8.5 kw shower

[beanzntoast]

Its me,
Hi all i'm abit confused at minute after reading past posts so it's just abit of clarification i'm looking for.Scenario an 8.5 kw shower is wired in 6mm2 cable (in plastic conduit or trunking) no insulation along it route.8500/230=37amp,Circuit length 30m so volt drop 8.1.So it is my understanding this should be protected by (Forgetting the RCD for now) a B40a Mcb.But this is where i get confused because i have also read that a 6mm2 cable should have no more than a B32amp(obvously because of CCC).If a B32amp is fitted then wouldn't the circuit be classed as overloaded(correct me if i'm wrong & explain please).So what is the solution apart from up grading the circuit to 10mm2(or would you all recommend this upgrade anyway for an 8.5 kw)Go easy on me,thanks in advance for any replies
Jason

 

[pushrod]

If you put your figures into one of those cable selector tables you get your 6mm cable size, but if you up it to just 9kW you get it giving a 10mm cable size. You are obviously right on the max for 6mm and it would be ok if you could get an mcb at that point. Far better to go for the 10 mm cable - which will allow for any slight problems and allow a future upgrade of the shower.

 

[beanzntoast]

Anymore advice from other people would be appreciated,could someone explain why it would be acceptable to use this shower on a B32amp MCB.I understand why a 32amp MCB is used to protect the 6mm2 cable but i was also at an understanding that you work out the design current of the load the you would use & select the next size rating up for the MCB ( i.e Ib should be less than or equal to In).Am i missing something here if so could someone please explain it to me.Maybe it could be my lack of hands on experience but when i learn something from a book & studying & it gets contradicted by experienced sparks i just like to know why ,thanks jason

 

[zupos40]

Anymore advice from other people would be appreciated,could someone explain why it would be acceptable to use this shower on a B32amp MCB.I understand why a 32amp MCB is used to protect the 6mm2 cable but i was also at an understanding that you work out the design current of the load the you would use & select the next size rating up for the MCB ( i.e Ib should be less than or equal to In).Am i missing something here if so could someone please explain it to me.Maybe it could be my lack of hands on experience but when i learn something from a book & studying & it gets contradicted by experienced sparks i just like to know why ,thanks jason

If 32A mcb tripped at current slightly over 32A then it would be a problem, but MCB take a certain amount of overload for a given period (this is called convential time and is 1 hour) it happen that for a type B breaker the factor is 1.45, as a result a 32A type B with take upto 32A x 1.45 = 46.40 A for upto 1 hour, so 37A would not cause it to trip

As a result most showers are only used for 10 to 20 minutes, as a result breaker will not come out on overload.

Personally I always install 10mm twin+ cpc and 40A type B mcb, so cable does not get as hot, less voltage drop, if also allows for client changing shower for a higher kW rating.

 

[pushrod]

The mcb is there to protect the installation so 6 mm is protected by anything less than its max current carrying capacity. In your scenario it is not ok to use the 32A mcb because it will not allow the design current to flow and so the shower could not work properly. If the design current was 28A you would still use 6 mm cable and then a 32Amcb would be used to protect the cable but it would still allow the Ib (design current) to flow. What you are saying below is completely correct.

"but i was also at an understanding that you work out the design current of the load the you would use & select the next size rating up for the MCB ( i.e Ib should be less than or equal to In)".

In your scenario you would need a 38A mcb, but they don't exist and i don't think (haven't checked) that the 40 A mcb would protect the 6mm cable, therefore you go up to 10mm cable and a 40Amcb.

 

[electricAl]

Good to ask the questions...better than guessing i always think

You`re quite correct in your reasonings. Altho fitting a derated MCB may be acceptable to ease the suffering of a customer who`s been left with a badly designed circuit (Pushrod`s point i suspect ), it certainly aint acceptable if you`ve installed it (which is obviously you`re point)

The over-generalising statements you may hear re: "a 6mm has a 32A MCB" should be discounted. Do as you`re doing - calculate. Each circumstance varies.

Whilst the shower won`t pull enough to trip a 32A in any great hurry (if atall in normal operating times) to do the job right, you`d need that 40, which is close on the limit of the 6mm`s ccc (46A)
However, the 46A is for Ref Method C - ideal, clipped direct installation conditions. As you stated conduit install then it`ll be Ref Method B, in which case ccc is only 38A - very tight. Too tight infact.

Truth is, in the real world, it`s far better to install 10mm - the small extra cost is well worth the saving in hassle to you (the designer) & also to your customer (who may wish to upgrade the shower in the future)

So 10mm it is & a B40


p.s > Sorry if i`ve repeated above points - London buses & all that. At least there`s a consensus for you Beanz