Routing 16sqmm SWA externally - recommendations?

[Rob_the_rich]

As I see it;

if an external bonding conductor is needed, it doesn't need to follow the same route as the SWA, so can be internally run.

If an external protective conductor is needed, eg due to high Zs on a long run, it will need to follow the same route. (543.6.1)

 

[Tony S]

I've had the argument loads of times about using the armour as combined CPC and bonding conductor.....Its clearly stated within the regs that it has to offer equivalent conductance of copper if other metals are used i.e the steel armour when used as a bonding conductor. 16mm 2 core hasn't got a hope in hell......Bit of a silly suggestion IMO!

The tables I’ve got would disagree with you.

 

[Sidewinder]

So where exactly in the op does the swa require to act as a bonding conductor sized in accordance with TN-C-S requirments?

This is so I understand a sub main to a section of the property, which will be attached to the main dwelling.

The existing earthing & bonding requirements must be acceptable, before this cable is installed.

Thus, no requirement for it to be a bonding conductor, with the information provided.

The issue is, as usual, not enough information being provided.

We know nothing of the supply type, it could be any af the common 3.

There is nothing wrong with using 2 core SWA & relying on the SWA as the cpc.

BS7671 specifically includes SWA as a suitable conductor.

 

[leesparkykent]

The tables I’ve got would disagree with you.

Yes there is nothing wrong with using the armour as the CPC if its of an adequate size. There is nothing wrong with using the armour as a bonding conductor if its of an adequate size to offer equivilant conductance. The armour of a 16mm 2 core SWA is adequate as a CPC but not as a bonding conductor no matter what them tables say. The resistivity of steel changes with carbon content and guidance note 8 gives guidance on the ratio between copper and steel to account for this.

 

[Sidewinder]

Yes there is nothing wrong with using the armour as the CPC if its of an adequate size. There is nothing wrong with using the armour as a bonding conductor if its of an adequate size to offer equivilant conductance. The armour of a 16mm 2 core SWA is adequate as a CPC but not as a bonding conductor no matter what them tables say. The resistivity of steel changes with carbon content and guidance note 8 gives guidance on the ratio between copper and steel to account for this.

Yes you are correct it does & table B2 in GN8 disagrees with you lee, in that most of the sizes are acceptable.

 

[leesparkykent]

Yes you are correct it does & table B2 in GN8 disagrees with you lee, in that most of the sizes are acceptable.

Table B2 disagrees with what?

 

[leesparkykent]

I only have the 16th edition GN8 but page 41-42 are worth reading for whoever doesn't understand the bit about equivalent conductance. 

 

[Sidewinder]

Table B2 disagrees with your statement that the armour for most SWA cables is inadequate as a bonding conductor.

I do have several versions of GN8, from the very first one published right up to the Amd3 version, both 17th Ed. versions disagree with your statement that the armour on most SWA is inadequate to comply with 54.7.

The Amd3 version table is very poor, surprise, surprise.

Why?

I have it on ES+!

Just sent a bug report in, again.

I'll post a pdf of the relevant Amd1 table shortly.

 

[Sidewinder]

As promised.

View attachment GN8-Amd1-Tbl-B2.pdf

 

[leesparkykent]

Table B2 disagrees with your statement that the armour for most SWA cables is inadequate as a bonding conductor.

I do have several versions of GN8, from the very first one published right up to the Amd3 version, both 17th Ed. versions disagree with your statement that the armour on most SWA is inadequate to comply with 54.7.

The Amd3 version table is very poor, surprise, surprise.

Why?

I have it on ES+!

Just sent a bug report in, again.

I'll post a pdf of the relevant Amd1 table shortly.

Table 54.7 is to calculate/select the cross sectional area of every protective conductor, other than a protective bonding conductor.

Regulation 544.1.1 states the minimum CSA of bonding conductors except where PME conditions apply. Where PME conditions supply the CSA should be selected using table 54.8.

Both 544.1.1 and table 54.8 state that these sizes are for copper conductors and if the bonding conductor is not copper then the other metal used needs to offer equivalent conductance.

0.004 is the simplified resistance coefficient per degree C at 20 degree C given by BS 6360 for copper and aluminium conductors.The resistivity of steel changes with carbon content hence the ratio given in guidance note 8.

The charts/tables you are referring to are all for using the armour as a circuit protective conductor not a protective bonding conductor.

 

[Sidewinder]

OK, fair cop, once I couldn't access the blinking things correctly in ES+, I didn't cross reference anything, I got hung up with the frustration of the thing not being correct again.

Therefore, I didn't check what table 54.7 was.

 

[leesparkykent]

OK, fair cop, once I couldn't access the blinking things correctly in ES+, I didn't cross reference anything, I got hung up with the frustration of the thing not being correct again.

Therefore, I didn't check what table 54.7 was.

Just given you a scoob :Salute . I cant remember the ratio off the top of my head but using the ratio phoenix provides in post 34 of 8.9 and using the pdf you've just posted then a 95mm 2core armoured would provide equivalent conductance if a 10mm copper bonding conductor was needed.

 

[Sidewinder]

I have written something up, but I can't paste it here, so I've pdf'd it and attached it.

Remember though, the large CSA is ONLY required if it is a bonding conductor, this is very, very often not the case.

I don't see why in the OP's case with the info I have read that the SWA needs to act as a bonding conductor, and there is no mention of TN-C-S/PME.

View attachment SWA as a bonding conductor Talk Forum Sept 16.pdf

 

[leesparkykent]

I have written something up, but I can't paste it here, so I've pdf'd it and attached it.

Remember though, the large CSA is ONLY required if it is a bonding conductor, this is very, very often not the case.

I don't see why in the OP's case with the info I have read that the SWA needs to act as a bonding conductor, and there is no mention of TN-C-S/PME.

View attachment 7356

Even if it wasn't PME the minimum CSA of the bonding conductor would have to be 6mm copper so if using the armour as the bonding conductor it would have to still provide equivalent conductance of the 6mm copper conductor. 6mmx8.5mm=51mm, so the armour would have to have a minimum CSA of 51mm. I agree this may have no bearing on the OP as bonding may be connected some where near the origin. 

 

[Simon1013]

Just for the records since there are a couple of references to the 'OP' I am happy my query has been resolved. I am the 'OP.

My question was simply recommendations for routing 16mm, 3 core. My sparky specified 3 core and will do the rest. As interesting as this topic sounds, why don't you open another thread so that anybody searching in future can find it based on a more appropriate thread title than the one I dreamt up....

...just a thought.