10mm bonding to water and gas

[m1ck_b87]

Hi all

Do main bonding conductors have to be upgraded to 10mm when changing a consumer unit, my own opinion is as long as i am not adding any additional circuits or extending circuits i can leave 6mm as long as theres no signs of thermal damage, this would be just noted down as deviation on electrical cert, or on the other hand if i am changing cu then installstion has to comply with 17th edition, what does every one else think

 

[kingeri]

I'd always check that the current bonding is adequate. If the main earth conductor is (and more importantly needs to be) 16mm then the bonding conductor SHOULD be at least half this....so 10mm is the one. I'd only not change it if I had a compelling reason not to, after confirming that the 6mm is adequate, of course!

You don't mention the earthing arrangement so can't be more specific.

 

[m1ck_b87]

I agree if its a tn-s earthing system and has 16mm earth then yeh the bonding conductor no less than half, but if its tnc-s then minimum on domestic will be 10mm.

 

[m1ck_b87]

Bs7671 states minimum requirement 6mm.

 

[Jay Sparks]

On a Tn-s system, if you do the adibiatic equation and it works out you only need say 9mm main earth, then, even if you fit a 16mm main earth, you can leave 6mm bonding in place if there no sign of thermal damage. The bonding conductor should be not less than half the calculated main earth, and not half the size of the main earth thats fitted.

Saying that, I will always try and upgrade the bonding to 10mm wherever possible.

Jay

 

[Octopus]

What is the end to end reading of the 6mm?

 

[Deleted member 26818]

I'm confused.
The OP is obviously aware that the minimum CSA for bonding conductors where the system is TN-S is 6mm.
Why are they then asking whether the conductors have to be upgraded to 10mm?

You would only contemplate upgrading bonding conductors, if they did not satisfy the requirements of the Regulations.
As for noting down that the existing bonding conductors satisfy the Regulations as some sort of deviation, what's that all about?

 

[Toonlad]

self doubt maybe

 

[onions1066]

Am I missing something is it tns or tncs?

 

[m1ck_b87]

Yes abit of self doubt, was kinda under the impression if you do work on domestic install you have to make sure it complies with 17th edition, ie if i install circuit buried in plaster it would have to be rcd protected. So i was thinkin same rule applies for change of consumer but if all is adequate ie 16mm tails protected by 60a fuse, they wont neccessary need replacing for 25mm which is only a recommendation so yes abit of self doubt as there is soooo many regulations with different opinions, i dont yet know what the system is but just a quick question how do you confirm that the csa of main bonding conductors are adequate enough.

 

[m1ck_b87]

And i am aware the measured value should be 0.05 omhs or less

 

[Jay Sparks]

Adibiatic equation.

 

[m1ck_b87]

Adibiatic equation.

Thats the equation for your earthing conductor

 

[Jay Sparks]

Thats the equation for your earthing conductor

And when you know the size of your earthing conductor you will know the size of your bonding conductor.

 

[m1ck_b87]

Brilliant ive learnt something new there thanks pal

 

[m1ck_b87]

Adibiatic equation is not used for main bonding conductors

 

[Deleted member 26818]

With installations up to and including 100A, there are very few instances where an earthing conductor with a CSA greater than 10mm² would be required.
Those instances are limited to 100A supplies, with a Ze such that disconnection times would be around the 5 second mark, and would often involve the use of multi core cables.
Even in those instances where a CSA greater than 10mm² is required for the earthing conductor, often 6mm² bonding conductors would be acceptable
As Jay Sparks says, conduct the adiabatic equation to determine what CSA is required for the earthing conductor, then size the bonding conductor accordingly.

 

[m1ck_b87]

Bgb book states minimum size conductor of 10mm in relation to neutral conductor upto 35mm on a tnc-s system. so basically if its a 6mm main earthing conductor the bonding conductor still has to be 10mm, now tn-s is different no less than half rhe size of earthing conductor , so my question to you is would it be acceptable as long as no thermal damage on a tnc-s system.

 

[m1ck_b87]

Would 6mm be acceptable

 

[onions1066]

10mm on a tncs

 

[Deleted member 26818]

Bgb book states minimum size conductor of 10mm in relation to neutral conductor upto 35mm on a tnc-s system. so basically if its a 6mm main earthing conductor the bonding conductor still has to be 10mm, now tn-s is different no less than half rhe size of earthing conductor , so my question to you is would it be acceptable as long as no thermal damage on a tnc-s system.

No, with TN-C-S the minimum CSA of 10mm² applies to both the earthing and bonding conductors.
It is only with TT and TN-S, where the earthing conductor can have a smaller CSA than the bonding conductors.

Yes 6mm² would be considered as acceptable if as you say, there are no indications that the conductors have been overheating.

 

[Midwest]

I'm trying to understand this particular issue. Surley the earthing conductor and main protective conductors are relative to the supply tails, not the size of the cut out fuse (which the DNO could change)?