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Topic Title: Parallel submain
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Created On: 11 April 2013 12:25 PM
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 11 April 2013 12:25 PM
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Parsley

Posts: 960
Joined: 04 November 2004

Hi all

Parrallel SWA 4c Submain directly buried in ground cables likely to be touching supplying a panel board protected by BS88 fuses.

Am I right in thinking Cc should be 0.9 from appendix 4 5.1.1 and Cg 0.8 from 4C1?

Regards
 11 April 2013 01:53 PM
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John Peckham

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Yes but you need to consider depth of burial and soil resistivity as well. If you email me the cable, load and CPD details I will do you a calc. on Amtech for you.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 11 April 2013 02:04 PM
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Parsley

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Thanks John

I will send you what I have, my Amtech and paper calc doesn't comply. The designer seems to have just used 4D4B value.

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 11 April 2013 02:25 PM
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Jaymack

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Originally posted by: Parsley
Parrallel SWA 4c Submain directly buried in ground cables likely to be touching supplying a panel board protected by BS88 fuses.
Am I right in thinking Cc should be 0.9 from appendix 4 5.1.1 and Cg 0.8 from 4C1?

I don't see the relevance of the fuse protection. Apart from the conditions of laying etc., I would just assume that the cable is designed to carry half the current, and treat it as a standard calculation for 2 independent cables. Ensure that instructions are given to accurately match the lengths, otherwise there will be circulating currents causing overheating.

Regards
 11 April 2013 03:00 PM
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Parsley

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Thanks Jaymack

I wasn't concerned about the 88's, I was just mentioning what was protecting the submain, it's not my design or install but I've been asked to look. It's for another division of the company I work for who are using a subbie.

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 11 April 2013 03:23 PM
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AJJewsbury

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I don't see the relevance of the fuse protection. Apart from the conditions of laying etc., I would just assume that the cable is designed to carry half the current, and treat it as a standard calculation for 2 independent cables. Ensure that instructions are given to accurately match the lengths, otherwise there will be circulating currents causing overheating.

I guess the mention of the fuses hints at overload protection.

The 17th has made calculating CCC of underground cables a lot more complicated, especially where overload protection is required. As far as I can tell, they've recalculated the tabulated values based on 20-degree surroundings rather than 30 - so tabulated values look higher. But that undermines the implicit 1.45x safety margin we usually rely on for overload protection, so if overload protection is needed, that needs to be factored back in - using Cc.

The of course, the two cables are grouped - each warming the other, so reducing its CCC a little.

There's a bit about it here: http://electrical.theiet.org/w...ix-4-csa.cfm?type=pdf

- Andy.
 11 April 2013 08:28 PM
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Parsley

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Andy that's useful

It will be interesting to see what correction factor values the designer used.

Thanks JP


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