8.5kw Shower on a 6.0mm cable

[HappyHippyDad]

it may just need the correct mcb to be fitted for the load

Not quite sure what you are getting at here? The load is 37A. What MCB would you suggest to make this situation ok?

 

[PEG]

....Is anyone going to clamp this,and have at least one known factor? :winkiss:

 

[davesparks]

Not quite sure what you are getting at here? The load is 37A. What MCB would you suggest to make this situation ok?

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

 

[HappyHippyDad]

I see your point happyhippydad but to determine how it's installed throughout its length can be difficult in a lot situations.
it was installed on 30amp before and there is no evidence of overheating to the cable. I know that don't confirm its right to leave it but does that mean we have say to the customer it will need rewiring when we come across this situation when we can't determine its installation method.

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!

 

[davesparks]

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!

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

 

[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?