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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

    What has caused me to review this as one of the certification bodies is putting this down as a non-conformity when inspecting contractors installations. There justification for doing this is it is in the OSG.



    Sounds to me like there's a problem with 'one of the certification bodies' rather than the OSG. The OSG is only meant to provide a set of pre-calculated general designs that will be safe in any circumstance (within scope) - not to provide the most economic compliant solution for a particular situation. If I were in a situation where I didn't know what the load was and I didn't know what size the DNO's fuse was (other than it can't exceed 100A) then I'd specify 25mm² tails too - but given better information a better honed solution can be obviously be produced.


    Unless of course that particular certification body has its own requirement that their members produce installations to the OSG rather than just BS 7671 - in which case they should still be challenged on the basis of wasting natural resources and forcing customers to accept uneconomic designs.


       - Andy.
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  • 0

    What has caused me to review this as one of the certification bodies is putting this down as a non-conformity when inspecting contractors installations. There justification for doing this is it is in the OSG.



    Sounds to me like there's a problem with 'one of the certification bodies' rather than the OSG. The OSG is only meant to provide a set of pre-calculated general designs that will be safe in any circumstance (within scope) - not to provide the most economic compliant solution for a particular situation. If I were in a situation where I didn't know what the load was and I didn't know what size the DNO's fuse was (other than it can't exceed 100A) then I'd specify 25mm² tails too - but given better information a better honed solution can be obviously be produced.


    Unless of course that particular certification body has its own requirement that their members produce installations to the OSG rather than just BS 7671 - in which case they should still be challenged on the basis of wasting natural resources and forcing customers to accept uneconomic designs.


       - Andy.
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