I would completely leave that up to the electrician and the code.8

I am away from my desk but minimum for a 125A residential panel would be #2 gauge power conductors and #6 bond cable.

But due to the fact of the sub panel with a (predetermined load of 80A) being approximatly 85m away.

I would size up to a 3/0 conductor & #3 bond. This is a signficant increase as this conductor is rated for 225A. The issue you must remember is voltage lose due to line lose (I^2 x R)..... and to correct this , increase the conductor size decreases the resistance.

You can either pipe this or find a cable that ca meet your application.

The typical voltage drop calc is Vd = KIL/Kcmil.

Vd = allowable amount of voltage drop.
K = the constant of your conductor Ohms per mil foot (as they say in the states) de-rated by the operating temperature.
I = the normal current flow of the conductor
L = the length of the conductor to and from
Kcmil = is the cross sectional area of the conductor

In order to answer your question correctly we need to know the allowable voltage drop, the operating temperature, type of conductor material and type of insulation of the conductors.

Maxximus98,

You also left off a significant piece of data--the circuit voltage. Good design will limit total voltage drop to 5% of the circuit voltage, with this being allocated 2% to the service and upstream feeders and 3% to the circuit in question. The higher your system voltage, the more loss is allowed on this panel feeder. Smaller wire sizes can work at higher voltages, when they would be harmful at lower voltages.

--JMM

It would _really really REALLY REALLY be nice to know where you are located OP! NEC information might make no sense at all if you are located in China, or India, or Europe, or many other places.

Much Thanks

Bill