Is 25mm 3 core swa good for earth and Bonding

[allybally69]

So me and a colleague were having a discussion about using 25mm 3 core swa as a live, neutral, earth and a Bonding conductor when relocating the position of the consumer unit, I suggested that using a 25mm 4 core as I can't see the 25mm earth being big enough for 16mm earth and 10mm bonding with the steel added, maybe I'm having an overthinking moment...? Can anyone clear this up for me?

It's TNCS and mains water and gas, both metal

Thanks

 

[timhoward]

I suggested that using a 25mm 4 core as I can't see the 25mm earth being big enough for 16mm earth and 10mm bonding with the steel added

Is the question whether a 25mm main earthing conductor is sufficient to support the required bonding?
I'm not quite sure why it would be any different to a more common installation where you have three 'normal' 25mm tails and the gas and water bonding was connected at the consumer unit.

Or is your question whether the 62 sq mm csa of steel armour in the 3 core SWA is sufficient to be a bonding conductor?
I believe the copper equivalent is 19.9 sq mm, taking k1 as 143 from table 43.1, and k2 as 46 from table 54.4. (62 * 46/143))

 

[telectrix]

3 core is fine. (even without the armour). 25mm cpc doubles as cpc and bonding conductor. for TNC-S, bonding conductor min. is 10mm.

 

[Pete999]

So me and a colleague were having a discussion about using 25mm 3 core swa as a live, neutral, earth and a Bonding conductor when relocating the position of the consumer unit, I suggested that using a 25mm 4 core as I can't see the 25mm earth being big enough for 16mm earth and 10mm bonding with the steel added, maybe I'm having an overthinking moment...? Can anyone clear this up for me?

It's TNCS and mains water and gas, both metal

Thanks

Look at the table in the OSG or 7671

 

[MJPD29]

25mm 3core is fine

 

[timhoward]

The original question is a little confusing as it isn't 100% clear where the suspected issue is.

maybe I'm having an overthinking moment...?

I suspect you might be...
Were you maybe thinking you needed 25mm main earthing + 10mm bonding = 35 total required as you were shifting it all downstream? Hence an extra core? It doesn't in fact work like that, but do ask away if questions remain.

 

[pc1966]

I was wondering about that, or if the thought was the bonding has to be independent back to the MET at the DNO source?

Short answer is no. Pipes, etc, can be bonded to the CU's earth bar in that case as the 25mm 3-core exceeds the 10mm minimum.

I would add though you should have the armour properly glanded and earthed at both ends in parallel with the 3rd core, keeps the Zs for any internal cable fault down so (hopefully) you can meet 5s disconnection on the supply fuse (DNO's if they agree, or easier the one in the switched-fuse you install for the sub-main).

 

[MJPD29]

25mm 3core is fine

 

[MJPD29]

Look at the table in the OSG or 7671

What more info do you need Pete? My answer is very self explanatory.

 

[allybally69]

Thanks for all the replies

Sorry if bits were confusing

So just to try and clear it up a bit...

If a consumer unit is within 3metres of the cutout you just run 25mm tails and 16mm earth, then from the Consumer unit 10mm to services etc...

If the consumer unit is say 30m away, is the correctly terminated swa (as pointed out 19.9 copper equivalent), with the 25mm earth cable sufficient for main earthing and Bonding for the job.

I thibk I know the answer but sometimes I doubt myself massively, hence this thread ?

 

[timhoward]

If the consumer unit is say 30m away, is the correctly terminated swa (as pointed out 19.9 copper equivalent), with the 25mm earth cable sufficient for main earthing and Bonding for the job

Yes.
If you use tables 4.4(1) and 4.4(2) in the on site guide they would tell you that for a 25mm line conductor you need a 16mm main earthing conductor, and 10mm protective bonding conductor (which can be connected downstream at the consumer unit if required.) You can also arrive at this from tables 54.7 and 54.8 in big blue book.
As someone stated above the 25mm 3rd core is in fact meeting all of this on it's own. The steel armour just makes it even better.

As a point of interest, 2 core 25mm XPLE only has copper equivalent csa of 13.5mm, so less than the 16mm the tables say you need. At first glance this is a non-starter.
You can in fact sometimes arrive at this meeting the regs by proving the the earthing conductor is adequate using the adiabatic equation (subject to PFC and fuse size) and as it also exceeds the 10mm main bonding requirement it would comply with the PME requirements too. However I wouldn't relish trying to explain this to an 'expert' during an assessment!

 

[allybally69]

Yes.
If you use tables 4.4(1) and 4.4(2) in the on site guide they would tell you that for a 25mm line conductor you need a 16mm main earthing conductor, and 10mm protective bonding conductor (which can be connected downstream at the consumer unit if required.) You can also arrive at this from tables 54.7 and 54.8 in big blue book.
As someone stated above the 25mm 3rd core is in fact meeting all of this on it's own. The steel armour just makes it even better.

As a point of interest, 2 core 25mm XPLE only has copper equivalent csa of 13.5mm, so less than the 16mm the tables say you need. At first glance this is a non-starter.
You can in fact sometimes arrive at this meeting the regs by proving the the earthing conductor is adequate using the adiabatic equation (subject to PFC and fuse size) and as it also exceeds the 10mm main bonding requirement it would comply with the PME requirements too. However I wouldn't relish trying to explain this to an 'expert' during an assessment!

Brilliant thank you so much for this, I know it's a easy to say I knew this was the answer... But like I said there was some doubt in my mind, really appreciate the help ?