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Topic Title: Over cautious with shower cable?
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Created On: 08 January 2015 08:39 AM
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 08 January 2015 08:39 AM
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ahoneahtwo

Posts: 87
Joined: 15 February 2014

replacing an existing 9Kw elec shower that is currently running on 6mm cable. Now, I realise under ideal conditions the cable is perfectly capable of taking the load, but I have no way of knowing any of the run, other than it starts off buried in the wall, so obviously it may be subject to de-rating. I am of the opinion an 8.5 might be a better bet, to be on the safe side. Anyone agree or am I being over cautious? There is no chance of examining the run without major upheaval and even less of running anything bigger.
 08 January 2015 09:40 AM
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mapj1

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Ii the breaker protects the cable, the load at the end is less important. If it has been running for years without issue, then its likely to continue.

If you take out a 9kW and its all fine, then putting another one like it or anything smaller back in its place is unlikely to make it suddenly worse. Don't go bigger than what you took out though.

But, in any case, worth noting on the invoice that old cable re-used and the route could not be confirmed and may not comply with current regs. For all you know it runs diagonally accross that wall with all the photos nailed up!! So long as customer OK with re-use of old cable, no reason you not to be.

Edited for clarity.

-------------------------
regards Mike


Edited: 08 January 2015 at 03:56 PM by mapj1
 08 January 2015 11:26 AM
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daveparry1

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I would stick with 8.5kW in this situation.
 08 January 2015 11:59 AM
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geoffsd

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I am puzzled.

Why is there any reason to doubt the replacement of the shower?

39.6A @ 240V (38A @ 230V) on 47A cable.
The OP seems to think a cable buried in wall warrants derating.

Shall we derate every replaced appliance because the cable installation method is not seen?
 08 January 2015 12:13 PM
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ahoneahtwo

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I said the cable started if buried in the wall, and that the rest of its run is unknown . It could quite easily pass through insulation, and no I am not suggesting we de-rate every appliance. The point I was making is that for a relatively high current consuming item I was going to err on the side of caution. It's not a 47a cable if it happens to be going through insulation or any other factor is it?
 08 January 2015 01:07 PM
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Zimmerman

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If clipped direct then the cable would carry 47A at best (Method C OSG) However, if any other method is applicable, then the cable is undersize.

Please remember, if we change the load placed on a cable then we are responsible for our actions and could be asked to prove our decisions, and calculations in a court of law when required. The answer to the question regarding 'Shall we de-rate every replaced appliance because the cable installation method is not seen? is of course YES if we are increasing the loading on the supply cable we need to make an assessment . That is a requirement not an opinion. It is our responsibility to assess the situation based on our knowledge, qualifications and experience.

If not clipped direct or if in any reasonable doubt then...de-rate. I would go for the like-for-like option in this case. We are de-rating the cable not the load.
 08 January 2015 01:12 PM
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ahoneahtwo

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Originally posted by: Zimmerman

If clipped direct then the cable would carry 47A at best (Method C OSG) However, if any other method is applicable, then the cable is undersize.



Please remember, if we change the load placed on a cable then we are responsible for our actions and could be asked to prove our decisions, and calculations in a court of law when required. The answer to the question regarding 'Shall we de-rate every replaced appliance because the cable installation method is not seen? is of course YES. It is our responsibility to assess the situation based on our knowledge, qualifications and experience.



If not clipped direct or in any reasonable doubt then...de-rate. I would go for the like-for-like option in this case.
Thanks Zim that's exactly along the lines I was thinking, although I am still going to err on the side of caution. I can see your train of thought though regarding like for like, but it still doesn't make it right if the cable route is unknown
 08 January 2015 01:49 PM
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Zimmerman

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That is true, but we are not magicians we can only do our best. You have done more than most :-).

If you put your concerns in writing to the client and recommend that they should consider a new circuit (provided that all tests have been carried out successfully with the old circuit and it is just your opinion given the reason above) you can prove that you have taken everything into consideration and have gone as far as anyone could reasonably ask.

