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CONSUMERS TAILS IN THE OSG

John Peckham
I have just been having a scan read of the latest edition of the OSG. On page 20 it says in respect of consumers tails, " Polarity should be indicated by the colour of the insulation and the minimum cable size should be 25mm".


Alongside this statement is a reference to 514.3.1. I am OK with the requirement in 514.3.1 for the colour of the insulation but where  does the requirement come from for the CSA to be 25mm?


I do know some DNOs representatives may require a minimum CSA of 25mm but I suspect that this is a personal preference. My thinking is this is one of the myths and legends of electrical installation work that has been repeated so often that it has become fact. My view is the tails need to be sized to take the maximum load and they also need to be co-coordinated with the supply fuse. In addition some older consumer unit terminals will not take a 25mm tail.


Can anyone direct me to a reference source that requires the minimum CSA of meter tails to be 25mm?


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  • 0
    It is not an ideal world. For a 100amp main switch we would expect  cable sizes of 25mm2. However, if the main fuse is 60amp and the meter is less than 100amp 16mm2 is acceptable.  A 60 amp main switch could also have 25mm2 tails, but 16mm2 are more common. Most meter guys these days changing a meter and finding existing 16mm2 consumers tails will fit Henly blocks and tail into the meter with 25mm2. We must also remember some cut-outs wont take 25mm2 so cut out to meter is 16mm2.at the end of the day it all comes down size of fuse at the origin. As regards G Kenyons query, we wont change 60 to 100 cut outs on demand because the supply may be limited. 


    Regards, UKPN
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  • 0
    It is not an ideal world. For a 100amp main switch we would expect  cable sizes of 25mm2. However, if the main fuse is 60amp and the meter is less than 100amp 16mm2 is acceptable.  A 60 amp main switch could also have 25mm2 tails, but 16mm2 are more common. Most meter guys these days changing a meter and finding existing 16mm2 consumers tails will fit Henly blocks and tail into the meter with 25mm2. We must also remember some cut-outs wont take 25mm2 so cut out to meter is 16mm2.at the end of the day it all comes down size of fuse at the origin. As regards G Kenyons query, we wont change 60 to 100 cut outs on demand because the supply may be limited. 


    Regards, UKPN
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