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Topic Title: The CPC conductor in T&E // & // Table 4D5
Topic Summary: csa query on CPC // & // 4D5 for cable doubling
Created On: 12 July 2013 12:15 PM
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 12 July 2013 12:15 PM
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burke3

Posts: 24
Joined: 12 January 2006

I don't know what I would do without this super forum. I have a couple of strange questions ....


(1) BS7671 table 54.7 indicates that the CPC & line conductors are same CSA upto 16mm2. This is true also for other standards like EN60204. So, what then makes T&E a valid cable to use anywhere when its CPC is less than the line conductors.

(2) table 4D5 gives the ampacity in free air for a single run of flat T&E prior to application of the list of derating ambient factors.
For a ring main, where the cable run is in effect a dual run, would this mean that the ampacity for the ring configuration is doube the value from the table prior to application of the derating factors.


thank you for your thoughts.


Joey
 12 July 2013 12:44 PM
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Parsley

Posts: 1090
Joined: 04 November 2004

Reg 543.1.3 normally enables a smaller cpc to be selected than table 54.7 requires.

Take a look at 433.1.1 and 433.1.103 for the ring final circuit.

Regards
 12 July 2013 01:03 PM
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AJJewsbury

Posts: 11772
Joined: 13 August 2003

(1) BS7671 table 54.7 indicates that the CPC & line conductors are same CSA upto 16mm2. This is true also for other standards like EN60204. So, what then makes T&E a valid cable to use anywhere when its CPC is less than the line conductors.

Table 54.7 is just one of the available options for obtaining a c.p.c. size - it's also allowed to be calculated according the the circumstances, which almost always results is a much smaller size. See reg 543.1.1.

(2) table 4D5 gives the ampacity in free air for a single run of flat T&E prior to application of the list of derating ambient factors.
For a ring main, where the cable run is in effect a dual run, would this mean that the ampacity for the ring configuration is doube the value from the table prior to application of the derating factors.

I don't think 4D5 covers 'free air' (method E), but I take your point that it applies to a single cable and a drop from socket would usually consist of two cables bunched - both of which could be carrying their full rated current (if the ring was fully loaded by that particular socket unused) - thermally that would be indistinguishable from having two 20A circuits grouped.

In theory you should apply a grouping factor (e.g. 0.8 or 0.85), but in practice it's usually ignored - which can be justified in domestic and similar if the ring isn't usually heavily loaded, or if the cables are spaced by a diameter or more.

- Andy.
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