Main earth sizing

[APE37]

Hi guys,
Just wondering if anyone can clear up an issue.
On tncs earthing systems, can you calculate the size of the main earth?
I thought you could but main bonding conductors have to be sized to the neutral.
Just had a conversation with another electrician and he said it has to be 16mm for main earth for tncs and showed me the onsite guide. But I was under the impression in regs book it says that all earth conductors need to be either selected or calculated except from bonding conductors which for tncs need to be sized according to the neutral, and on tns bonding conductors need to be sized at least half of the main earth but no smaller than 6mm? Just wonder if you can calc main earth really, just made me doubt myself and thought I would get the opinions of others.
Cheers.
Cheers

 

[mhar]

16mm is the DNO requirement for a pme system.
The 'scientific' calculation is the adiabatic equation but you need to know what the DNO ocpd is

 

[westward10]

It should be not less than half the size of the supply neutral conductor.

 

[Ian1981]

The protective bonding conductors in pme arrangements are sized to the PEN conductor or to The dno specific requirements so the main earthing conductor needs to be at least the same size but more often than not it will be larger than the protective bonding conductors.
Your earthing conductor is often sized in relation to the line conductor either by using table 54.7 or by calculation.

For new builds that the company work on ,the dno requires us to install 25 mm live conductors and a 16 mm earthing conductor.

 

[Deleted member 26818]

According to BS7671:
The minimum CSA for an earthing or circuit protective conductor which is not part of a multi core cable, is not buried in the ground and has mechanical and corrosion protection, is 2.5mm2.

The minimum CSA required can be obtained by use of the adiabatic equation.

Where PME conditions apply, the minimum CSA for a Main earth, is no less than that required for the bonding conductors.

According to most DNOs:
The minimum CSA for a Main earth is 16mm2.

 

[APE37]

Cheers guys. Looks like I need to brush up.

 

[Des 56]

Ok thats the "main" earthing conductor sorted

 

[UKPN]

Hi guys,
Just wondering if anyone can clear up an issue.
On tncs earthing systems, can you calculate the size of the main earth?
I thought you could but main bonding conductors have to be sized to the neutral.
Just had a conversation with another electrician and he said it has to be 16mm for main earth for tncs and showed me the onsite guide. But I was under the impression in regs book it says that all earth conductors need to be either selected or calculated except from bonding conductors which for tncs need to be sized according to the neutral, and on tns bonding conductors need to be sized at least half of the main earth but no smaller than 6mm? Just wonder if you can calc main earth really, just made me doubt myself and thought I would get the opinions of others.
Cheers.
Cheers

If you are talking PME it's the DNO. It's 16mmsq earth lead. Of course, the endless collection of IET "guide books" ( "18th Ed regs" book, on site guide, wiring matters, guidance notes, commentary etc etc etc) will tell you their opinion, but that's all it is opinion and not an actual legal regulation.