The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 25 replies
  • Subscribers 31 subscribers
  • Views 2862 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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?


Parents
  • 0
    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.
Reply
  • 0
    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.
Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines