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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

    John Peckham:

    Happy to having the OSG saying that when replacing the consumer unit, a new installation or a rewire "it is recommended that the tails be upgraded to 25mm or even 35mm to accommodate a future increase in installation loading such as EV charging". But I am not happy with the statement "the minimum cable size should be 25mm".



    I cannot argue with that.


    Perhaps make it simpler: "It is recommended that new or replacement consumer's tails have a minimum CSA of 25 mm2 to accommodate ..."


    The point is that yes, the consumer's tails are provided by the installer in a new installation when they are left dangling for the meter fitter to fit, but not otherwise. I suspect that most sparks would happily remove the supplier's fuse in order to do a CU change, but be more reluctant to open the meter and fit new tails.

     
    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. Also contractors are putting less than 25mm tails down as a C2 non-compliance regardless of the company fuse size. I have to ask when I see this is "how do you know what size fuse is fitted inside the head"?


    ...


    Lots of very good practical advice in the OSG for installers but the advice needs qualification to avoid it being considered as superseding or replacing the regulations set out in BS 7671.

    That is a very real concern. It would be particularly unfortunate if that is being taught on courses or to apprentices.
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  • 0

    John Peckham:

    Happy to having the OSG saying that when replacing the consumer unit, a new installation or a rewire "it is recommended that the tails be upgraded to 25mm or even 35mm to accommodate a future increase in installation loading such as EV charging". But I am not happy with the statement "the minimum cable size should be 25mm".



    I cannot argue with that.


    Perhaps make it simpler: "It is recommended that new or replacement consumer's tails have a minimum CSA of 25 mm2 to accommodate ..."


    The point is that yes, the consumer's tails are provided by the installer in a new installation when they are left dangling for the meter fitter to fit, but not otherwise. I suspect that most sparks would happily remove the supplier's fuse in order to do a CU change, but be more reluctant to open the meter and fit new tails.

     
    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. Also contractors are putting less than 25mm tails down as a C2 non-compliance regardless of the company fuse size. I have to ask when I see this is "how do you know what size fuse is fitted inside the head"?


    ...


    Lots of very good practical advice in the OSG for installers but the advice needs qualification to avoid it being considered as superseding or replacing the regulations set out in BS 7671.

    That is a very real concern. It would be particularly unfortunate if that is being taught on courses or to apprentices.
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