Rob
 08 January 2015 03:02 PM
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Parsley

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Is the shower rated at 9KW with 240V or 230V? I can't make your figure work Geoff.
If there's no signs of overheating like Mike I wouldn't be too concerned about the 6mm2. You could argue that overload protection isn't required it's a fixed load isn't it. How long is the average shower on for each time it's used even with a largish family.

However if it's rated at 9KW at 230V and you have 240V+ on site then it might be closer to a 10KW shower.

Regards
 08 January 2015 03:23 PM
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Zimmerman

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You pays you money...he who signs wins (or do not pass go)
 08 January 2015 04:01 PM
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mapj1

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Without knowing the construction, yes it may go though a cavity wall that is now full of insulation, or a loft or partition.
Plastered against the bricks is quite well cooled.

But, you are not breaking any rules if you start with a non-complaint installation so long as you make it no worse. If the users report a burning smell when showering or there are burnt bits in the wall behind, then rethink, but I think its OK - maybe not ideal from a voltage drop perspective if its a long run - have you done a PSSC/ ELFI test? but scarcely the worst job in the world.

-------------------------
regards Mike
 08 January 2015 04:17 PM
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ahoneahtwo

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Originally posted by: Parsley

Is the shower rated at 9KW with 240V or 230V? I can't make your figure work Geoff.

If there's no signs of overheating like Mike I wouldn't be too concerned about the 6mm2. You could argue that overload protection isn't required it's a fixed load isn't it. How long is the average shower on for each time it's used even with a largish family.



However if it's rated at 9KW at 230V and you have 240V+ on site then it might be closer to a 10KW shower.



Regards

"How long is the average shower on for each time it's used even with a largish family". - I had a call from a customer that turned her shower on and then went to answer the door (she didn't mention her state of dress) and then realised an hour and a half later it was still running!! Only damage was the 30A rewirable popped. Must be made of money..........
 08 January 2015 04:31 PM
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geoffsd

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Originally posted by: Parsley

Is the shower rated at 9KW with 240V or 230V? I can't make your figure work Geoff.


My figures were for 9.5kW @ 240V


However, my apologies to the OP and others.

I thought the original was 9.5kW as well which made me puzzled why there was a query.
 09 January 2015 06:55 AM
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davezawadi

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Gentlemen, again you misunderstand how cable ratings work. Worrying whether this cable is satisfactory because it could pass the odd amp or two over the rating which you do not know (because you do not know the path) is foolish and the stuff of bad dreams. Lets protect the circuit with a 40A breaker (type B), and now what do you think?

Say we move to a 50A breaker, then what?

Say there is a 300mm bit in insulation, does this change anything?

If the whole length is in a cavity filled with insulation, then what do you think?

Lets try again, does a 500W change in load change anything? Not really.
Say I fit a 12kW shower and the same 40A breaker, will the customer complain? Almost certainly not unless they live in the shower.
Is the installation likely to catch fire? Not at all, it is very unlikely the cable will ever get anywhere near 70C conductor temperature.

Answers can now be discussed, or on a postcard to me if you want the full half hour!

Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 09 January 2015 09:16 AM
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Zimmerman

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Thanks Dave. I'll stick with the guidance mate. That way I'll be almost sure to stay on the right side of the EWR and not rely on a few lines by an unknown writer on an electrical forum. I have little doubt that you know what your talking about and in no way do I wish to dispute what you say but I'll stick to my way of doing things and follow like a sheep if need be. I'll stand by what I know. If people have not got the 'magic formula' of being wiser then the official guidance take things into their own hands all sorts can, and do happen. It is foolish to advice people to step outside those guidelines. I respectfully suggest that you make your feeling known to the IET and they can then give us yet another revision of BS7671 as soon as they are short of money for the next Christmas party.

