You dont mention voltage drop limitations but at around 1.5%, unless the circuit is commencing at a DB where the Ud is already stretched, it is well within deemed to satisfy constraints.

I do not think your calculation is wrong. I think that you have set out your calculation neatly, although if you are doing a written exam, I would advise you to indicate the table from which you are extracting your data. For example, Installation method B from Table 4A2, Cg selected from row 1 of Table 4C1, It from column 4 of Table 4D1A, mv/A/m from column 3 of 4D1B etc. That way, if a value is wrongly selected, at least the examiner can quickly see your mistake and award some marks, even if the calculation is wrong.

For those "knowledgeable individuals" who would determine you to be wrong, ask them for their computations.

Below, row 1 employs overload and assumes fault protection by virtue of 435.1, row 2 employs only fault protection as overload may not be needed, row 3 uses the X1.8 that Sparkingchip referred to.

Below that is how Ud is calculated in Ireland.

Unless there is some kind of I2t issue, Ud or Zs limitation, you can see that you could even use 1.0mm2

You dont mention voltage drop limitations but at around 1.5%, unless the circuit is commencing at a DB where the Ud is already stretched, it is well within deemed to satisfy constraints.

I do not think your calculation is wrong. I think that you have set out your calculation neatly, although if you are doing a written exam, I would advise you to indicate the table from which you are extracting your data. For example, Installation method B from Table 4A2, Cg selected from row 1 of Table 4C1, It from column 4 of Table 4D1A, mv/A/m from column 3 of 4D1B etc. That way, if a value is wrongly selected, at least the examiner can quickly see your mistake and award some marks, even if the calculation is wrong.

For those "knowledgeable individuals" who would determine you to be wrong, ask them for their computations.

Below, row 1 employs overload and assumes fault protection by virtue of 435.1, row 2 employs only fault protection as overload may not be needed, row 3 uses the X1.8 that Sparkingchip referred to.

Below that is how Ud is calculated in Ireland.

Unless there is some kind of I2t issue, Ud or Zs limitation, you can see that you could even use 1.0mm2

the voltage drop limitation is less than 5%. (voltage drop calculated is 1.147%)

for derating factor, Cg = 0.7 based on Table 4C1 | Item 1. Bunched in air, on a surface, embedded or enclosed | number of circuits or multicore cable is 3 | ..... assuming my cable laid from the DB to the load through cable trunking then conduit , so other coefficient = 1 / not included

cable current carrying capacity , 17.5A for 1.5sqmm based on Table 4D1 Single core non-armoured cable, PVC, reference method B (enclosed in conduit on a wall or in trunking, 2 cables, -single phase)

voltage drop, (mV/A/M)=29  for 1.5sqmm based on Table 4D2 Single core non-armoured cable, PVC, reference method B (enclosed in conduit on a wall or in trunking, 2 cables, -single phase)

I did ask them on the justification why 1.5sqmm is not suitable, but it seems like most of the answer is through their experiences, hence there is no computation provided. But I did not say that they are wrong and I'm correct, since my previous project also used 2.5sqmm for floodlight. But I believe as a design engineer / electricians there must be a reason / fundamental behind the design either through calculation or assumption (which I might have overlooked).