Size of Earthing/Bonding Conductors - Domestic Installation TN-C-S

[seftonbarn]

Hyperthetically.

TN-C-S in a Domestic - Earthing Conductor 16mm. Bonding conductors 6mm. I understand that the bonding conductors must be not less that half the earthing conductors meaning the the 6mm bonding in this situation is too small but they are a pain in the backside to change. However if I apply the adiabatic to the earthing conductor I can get away with a smaller size. So I reduce the the Earthing conductor to 10mm and I am now compliant.

Is the installation more safe or just more compliant?

 

[telectrix]

you don't reduce the earthing conductor. if using the adiabatic confirms that the earthing conductor can be 10mm, you then check the 6mm bonding for signs of damage. if it is OK, then it is acceptable to leave as is, but note on your cert. that it is undersized for compliance with current. regs., but leave the 16mm maing earthing conductor alone.

 

[yellowvanman]

I understand that the bonding conductors must be not less that half the earthing conductors meaning the the 6mm bonding in this situation is too small but they are a pain in the backside to change.

This is not applicable to TN-C-S, sizes are governed by Table 54.8

 

[Engineer54]

It's actually the incoming Neutral conductor that is used in a PME system, not the main earthing conductor!!

 

[seftonbarn]

This is not applicable to TN-C-S, sizes are governed by Table 54.8

I thought the main protective bonding conductors still needed to be not less than half the csa of the earthing conductor where PME conditions apply and that Table 54.8 is used to determine the minimum size of the main protective bonding conductors compared to the csa of the incoming supply neutral conductor. The hypothetical debate of reducing the size of the earthing conductor so that the bonding conductors meet the "not less than half....." criteria, I think still applies (apart from the fact that the min csa of the bonding is 10mm(forgive me that one for the general purpose of the debate)).

 

[telectrix]

The hypothetical debate of reducing the size of the earthing conductor so that the bonding conductors meet the "not less than half....." criteria, I think still applies.

not quite. you are correct in your statement that the bonding conductors need to be 50%. but that is 50% of what the main earthing conductor needs to be as a calculation with the adiabatic. what i mean is, the adiabatic may give you 10mm for the main earthing conductor as a calculated size, but the actual main earthing conductor can be bigger, 16mm, 25mm, or even 70mm. it's the required size that you take your 50% off, not the actual size.
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[SirKit Breaker]

I thought the main protective bonding conductors still needed to be not less than half the csa of the earthing conductor where PME conditions apply and that Table 54.8 is used to determine the minimum size of the main protective bonding conductors compared to the csa of the incoming supply neutral conductor. The hypothetical debate of reducing the size of the earthing conductor so that the bonding conductors meet the "not less than half....." criteria, I think still applies.

The not less than half is a good rule of thumb. And if you are going to rewire a 3 bed semi, then the cost implication of using an oversized protective cable in any application is negligible, so the rule of thumb will do. However, if you are going to wire up a large hospital or hotel, then you would use the adiabatic equation, because the cost of installing a 10mm bonding cable when a 6mm will do, could be a very significant amount of money.

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

 

[Engineer54]

The not less than half is a good rule of thumb. And if you are going to rewire a 3 bed semi, then the cost implication of using an oversized protective cable in any application is negligible, so the rule of thumb will do. However, if you are going to wire up a large hospital or hotel, then you would use the adiabatic equation, because the cost of installing a 10mm bonding cable when a 6mm will do, could be a very significant amount of money.

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

If you were involved with these types of installations, you would be undoubtedly, working to a contractual specification. I can tell you now, that there is no way, would you be installing 6mm bonding cables to save money!!! lol!!

 

[tony mc]

I thought the main protective bonding conductors still needed to be not less than half the csa of the earthing conductor where PME conditions apply and that Table 54.8 is used to determine the minimum size of the main protective bonding conductors compared to the csa of the incoming supply neutral conductor. The hypothetical debate of reducing the size of the earthing conductor so that the bonding conductors meet the "not less than half....." criteria, I think still applies (apart from the fact that the min csa of the bonding is 10mm(forgive me that one for the general purpose of the debate)).

I think you are getting yourself confused !

Have another read of reg 544 and you will see that see that the first paragraph say Except where PME conditions apply!

It is your main bonding conductors that you are sizeing so the DNO neutral conductor is what your looking at with regards to the size of your bonding conductor so in theory you Main earth will have to be equal to that size or bigger !


If you did do a adiabatic calc on your hypothetical instalation and it came out as 6mm what would you do Hypothetically re the sizeing of your Main Earth & Main Bonding conductors ?

 

[seftonbarn]

I think you are getting yourself confused !

Yeah I think you could be right. Getting mixed up between TN-S and TN-C-S. I think Tel gave me the most logical answer to my misunderstanding " The hypothetical debate of reducing the size of the earthing conductor so that the bonding conductors meet the "not less than half....." criteria, I think still applies.

not quite. you are correct in your statement that the bonding conductors need to be 50%. but that is 50% of what the main earthing conductor needs to be as a calculation with the adiabatic. what i mean is, the adiabatic may give you 10mm for the main earthing conductor as a calculated size, but the actual main earthing conductor can be bigger, 16mm, 25mm, or even 70mm. it's the required size that you take your 50% off, not the actual size"
​

 

[oldtimer]

Funny how this subject will not die if you go into a 20 year old PME or TNCS house with 16mm tails you find a 10mm main earth and 6mm to gas and water so no doubt in two weeks time this question will get asked again