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Topic Title: Underground Cables
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Created On: 21 December 2012 08:40 AM
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 21 December 2012 08:40 AM
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NickEbsworth

Posts: 6
Joined: 19 December 2011

When I select a cable size for underground use (burried in a duct) I select the current carrying capacity from table 4E4A, reference method D (direct in ground or in ducting in the ground or in or around buildings)

Then I look at the section 5.1.1 and then have to apply a correction factor of 0.9 because my cable is "in a duct in the ground"

Any ideas why I have to apply the 0,9 correction factor when the table has already taken this into account? Is this an example of 'double accounting"?
 21 December 2012 09:17 AM
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AJJewsbury

Posts: 11373
Joined: 13 August 2003

I've a feeling that Cc isn't because the cable is buried in a duct, but because the buried cable tables are for an ambient temperature of 20 degrees rather than the usual 30 degrees - hence they appear to have a higher rating than they otherwise would. This however undermines the cable's overload safety margins somewhat, so where overload protection is required the 0.9 factor is needed to bring it up to par with the normal tables.
- Andy.
 21 December 2012 09:26 AM
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NickEbsworth

Posts: 6
Joined: 19 December 2011

Ah! That makes some sense. In my case we do not need overload protection due to the nature of the load and the nature of the fault conditins that may be present, so I probably can safely ignore the 0.9 factor I guess.
 21 December 2012 09:53 AM
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OMS

Posts: 19593
Joined: 23 March 2004

so I probably can safely ignore the 0.9 factor I guess.


to safely ignore it you would need a bit of knowledge of the ground thermal resistivity.

Given that cable is cheap compared to trenching and if buried direct you have no room for future change, I'd leave the 0.9 factor in place regardless.

Do the calc a few times - if you find the difference in Ib and It gives you a specific cable size and then you test that with or without the 0.9 factor to see if the cable size grows or not. If it does, then your margin between Ib and It is probably too small anyway.

If you are revalidating an existing cable, then a bit of fiddling with the 0.9 factor and temperature is often useful to get the anser you (or your client) wants

regards

OMS

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