CONSUMERS TAILS IN THE OSG

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.

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 !

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.

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.

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.

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.

There is a problem with some assessors, more likely not the usual Area Engineers, but someone they call in to get rid of a backlog. Some of these can be QS from a contractor. Mostly it seems these assessors deal with purely domestic electricians and default to OSG instead of the regs

Several years ago I went to a NAPIT trade association meeting and John Peckham was the guest of honour, in return for tea and biscuits he gave us an earthing and bonding CPD, this was followed by a Q&A session.


The subject of requirements for 25 mm meter tails came up and was the debated.


My comment was that I had stopped worrying about it, as I had attended a electric meter box fire to reinstate the customers damaged, totally wrecked, main switch and tails and metering services installed 16 mm tails as the replacements.


I had actually used 25 mm tails. then metering services arrived to install a new meter and make the connection to the new cut out installed by WPD and used 16 mm tails. I queried the use of 16 mm tails and was told that meets the requirements and actually is all that is needed with the standard 60 amp fused supply that you get.


So on new work I still use 25 mm though the DNO side of the meter may be 16 mm, on existing jobs if there is 16 mm tails I don’t break into a sweat about it and I definitely won’t be phoning the DNO to try and get the tails upgraded, a couple of years ago I tried unsuccessfully to get a neutral meter tail that had been rubbed by a garage door exposing the copper replaced and gave up leaving it bound up with insulation tape along with instructions to the customer to try and get it replaced. Bear in mind had it been the live phase there would have probably been a body on the floor.


If I couldn’t get a damaged tail replaced I don’t stand a cat in hells chance of getting 16 mm tails upgraded to 25 mm on the grounds that they are dangerously undersized.


Pick your battles, and this is not one worth fighting.


Andy Betteridge

Man and boy, (but not in that order obviously) I have seen hundreds of 16.0mm2 meter tails in homes and small shops and offices etc. I have yet to find 16.0mm2 meter tails damaged or showing signs of melting due to overloading. 16mm2 copper is very chunky  and the tails are short. Save materials and the planet and downsize.


Z.