DoktorB wrote: I don't think I have a choice.
It will apply to change the connection capacity from 5 to 15kW and in 3 phases. Pre-meter protection will be changed from 1x25A probably to 3x25A. Therefore, the load on a single phase / conductor will not change.
The hob has a power of 9kW. It is powered by a 5x2.5 mm2 cable, so it should be enough.
From the meter to the distribution box goes 3x6 mm2. And here the question arises: Can I adapt this cable to a 3-phase installation by adding two 6 mm2 wires, or do I have to replace the entire cable with 5x6 mm2? Laying the two wires will be cheaper and easier. It is enough to make a groove in the plaster to the thickness of the grinding disc and that's it. Or am I wrong? Is there any rule that the wire must be shared, or can it be caused by separate wires?
You ask about such matters that it would probably be better for you to entrust this work to an electrician from the installation. Well
you can not place the DY cables directly in the plaster. So, it is excluded to make grooves in the thickness of the grinding wheel. The cables must be inserted into a conduit, or a YDY cable should be used, in addition, it will be necessary to use cables with appropriate insulation colors. That is why I think that it is not worth saving a few zlotys and you have to put all the new ones, because otherwise the old ones have to be disconnected from the power supply and we will deprive ourselves of power, even for power tools and light. And I do not think that a cross section of 6 mm is necessary. 4 mm is enough.
Besides, one note: for each kW of allocated power, you must pay. It is from 120 - 150 PLN. Therefore, it is not profitable to apply excessively, because you pay without need. Consider how many times you will need the stove at full power and whether you need to use other high - energy appliances at the same time.
Or maybe it is enough to use a priority relay? I recently connected an induction cooker (without an oven), where the manufacturer only wanted a 4 x 1.5 mm cable for a 3-phase power supply. So maybe it is worth for an experienced electrician to calmly look at all of this.