Garage again....

[baldersj]

Hi all,

I'm doing some work at a mate's, he wants power to his metal framed garage which is about 20m from the house and then on to the shed, about another 10m from that. NOW...he's on a PME system, I know the requirements for the earthing conductor state that it should be 16mm, what I need to know is what size armoured do I need to obtain this? It would be far easier than sinking an earth rod, the ground is like rock!

Sorry if this sounds dense, I'm fully qualified, just never done a garage installation before...

 

[THRIPSTER]

Size of the SWA depends upon the anticipated load.

As the ground is `like rock' then you may have problems digging the necessary trench?

 

[baldersj]

It's only the last bit which needs trenching, the rest is a concrete path which he obligingly left a a gap down the side of for the cable, talk about doing things in the wrong order!
I know the size of the cable depends on the load, what I was asking was did anybody know the smallest CSA of armoured cable which would provide the equivalent of a 16mm earth, to comply with the regs on PME. Any ideas?

 

[RF Lighting]

Size of the SWA depends upon the anticipated load.

PME earth requirements have nothing to do with sizing your armoured.

 

[baldersj]

And the answer to the question is...

 

[RF Lighting]

And the answer to the question is...

Size of the SWA depends upon the anticipated load

 

[baldersj]

OK, I'm opening myself up to all sorts of ridicule now, but I would assume that a metal shed would need bonding (although there's no earthy pipework) and that this would need a reliable connection back to the intake MET. So would bonding 10mm to the earth bar in the garage CCU, and then back via the armour of 6mm SWA be adequate?

P.S. RF sorry about the , this has been bugging me for days!

 

[securespark]

It's only the last bit which needs trenching, the rest is a concrete path which he obligingly left a a gap down the side of for the cable

Where you can dig a 450mm trench?

 

[Spark123]

OK, I'm opening myself up to all sorts of ridicule now, but I would assume that a metal shed would need bonding (although there's no earthy pipework) and that this would need a reliable connection back to the intake MET. So would bonding 10mm to the earth bar in the garage CCU, and then back via the armour of 6mm SWA be adequate?

It depends wether or not the CSA of the steel armour is equivalent to the size of copper for the main equipotential bonding conductors.

The ratio of K factors for steel armour with 90ºC insulation to copper is [143:46] therefore a 10mm² copper cable is equivalent to a 31.09mm² steel armouring of a cable. As a 2 core 6mm² only has a steel CSA of 22mm² it is inadequate for use as a 10mm main equipotential bonding conductor and an additional one is required, or revert to using TT.

Just a thought, although a 2nd opinion would be nice . I'm thinking with a 3 core the 23mm² armour equates to 7.4mm² copper equivalent, add that to the spare core of 6mm² and it should be large enough for a 10mm² MEB.
Another thing worth considering for a 30m run, is the resistance of the MEB going to be low enough i.e. 0.05ohms??

 

[baldersj]

Thanks Spark123, I'd considered that, just wanted to bounce it off some other people first, I'm still finding my feet and there's always something that you've never done before, no matter who you are! i wanted to clear up the earth problem before I did the calcs, so will see how it pans out...

Securespark, there will be a 450mm trench with tape all the way down, the gap down the side of the path will be that depth by the time the slabs and all go down, and the rest will be pure brute force...p.s. I thought the 450mm was a "rule of thumb" and not actually laid down anywhere?

Correct me if I'm wrong, thanks guys

 

[plugwash]

Just a thought, although a 2nd opinion would be nice . I'm thinking with a 3 core the 23mm² armour equates to 7.4mm² copper equivalent, add that to the spare core of 6mm² and it should be large enough for a 10mm² MEB.

just be carefull, the split of the current will depend on the resistances not the K values.

probablly best to just use 10mm with a core for earth throughout