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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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    gkenyon:

    Well, my understanding is that DNOs have to upgrade the supply to 100 A on request, meaning that 25 mm2 or 35 mm2 tails would be required (and a 100 A CU and main switch). If you are fitting a 100 A CU, why not fit tails to suit - regardless of the maximum demand today, the consumer may want an EV charging point fitted tomorrow.




    Where does it say that DNOs are compelled to upgrade?

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    Graham


    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". 


    Some DNOs don't upgrade fuses in heads and will often reduce supply fuse ratings from 100A to 80A or 63A regardless of the CSA of the supply tails. Perhaps our forum member "UKPN" could comment?


    That is not a requirement of BS 7671.  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"?


    I have the same problem with the similar direction given on Page 48 in respect of earthing conductor CSAs. Apart from the advice on PME sizes being wrong, it is helpful to give some helpful advice on CSAs but it needs to be qualified with a mention of calculating the CSA using the adiabatic equation and reference to table 54.8.


    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.
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    I think Andy sort of has the clue to the right answer.


    I believe the answer is in Table 4D1A - Why?


    Well, my understanding is that DNOs have to upgrade the supply to 100 A on request, meaning that 25 mm2 or 35 mm2 tails would be required (and a 100 A CU and main switch). If you are fitting a 100 A CU, why not fit tails to suit - regardless of the maximum demand today, the consumer may want an EV charging point fitted tomorrow.


    As a result, I think the OSG has for some time been recommending 25 mm2 tails.
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    Morning Andy


    Yes maybe but if this is the case perhaps the assumption should be qualified. However if this was the case it would not tie up with the tables for the CSA of earthing conductors relating to supply conductors on page 48 which has CSAs for supply conductors for 6-70mm.


    I do have a big problem with the CSAs of bonds in Table 4.4ii !
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    Isn't the 25mm² requirement just a consequence of the OSG being intended to be applied to any installation of up to 100A without further calculation?


    I'm sure the 100A limit was stated in the scope of earlier editions - perhaps it's wandered off in later ones? (but the underlying assumption remains)


      - Andy.

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