What size should the Tails be to a domestic fuse board?
What size tails?
- Thread starter markymark2406
- Start date
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OSG 2.2.3
Surely you must know this, if you are messing around with Meter Tails?
Surely you must know this, if you are messing around with Meter Tails?
OSG is just a guide though.
Nothing wrong with using 16mm Tails on a 60 or 80amp service, which is the standard sized service WPD provide in my area. If you want a 100amp service, they want to increase the service cable size and cutout type, and 25mm tails would then be used.
If you fit a one way CU for a 32amp circuit, there would also be nothing wrong in using 4mm tails if you wanted. I think everyone these days would fit 16mm though.
Nothing wrong with using 16mm Tails on a 60 or 80amp service, which is the standard sized service WPD provide in my area. If you want a 100amp service, they want to increase the service cable size and cutout type, and 25mm tails would then be used.
If you fit a one way CU for a 32amp circuit, there would also be nothing wrong in using 4mm tails if you wanted. I think everyone these days would fit 16mm though.
Yes that what I thought just having a conversation with a mate and the earth to PME the system 16mm ? Obviously from the CU to N in cut out ?
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If ever (we) I change or move a CU we always put in 25mm tails and a 16mm main bond as a matter of course. Just think it future proofs it to a certain extent.
OSG Figure 2.1
And its specified in BS7671 somewhere around 542.
Don't you have copies?
As has been said, the OSG is a guide for one size fits all without thinking.
In it all the diagrams show 100A main fuse but no tail size and 2.2.3 states tails should be a minimum of 25mm².
(The diagrams also show 16mm² earthing conductor of TT system (this was not in the previous edition and so may actually be a mistake)).
Tails are no different than any other conductors and should/could be sized appropriately with regard to main fuse.
10mm² tails would be satisfactory with a 60A main fuse.
So, blindly fitting 25mm² tails as a matter of course to 60A and 80A supplies is, in my view, irresponsible.
Perhaps someone could calculate the amount and cost of extra copper needed when installing cables 50% or 150% larger than necessary.
'Future-proofing' may be used as an argument for cooker circuits but hardly applies with regard to meter tails and, don't forget, the supply cable is still going to be the same and don't the DNO use 16mm² for 100A supplies?
In it all the diagrams show 100A main fuse but no tail size and 2.2.3 states tails should be a minimum of 25mm².
(The diagrams also show 16mm² earthing conductor of TT system (this was not in the previous edition and so may actually be a mistake)).
Tails are no different than any other conductors and should/could be sized appropriately with regard to main fuse.
10mm² tails would be satisfactory with a 60A main fuse.
So, blindly fitting 25mm² tails as a matter of course to 60A and 80A supplies is, in my view, irresponsible.
Perhaps someone could calculate the amount and cost of extra copper needed when installing cables 50% or 150% larger than necessary.
'Future-proofing' may be used as an argument for cooker circuits but hardly applies with regard to meter tails and, don't forget, the supply cable is still going to be the same and don't the DNO use 16mm² for 100A supplies?
Always know what the DNO is doing, a 16mm copper (25mm Aluminium) (Imperial .0225 in) service cable is rated to carry over 100A in our tables and may well be fused accordingly.
16mm Tails would require a 16mm earth, 10mm main bonds.
That's where WPD seem to differ. If you want a 100amp service, they will want to install a 25mm copper drop, replacing the existing 16mm. They will also not use the standard '100amp' heads.
Good advice this. Where possible I use 25 mm tails as it's future proof. If the cut out and meter ever gets changed, the DNO never replace your tails, and since their equipment will often be sealed it's difficult for you to change them.
IF - meanwhile...
At least in the context of this thread (meter tails) I think one of the problems is that BS7671 itself does not give CCCs for non-touching singles for CSA's <25mm², which one imagines will be a little higher than for touching singles or muticore cables. For 16mm² 'clipped direct' touching singles it gives 87A (the 'in free air' {touching and non-touching} columns are blank for <25mm²!). For 2-core 16mm², it gives 94A for 'in free air'. One therefore imagines that the corresponding rating for 16mm non-touching singles in free air, if they gave it, probably would, indeed, be of the order of 100A, if not more - so perhaps no real inconsistency there.
Kind Regards,
I suspect that is the case, and as the tails between the cut-out & the meter are "ours" they will be asigned our rating which will probably be higher than BS 7671. (I can check as I am off work following an operation)
Amazing how a meter can change a cable rating depending upon which side it is on!
Somewhere around my desk is an old BICC ratings book It would be interesting to see what ratings that assignes
Indeed. I hope you're making a good recovery.
As we've discussed many times before, I think one of the issues is that virtually all these tables of 'ratings' tend to be very conservative - so the variation we see is probably nothing more than varying degrees of conservatism.
Kind Regards, John
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