Minimum bonding size TNC-S

[robd]

Afternoon all,

Just trying to clear something up, going through regs book at present.

Basically have got a TNC-S/PME installation, 16mm tails, 60/80a main fuse (think it is 60A), 6mm bonding to water and 6mm coming from service head to block then into CU as main earth.

Am I right that being as it is PME I am obliged to upgrade bonding to 10mm as per table 54.8 and that the note about bonding must be not less than half size of CSA for main earth only applies to conditions other than PME.

Just wanted to be clear as that's how I'm reading it, the job is a mains change. Thanks in advance.

 

[Deleted account]

No you are not obliged to do this unless you do the adiabatic equation and the 6 MM is not suitable.
If it's under 6mm then you are obliged to increase to 10mm for bonding and 16 mm for main earthing.

 

[westward10]

Minimum size is 10.0. People will state that 6.0 at one time was acceptable but you will not find this in a copy of the Regulations. The 15th Ed quoted you should consult your supplier with regards to minimum sizing so the chances of knowing whether your 6.0 conductors were acceptable at the time of installation is zero.

 

[Octopus]

With TNCS the 10mm minimum is the norm ........... TNS 6mm can be acceptable

Tin hat on.

What you need to be aware of is that lots of houses don't have any bonding at all.

 

[Wilko]

10 and 16 from me. The 16 comes from Table 54.7

 

[robd]

So when BS7671 refers to main protective bonding conductors it is referring to bonding of services? If so table 54.8 says minimum 10mm.

Up until this point if I have done a mains change and there's a set up of 60a fuse, 16mm tails and 10mm earth and 6mm bonding I haven't upgraded bonding (depending on circumstances) and have had a job like this looked at on an NIC annual visit, but this particular job has now had solar panels and it is the governing body of the solar company asking for the 10mm bonding. Will be tricky getting 10mm to the water under the sink so that's why I'm trying to be 100% clear.

Thankyou all for your replies

 

[westward10]

Proving the existing 6.0 is or ever was adequate will not be possible.

 

[robd]

Thanks I think I see where this is heading

 

[Octopus]

Proving the existing 6.0 is or ever was adequate will not be possible.

And proving 10mm is mandatory and essential is equally impossible.

 

[Midwest]

Isn't the size of a protective conductor either determined or selected, whereas a main protective bonding conductor, is selected from table 54.8, where PME conditions apply?

 

[westward10]

And proving 10mm is mandatory and essential is equally impossible.

??

 

[Ian1981]

No you are not obliged to do this unless you do the adiabatic equation and the 6 MM is not suitable.
If it's under 6mm then you are obliged to increase to 10mm for bonding and 16 mm for main earthing.

Adiabatic equation has nothing to do with protective bonding conductor sizes

Edit.
I should read the full post first

 

[robd]

I agree about the adibiatic not applying to bonding that's how I read it

 

[Deleted account]

I agree about the adibiatic not applying to bonding that's how I read it

I Should have worded that one better. Cheers

 

[Andy78]

I will leave 6mm in every time if the customer chooses that option.

 

[andyb]

I agree about the adibiatic not applying to bonding that's how I read it

It does not apply to a PME supply, but it could be used for a TNS suppy.

 

[davesparks]

but it could be used for a TNS suppy.

Not really, the adiabatic is can be used to select the size of earthing conductor, then the bonding conductor is worked out based on the earthing conductor. The size of the bonding conductor isn’t a direct result of the adiabatic calculation. And if the earthing conductor size has been selected according to the table then the adiabatic has nothing to do with the bonding conductor size.
For example if the adiabatic shows that a 19.7mm earthing conductor is needed you would use 25mm, then the bonding would be 16mm (next size up from half of earthing conductor) if the adiabatic result was used for bonding it would be 10mm.

 

[Deleted member 26818]

The minimum CSA of an earthing conductor is 2.5mm2.
The minimum CSA of a bonding conductor is 6mm2.
Where PME conditions apply, the minimum CSA of both the earthing and bonding conductors is 10mm2.
There is no requirement to upgrade the Earthing conductor to 16mm2 unless the adiabatic equation dictates a CSA greater than 10mm2 is required.

 

[andyb]

Not really, the adiabatic is can be used to select the size of earthing conductor, then the bonding conductor is worked out based on the earthing conductor. The size of the bonding conductor isn’t a direct result of the adiabatic calculation. And if the earthing conductor size has been selected according to the table then the adiabatic has nothing to do with the bonding conductor size.
For example if the adiabatic shows that a 19.7mm earthing conductor is needed you would use 25mm, then the bonding would be 16mm (next size up from half of earthing conductor) if the adiabatic result was used for bonding it would be 10mm.

I think it can Dave, the reg's state 'not less than half the csa required for the earthing conductor'.
So the way see it is, if you are adding to an installation that has 25mm tails and a 16mm main earth but only say a 6mm main bonding conductor, then in that case you could use the adabatic equation to determine the required csa for the main earth then go from there. So if the equation gives you a requrement for say an 9mm main earth, despite the fact that someone has installed a 16mm earth you can say half 9mm = 4.5mm so the 6mm installed is adequate.

 

[Risteard]

No you are not obliged to do this unless you do the adiabatic equation and the 6 MM is not suitable.
If it's under 6mm then you are obliged to increase to 10mm for bonding and 16 mm for main earthing.

The adiabatic equation is not relevant to main protective bonding conductors, or any bonding conductors for that matter.