Hi All,

Currently working on an early stage design for a project with a 3200A main service from a 2MVA substation. Main cable is 8x 4x 630mm2. Earthing is TNS.

Following the rule of thumb, the earthing conductor (half line conductor) would be 4x 4x 630mm2 and the main bonding (half earthing conductor) would be 2x 4x 630mm2.

We have multiple earth bars in switchrooms, plant rooms and comms rooms across the project. It would be unrealistic to run this much copper to each item bonded to each earth bar, but BS7671 doesn't explain the correct method to size bonding in a more complex installation.

IET GN8 table 5.1 states main bonding for a 70mm2 - 400mm2 conductor can be 25mm2 for a TNS system. Does this rule continue to apply as the line conductor increases? Is a blanket size of 25mm2 across the project sufficient?

All submains have their own correctly sized CPCs, this is purely for bonding to structure, mechanical services, lightning protection, comms equipment etc.

Any help much appreciated.