Minimum CSA Tails?

mikeyb1000 · Oct 22, 2013

Hi everyone, quick question. In the OSG it states minimum tails from the meter and supply head must be 25mm. Now lets say there's a flat on the third floor and the current tails are 16mm. Would this then mean that the they would need to be replaced making damage all the way up a communal halway up three floors, meaning its going to cost a customer considerably more just to bring a board up to scratch if a new board was being put in. I have someone telling me the abadiatic equation will prove it can stay 16mm and another saying no, no ifs no buts it must be 25mm. Whats the deal here? Opinions?

telectrix · Oct 22, 2013

if the fuse protecting those 16mm tails is 60A or 80A then they're OK to remain. adiabatic needs to be done to calculate the size of the earthing conductor. it's new installations where 25mm should be used.

mikeyb1000 · Oct 22, 2013

Quick addition, yes I have said OSG says, but I have been told OSG is maximum measured values and to stick to to be 100% safe and in reg. And that the Regs is tabulated values and therefore more accurate and specific. So please if you feel like saying "you just said OSG says"... please dont. Id like constructive feed back and im not trying to engineer this so I get the answer I want. Im just looking for constructive feedback wiith explanations not quippy answers with things like "the answer is in the question" etc... Thanks

MDJ · Oct 22, 2013

mikeyb1000 said:
Quick addition, yes I have said OSG says, but I have been told OSG is maximum measured values and to stick to to be 100% safe and in reg. And that the Regs is tabulated values and therefore more accurate and specific. So please if you feel like saying "you just said OSG says"... please dont. Id like constructive feed back and im not trying to engineer this so I get the answer I want. Im just looking for constructive feedback wiith explanations not quippy answers with things like "the answer is in the question" etc... Thanks

Please don't take this the wrong way, but Tel has given you the answer fella.

SirKit Breaker · Oct 22, 2013

Both Tel and MDJ are right in what they say. But if youre fuse is 80a or lower, then the 16mm tails are fine.

the adiabatic is the best way to calculate the size of the main earthing conductor, but if we look a bit deeper into the regs, we can do it by another formula, and to cut a long story short, you can use a main earthing conductor that has a minimum CSA of 50% of your tails, so in your case 10mm would be the best fit. It is covered in parts 4 and 5 of the regs.

Cheers.................Howard

Octopus · Oct 22, 2013

You need to remember that the OSG is really written for new installations........

mikeyb1000 · Oct 22, 2013

No MDJ I wrote that as Tel was replying and didn't realise he already had..

Engineer54 · Oct 22, 2013

mikeyb1000 said:
Hi everyone, quick question. In the OSG it states minimum tails from the meter and supply head must be 25mm. Now lets say there's a flat on the third floor and the current tails are 16mm. Would this then mean that the they would need to be replaced making damage all the way up a communal halway up three floors, meaning its going to cost a customer considerably more just to bring a board up to scratch if a new board was being put in. I have someone telling me the abadiatic equation will prove it can stay 16mm and another saying no, no ifs no buts it must be 25mm. Whats the deal here? Opinions?

If this CU has been in use for a number of years, with absolutely no detrimental signs etc why would you even be thinking or considering changing 16mm for 25mm tails, if the cut out fuse is known to be 60/80A??
Because it say's so in an OSG (the G standing for Guide)? More fool you, because it say's nothing about minimum tail size in BS7671

How is the adiabatic equation being brought into this, that equation is to size main earthing conductor, rather than the supply tails...

If i were supplying tails for a new installation that had a DNO fuse rated at 60A, i can tell you now, i would not be providing 25mm tails!!