Earthing Conductor Sizing

Dear All,

I have done electrical design for a 7 apartment block. A 400A panel boad fitted with 250/400A adjustable MCCB incomer, TT earthing system. The main suppy cable is 4*185mmsq pvc/swa/pvc.
No earthbar, so the protective cable, in this case earhing cable, by the IEE reg. should be 95mmsq to the earthrod.

Is this too big or ok, as one would argue it doesn't matter much since it is path of least resistance and that the fault current would be conducted to ground in some microsecs.

Also, the same specs as above runs for the genset standby supply.
Is it imperative to bond their earths together?

I look forward to your responses soon.

regards,
kebian