11kw shower cable size?

[alanl]

6mm clipped direct just on the limit, but you'd need a OCPD of 50A, so 6mm is out of the equation.

Why?
433.3.1 (ii)
A shower is a fixed resitive load, so very unlikely to be subject to overload current apart from a fault.
It has fault protection via the RCD (and the 50A CB), any fault will be cleared in less than 0.3 seconds that the RCD takes, so a 50, or even 100A CB will be compliant.
(of course so long as cable length, Zs, grouping/reference all comply too)

 

[telectrix]

the shower is 47A so the OCPD needs to be >47A but less than the Iz of the cable. so a 50A MCB. this means that the cable Iz must be > 50A so it's got to be 10mm.


what's the RCD got to do with overload protection?

 

[alanl]

No, sorry, you are wrong.
It is a fixed resistive load, so does not need overload protection, it must have fault protection, which the RCD will provide.

 

[telectrix]

nothing to do with the type of load. it's the cable that needs overload protection. what's the RCD going to do in the case of a short circuit L-N?

 

[alanl]

433.3.1
There is no need for circuit protection if the load is unlikely to carry overload currents. A shower will not overload unless it has a fault. No-one is likely to fix the circuit directly to the outgoing RCD terminals, so fixing it to a CB rated higher than the current carrying capacity of the cable is quite acceptable in such circumstances, so long as the cable can carry the current of the appliance.
A L-N fault will cause many hundreds of amps to flow (in most installations), meaning any CB fitted, whether 50 or 100A will trip virtually instantly. It is only in fault conditions that a L-N short will occur.
Hence it is compliant to run a cable with a 47A capacity, feeding a 45A fixed load appliance, fed via a 50A CB, as there will be fault protection, both via any CB fitted, and via the RCD which will have to be fotted in any domestic premises. There will be no overload protection, but that isnt required, as stated in 433.3.1.

 

[Toonlad]

Raises eyebrow........mmmmmmmmmmmm........:thinking2:

 

[44ALLAN]

Raises eyebrow........mmmmmmmmmmmm........:thinking2:

Lol.

Wheres me books....................................

 

[Toonlad]

mines in front of me. Im thinking BS1361 60/80 amp fuse now

 

[telectrix]

i'll accept that if , and only if, the circuit Zs is sufficiently low enough to cause the OCPD to trip within the required time. e.g. < 0.37ohms for a 100A type B MCB. say the Ze was 0.2, then your R1+R2 for the 6mm cable would need to be < 0.17, i.e a circuit length of not more than 15m. if the Ze was > 0.3ohms, then you're stuffed.

 

[44ALLAN]

mines in front of me. Im thinking BS1361 60/80 amp fuse now

I'm thinking gravity fed shower off the boiler sod all this palava :shocked3:

 

[Darkwood]

No, sorry, you are wrong.
It is a fixed resistive load, so does not need overload protection, it must have fault protection, which the RCD will provide.

If your relying on this method the fault current characteristics of the overload device also need to be calculated and complied to as rcd protection only covers down to earth also to note that the characteristics of the fixed load should be noted that breakdown or aging of element can't cause long time small overloads (been a shower it will usually have thermal cut-out), lastly the cable natural deterioration shouldn't create small overload risk nor should you run the cable where thermal insulation maybe added at a future date.. i.e. running it through an un-insulated loft space.

 

[telectrix]

I'm thinking gravity fed shower off the boiler sod all this palava :shocked3:

much rather have a bath. showers are terrible for diluting your scotch.

 

[Outspoken]

433.3.1
There is no need for circuit protection if the load is unlikely to carry overload currents. A shower will not overload unless it has a fault. No-one is likely to fix the circuit directly to the outgoing RCD terminals, so fixing it to a CB rated higher than the current carrying capacity of the cable is quite acceptable in such circumstances, so long as the cable can carry the current of the appliance.
A L-N fault will cause many hundreds of amps to flow (in most installations), meaning any CB fitted, whether 50 or 100A will trip virtually instantly. It is only in fault conditions that a L-N short will occur.
Hence it is compliant to run a cable with a 47A capacity, feeding a 45A fixed load appliance, fed via a 50A CB, as there will be fault protection, both via any CB fitted, and via the RCD which will have to be fotted in any domestic premises. There will be no overload protection, but that isnt required, as stated in 433.3.1.

You are a fool Sir, and a very dangerous one too boot. Reading the regs and understanding the regs are two totally different things. It states on my driving license that I should tear along the dotted line, but does that mean 70 down the middle of the road...of course not (although MB's seem to do it!), if you are going to quote a Reg, namely 433.3.1, then please read it properly and expect those who have functioning brains and experience on their side to interpret it as is intended and not as an excuse for shoddy and dangerous work.

