Connecting to an earth rod

[Martin Wiseman]

Hi,

I'm putting in a 20A supply to my garden shed. 10m of 4mm SWA cable from the house to a small CU containing a 30mA RCD and using a local earth rod to provide TT earthing.

Clearly the cable connecting to the earth spike doesn't need to have a particularly great CSA for operational purposes (even the resistance of

1mm cable would be vastly less than the earth resistance of the rod and more than capable of carrying 30mA), however I was wondering if the regulations actually specify a minimum cable CSA.

In the absence of specific guidance I will probably use 2.5mm or 4mm cable.

Can anyone confirm what the regulations have to say on the subject.

Thanks, Martin.

[Andy Wade]

[Earthing conductor for TT installation]

They certainly do. As you say, current carrying capacity is not the issue - but mechanical robustness and longevity is.

Above ground the size needs to be 4 mm^2 or larger, unless protected against corrosion and mechanical damage (e.g. by being run in a conduit or trunking system), in which case the size can be reduced to 2.5 mm^2.

The minimum size for an earthing conductor buried in the ground is 25 mm^2, or 16 mm^2 if protected against corrosion (e.g. by PVC insulation). If protected against both corrosion and mechanical damage, as above, the minimum size falls to 2.5 mm^2 again.

Normal practice is to use at least 16 mm^2 6491X green/yellow insulated conduit cable - just as you would for the main earthing on a TN-earthed installation. Remember that the connection between the earthing conductor and the earth rod needs to be accessible, unless welded.

HTH