours (looking at the tables), so this could be your answer to the lower CCC of the cable due to reference method!

That won't solve anything! The load will still be 35.4A regardless of what size mcb you fit.
The cable needs to be rated for the load current.

 

[davesparks]

Agreed - we know it's non-compliant - but it raises the question: is it dangerous to have In<Ib when in practice we know it 'never' trips? Especially if In<Iz so the cable is theoretically protected.

My point is more to do with whether Ib<Iz as In does not need to be <Iz for a fixed load

 

[Lucien Nunes]

Sure, but follow the twisted logic here. Suppose the cable has an Iz that on paper is inadequate. It is protected by an MCB having In<Iz. This means In is also inadequate, but suppose also that we know from the curves that it will never trip. Is the cable overloaded?

 

[Marvo]

Agreed - we know it's non-compliant - but it raises the question: is it dangerous to have In<Ib when in practice we know it 'never' trips? Especially if In<Iz so the cable is theoretically protected.

This is where I was going but you put it better than I could. I guess the question in the UK would be how would you code it or would you even code it if you were doing an EICR type inspection? I personally couldn't class this as dangerous in any way and I'd be hard pushed to even say it needed improvement from a purely technical viewpoint.

 

[davesparks]

Sure, but follow the twisted logic here. Suppose the cable has an Iz that on paper is inadequate. It is protected by an MCB having In<Iz. This means In is also inadequate, but suppose also that we know from the curves that it will never trip. Is the cable overloaded?

If the Iz is inadequate then that implies it is less than the load current so yes it is overloaded.

 

[davesparks]

This is where I was going but you put it better than I could. I guess the question in the UK would be how would you code it or would you even code it if you were doing an EICR type inspection? I personally couldn't class this as dangerous in any way and I'd be hard pushed to even say it needed improvement from a purely technical viewpoint.

Then what is the point of having the ratings that we do have?

 

[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.