To change the MCB rating could involve a problem with max Zs and please don't go on about 1667 Ohms as it's on a RCD as that old chestnut is way past it's 'sell by date'.

I was under the impression that cavity wall insulation had a detrimental effect on PVC and therefore if a cable was run in the cavity, and in contact with the insulation that would be a problem in itself.
 09 January 2015 01:01 PM
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davezawadi

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No Zimmer you have it wrong. I don't know your provenance either, but clearly the following need to be addressed.

Thanks Dave. I'll stick with the guidance mate. That way I'll be almost sure to stay on the right side of the EWR and not rely on a few lines by an unknown writer on an electrical forum. I have little doubt that you know what your talking about and in no way do I wish to dispute what you say but I'll stick to my way of doing things and follow like a sheep if need be. I'll stand by what I know. If people have not got the 'magic formula' of being wiser then the official guidance take things into their own hands all sorts can, and do happen. It is foolish to advice people to step outside those guidelines. I respectfully suggest that you make your feeling known to the IET and they can then give us yet another revision of BS7671 as soon as they are short of money for the next Christmas party.

I am sure that you understand the requirements for a CPD for a circuit? What is wrong with fitting a CPD with a tripping current less than the cable rating? I refer you to the BYB (or green if you haven't got it yet). Look at the charts of MCB characteristics at the back. Do you notice something about my post?

To change the MCB rating could involve a problem with max Zs and please don't go on about 1667 Ohms as it's on a RCD as that old chestnut is way past it's 'sell by date'.

What is that supposed to mean, do you not know the regulations? Zs is not an issue if there is an RCD in the circuit. What does "old chestnut" mean? Zs is easily measured and then you can make an informed decision.

I was under the impression that cavity wall insulation had a detrimental effect on PVC and therefore if a cable was run in the cavity, and in contact with the insulation that would be a problem in itself.

It depends on the exact insulation but the effect is not serious to the cable performance as long as it is not flexed, which it cannot be if in polystyrene! The actual effect is leaching of the plasticiser which leaves the cable rigid and somewhat brittle. Other insulations have no effect. The plasticiser attacks the polystyrene and reduces the insulating value locally, but effect minor and not electrical.

Clearly you are miles away from knowing the basics, and even the OSG is probably too difficult for you. I suggest you contact a competent electrician, many of whom you will find subscribing to this forum. Why ask the question if you don't want or understand the answer?

I am not being any wiser than the "official" guidance, I am simply using BS7671 to answer the question you asked.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 09 January 2015 03:04 PM
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Zimmerman

Posts: 96
Joined: 31 July 2009

Sorry Dave, I thought you had written 'change a 40A for a 50A unit.

TN system: regarding the 1667 Ohms of a 30mA RCD. The reason for looking at the max Zs is that in the case of a failed RCD the circuit would at least open in a live to earth fault. Again my mistake.

I bow to you wisdom on the subject of electricity.
You view of me =

'Clearly you are miles away from knowing the basics, and even the OSG is probably too difficult for you. I suggest you contact a competent electrician, many of whom you will find subscribing to this forum. Why ask the question if you don't want or understand the answer? '

Edited: 09 January 2015 at 04:37 PM by IET Moderator
 09 January 2015 03:18 PM
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Parsley

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Ouch!
 09 January 2015 03:24 PM
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Zimmerman

Posts: 96
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This bloke should be on the stage. A one-man-regs-book (his own version by the look of it) It should only be about 5 pages including the index. Who needs guidance when the pin-ball-wizard is about ;-)


Just had a look at some of your posts Dave. 50 years in the trade and you've not been found out yet... a good run son!

Edited: 09 January 2015 at 03:45 PM by Zimmerman
 09 January 2015 04:06 PM
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davezawadi

Posts: 3848
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Well I never!

Pity there were no answers to my simple questions, then we could really have had some fun. Rude gets the usual treatment.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
IET » Wiring and the regulations » Over cautious with shower cable?

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