Main supply and earthing conductor

[sherman0251]

I am in the process of completing a new build where the main suppliers fuse is at the bottom of the garden in an external meter cabinet. The supply is 80A 3ph. I need to run my supply cable to the consumer unit in a utility room which is about 30m away. My builder has dug a trench 600mm deep ready for this. I am proposing to use a 4c 35mm SWA, my question is should I install a separate main earthing conductor alongside the swa or is the swa armouring sufficient?
Cheers for any advice

 

[Simon35]

Are you an electrician?

 

[sherman0251]

Yes I am.

 

[securespark]

What are the results of your calcs?

 

[bernardgreen]

Are you an electrician?

Yes I am.

Then why are you asking the question ?

The steel wire is armour, not a conductor although it does conduct. It can also rust to the point of being no longer conductive if the outer sheath is damaged. There are three sheaths protecting the copper conductors. The end connection glands are another weak point and may fail to maintain low impedance earth continuity.

So yes run an earth conductor to provide the earth function but include it in the cable by using 5 core SWA

 

[RF Lighting]

35mm² SWA is fine without an additional CPC assuming your calculations for Ze allow the resistance of steel.

I don't know where this recent trend of being afraid to use steel for earthing has come from?

 

[bernardgreen]

I don't know where this recent trend of being afraid to use steel for earthing has come from?

My "fear" started when a 150 yard run of buried SWA 3 phase + N cable was found to have several hundred ohms impedance along the armour and also a few hundred ohms from SWA to ground. The assumption was the cable's outer sheath had failed and the armour had rusted away.

 

[sparkwright]

Can't help thinking the meter box is in a dopey place anyway, and needs to be nearer the house, perhaps on the outside wall.

Op, is the meter box in a dopey place?

 

[EFLImpudence]

Where is the MET?

 

[sparkwright]

Exactly, what's the earthing arrangement?

Do you need 3 phase to the new property?

May prove handy later.

 

[Adam_151]

View attachment 87756

35mm² SWA is fine without an additional CPC assuming your calculations for Ze allow the resistance of steel.

Rob, as a CPC, yes [AFAIK if using BS88 fuses, and the fuse is suitably sized to provide overload protection, then the armour of 4C will always be fine as a CPC]

But.... You run into problems where it is required to act as bonding conductor on a TN-C-S supply, tabulated sizes in BS7671 give minimum copper CSA in relation to the suppliers neutral... and the wording says that if it is any other material that it must have an equivelent conductance. With Stell that involves an 8.9 factor.

So generally 10mm² of copper for a 100A supply or 89mm² of Steel. The armouring on the 35 4core is ever so slightly undersized! (25mm even more so)

 

[bernardgreen]

The supplier was probably installed as a "builder's supply" with the intention of it beinf relocated to the hosue when the house was finished. In which case the DNO may move the cut out and extend the cable to it when the house is finished. If that is acceptable to the DNO then ask them what size duct to install so they can pull in their cable. Also consider a duct fro ELV services such as telephones, alarms and other services.

 

[ban-all-sheds]

Are you an electrician?

Yes I am.

 

[JohnW2]

I don't know where this recent trend of being afraid to use steel for earthing has come from?

Maybe from a (rather late) realisation that, given appropriate circumstances, steel can 'rust away'?

Kind Regards, John

 

[Spark123]

Are you installing a consumer unit or a 3 phase distribution board?