 

[Toonlad]

Well we could take it to another level,, Whats cleaner a shower or a bath ?? Smartas5 comments welcome also !

 

[44ALLAN]

Heaven forbid....................never seen any smart as% comments on here !

I used to rinse my hair with the shower whilst I was in the bath so can't comment lol

 

[Toonlad]

Heaven forbid....................never seen any smart as% comments on here !

I used to rinse my hair with the shower whilst I was in the bath so can't comment lol

Got to state the obvious, the guys tend to overlook the obvious at times.

 

[alanl]

if you are going to quote a Reg, namely 433.3.1, then please read it properly and expect those who have functioning brains and experience on their side to interpret it as is intended and not as an excuse for shoddy and dangerous work.

Please explain why it is wrong then?
433.3.1"A device for overload protection need not be provided:
"for a conductor, which because of the characteristics of the load.. is not likely to carry overload current, provided it is protected against fault current..."
A shower is a fixed resistance load. You cannot overload it above the rated current, except under the most extreme conditions, if at all. If it fails, there will be a fault. It will be protected by the CB, even if that CB is rated higher than the current carrying capacity of the cable.
Of course, Zs, volt drop and other factors come into play, but using a 50A CB on a 6mm clipped direct cable for a shower DOES comply (if other factors permit).If you think it does not, then please, rather than plain abuse, explain how it cannot be used.
Then, maybe go onto the IET forum, and ask the people there to see if they agree with your view. Some people there contribute and write the Regs, so you will get the right answer.

 

[Outspoken]

alanl, please quote all sections and statement contained within 433.3.1 and then in writing it out, word for word, you may understand why your incorrect. The Regs are not multiple choice, although the way they are written you sometimes wonder.

I am specifically directing you to 433.3.1(i) and would ask that you read and absorb that before you then read 433.2.1, especially the note underneath it.

I would ask that you explain why you believe your statement is correct after reading and absorbing those regulations.

 

[Outspoken]

In case your brain is struggling alanl...here you are;

433.3.1 General

A device for protection against overload need not be provided:

(i) For a conductor situated on the load side of the point where a reduction occurs in the value of current carrying capacity, where the conductor is effectively protected against overload by a protective device installed on the supply side of that point.

​

(ii) For a conductor which, because of the characteristics of the load or supply, is not likely to carry overload current, provided that the conductor is effectively protected against fault current is accordance with the requirements of section 434

(iii) At the origin of an installation where the distributor provides an overload device and agrees that it affords protection to the part of the installation between the origin and the main distribution point of the installation where further overload protection is provided.

​

433.2.2

Except where regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current carrying capacity of the conductors of the installation.

Note: A reduction in the current carrying capacity may be due to a change in the cross sectional area, method of installation, type of cable or conductor, or in environmental conditions.
434.2 as mentioned above in 433.2.1(ii) deals with the correct positioning of devices detailed with providing protection against fault currents and follows;

433.2 Position of devices for the protection against fault currents

A device for providing protection against fault currents shall be installed at the point where a reduction in the cross sectional area or other change causes a reduction in the current carrying capacity of the conductors, except where Regulation 434.2.1, 434.2.2 or 434.3 applies.
Now under definitions BS7671 states the following;

Residual Current Device (RCD). A mechanical device or association of devices intended to cause the opening of contacts when the residual current attains a given value under specified conditions.

Overcurrent: A current exceeding the rated value. For conductors the rated value is the current carrying capacity.

Overload Current. An overcurrent occurring in a circuit which is otherwise sound.

Now whilst the resistance of a shower element(s) may be fixed, it is possible for them to overcurrent due to influences that have so far not introduced an fault current, such an occurrence can occur when the shower is installed in a hard water area and they become heavily scaled, causing the element to overheat due to the lack of convection of the heat generated, this increase in heat will alter the electrical properties of the resistive load and can cause it to draw more energy from the supply causing an overload, this is a fault that will eventually cause a failure of the element(s) and would cause an RCD to operate, however this is not what an RCD is there to provide, they are for additional or secondary electrical safety where a fault condition occurs that is insufficient to operate an over current device but may present a danger to users.

Now I would like you to please explain to this gaggle of highly qualified, experienced and knowledgeable Electricians and Engineers why you now believe you are still correct, and if your going to quote Regs, please do not snip them, but quote the whole sections and then attempt to argue your case.

 

[telectrix]

i assume you refer to the reduction in conductor size at the OCPD, where the incoming conductor is the busbar, and ther outgoing is the 6mm.????