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Topic Title: Current rating of 6.0mm cable for shower
Topic Summary: Disagreement with another spark
Created On: 05 December 2008 06:35 PM
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 05 December 2008 06:35 PM
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Mark30

Posts: 81
Joined: 20 March 2004

Hi

Been asked to wire a 10.8kw shower at someones house.
They said there is already a cable up in the attic ready to be used. When checking the cable for size i noticed it was 6.0mm so told the customer to either down rate the shower or replace the cable with a 10mm.

Got a phone call later on saying they the customer had spoken to another spark who said the 6.0mm cable would be ok and that it could handle the current up to 29m in length.

Who's right?

Doing a quick calc 10.8kw divided by 240v = 45a

Then there are all the derating factors to be applied thermal insulation, distance etc bringing the current carrying capacity down.

So say the 6.0mm will take 45 amps the next breaker applicable is 50a am i right in saying the cable won't take it?
 05 December 2008 06:41 PM
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daveparry1

Posts: 6162
Joined: 04 July 2007

6.0mm may well be ok but I wouldn't use it on 10.5Kw shower. I would just about use 6.0mm on 9.5Kw in favourable circumstances.
 05 December 2008 06:43 PM
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spinlondon

Posts: 4437
Joined: 10 December 2004

I'd go with 10mm2, 46.96amps VD of 6v. 6mm2 for a 8.5kw 36.96amps VD 7.8v.

Edited: 05 December 2008 at 06:50 PM by spinlondon
 05 December 2008 09:02 PM
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rocknroll

Posts: 8837
Joined: 03 October 2005

Got a phone call later on saying they the customer had spoken to another spark who said the 6.0mm cable would be ok and that it could handle the current up to 29m in length.

Who's right?


Another spark

Perhaps he didnt do a guesstimate and considered these options.

Fixed resistive load.
No need for overload protection so Iz <= In.
Adjustment for Ambient temperature
etc;

Fault protection.

The maximum length for a 6mm cable using a 50A MCB is;

Z41 - Ze / (R1 + R2)Cr

Z41 from table 41.3 for a 50A MCB = 0.92

Assume Ze at max TNCS 0.35

R1 + R2 from OSG 10.49 / 1000 = 0.01049

0.92 - 0.35 / 0.01049

So the maximum length for a 6mm cable to be covered by a 50A MCB is 54 metres, but for fault protection you have an RCD anyway

----------------------------------------------------------------------------------------------
Short circuit protection.

Using the same parameters.

Z41 - Ze / 2R1 Cr

R1 = 3.08 / 1000 = 0.00308 x 2 = 0.00616

0.92 - 0.35 / 0.00616

So for short circuit protection only a 6mm cable covered by a 50A MCB gives you a circuit length maximum of 92 metres.

Rockin! whats the problem

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 05 December 2008 at 09:12 PM by rocknroll
 05 December 2008 09:23 PM
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daveparry1

Posts: 6162
Joined: 04 July 2007

Very impressive rock but i'd still go for 10mm on 10.5 Kw!
 05 December 2008 09:32 PM
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beaver74

Posts: 361
Joined: 07 February 2007

Originally posted by: rocknroll

Got a phone call later on saying they the customer had spoken to another spark who said the 6.0mm cable would be ok and that it could handle the current up to 29m in length.



Who's right?




Another spark



Perhaps he didnt do a guesstimate and considered these options.



Fixed resistive load.

No need for overload protection so Iz <= In.

Adjustment for Ambient temperature

etc;



Fault protection.



The maximum length for a 6mm cable using a 50A MCB is;



Z41 - Ze / (R1 + R2)Cr



Z41 from table 41.3 for a 50A MCB = 0.92



Assume Ze at max TNCS 0.35



R1 + R2 from OSG 10.49 / 1000 = 0.01049



0.92 - 0.35 / 0.01049



So the maximum length for a 6mm cable to be covered by a 50A MCB is 54 metres, but for fault protection you have an RCD anyway



----------------------------------------------------------------------------------------------

Short circuit protection.



Using the same parameters.



Z41 - Ze / 2R1 Cr



R1 = 3.08 / 1000 = 0.00308 x 2 = 0.00616



0.92 - 0.35 / 0.00616



So for short circuit protection only a 6mm cable covered by a 50A MCB gives you a circuit length maximum of 92 metres.



Rockin! whats the problem



regards


my head hurts

Edited: 05 December 2008 at 09:33 PM by beaver74
 05 December 2008 10:18 PM
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spm

Posts: 194
Joined: 23 October 2005

but if manufactures state 10mm then you should use 10mm so far as i know
otherwise 6mm is ok infact useing a 40A mcb is ok cos it will run at 46A for hours so 50A mcb not needed
 05 December 2008 10:34 PM
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Testit

Posts: 2962
Joined: 06 August 2007

Well I might look at it this way..

If installing a new cable I might go for 10mm

For an existing cable and querying a 10.5KW shower a quick glance at OSG/BS7671 gives 47amps clipped direct.. you can get away with the overload protection by applying 433.3.1 assuming the PSCC is catered for by the cable, which for 6mm it likely is.. Its a bit of a fine comfort line mind, but you could argue its OK but not ideal.

I would likely go for a 40amp mcb though.. to give some measure of protection that would be safer than a 50amp MCB and it would be unlikely to trip for the duration showers are on and provide adequate functioning in daily use.. I wouldnt personally put an mcb in place that made the primary circuit attached to it without overload protection... a 40amp mcb I would consider a better option that would retain functionality.

In short I would install the shower and make a statement of reasoning on any certification provided.

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 06 December 2008 01:55 AM
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unshockable

Posts: 845
Joined: 18 May 2007

433.1 Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

You may say it ain't long, but don't say it ain't likely.

I love to make use of omission, particularly for showers, but aside from taking account of the power quoted being for 240V as opposed to our actual 230V, and therefore lower, I keep that drawn power<= Iz and select the next MCB up for fault protection.

One day sweet lord, I will understand what RnR is saying. Today is no that day and tomorrow looking unlikely.

Simon
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