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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 Status: Post and Reply Linear : Threading : Single : Branch
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 12 July 2013 12:15 PM 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 Parsley Posts: 1254 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 AJJewsbury Posts: 13117 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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