I would look at the factors for 0.125m, 0.25m and 0.5m and use a bit of extrapolation to determine where the grouping distance is sufficiently big enough to be ignored. At the end of the day, you have an  internally heated cylinder (ie the cable) - so a bit of 3 dimensional heat transfer analysis should show you that the heat flux is being lost to an equivalent cylinder of increasing diameter as you move away from the cable


For BS 7671 compliance, you can assume no grouping factors beyond 0.5m however


That said, unless you want to pay a fortune for trenching costs (in your example, you already have a trench approx. 1.8m wide) then it's often far more cost effective to dig a narrower trench, accept some grouping factors at say 0.25m spacing and use selected backfill to get the ground thermal resistance down to get you back to the cable size you intended to use.


Narrower trench, less excavation disposal and a smaller amount of imported sand or stonedust bedding is cheaper from a civil point of view, and still shouldn't result in larger cables due to narrower spacing as you've compensated by reducing the soil thermal resistivity


Regards


OMS

I would look at the factors for 0.125m, 0.25m and 0.5m and use a bit of extrapolation to determine where the grouping distance is sufficiently big enough to be ignored. At the end of the day, you have an  internally heated cylinder (ie the cable) - so a bit of 3 dimensional heat transfer analysis should show you that the heat flux is being lost to an equivalent cylinder of increasing diameter as you move away from the cable


For BS 7671 compliance, you can assume no grouping factors beyond 0.5m however


That said, unless you want to pay a fortune for trenching costs (in your example, you already have a trench approx. 1.8m wide) then it's often far more cost effective to dig a narrower trench, accept some grouping factors at say 0.25m spacing and use selected backfill to get the ground thermal resistance down to get you back to the cable size you intended to use.


Narrower trench, less excavation disposal and a smaller amount of imported sand or stonedust bedding is cheaper from a civil point of view, and still shouldn't result in larger cables due to narrower spacing as you've compensated by reducing the soil thermal resistivity


Regards


OMS