I am running electricity (for light and a couple of outlets) into my garden. I am trying to decide on which wire thickness I need to choose. FWIW, I am in Europe.
The whole house is wired with two types of wires: outlets with 2.5 mm2 (roughly 13 AWG) and ceiling lights and corresponding switches with 1.5 mm2 (roughly 15–16 AWG). All breakers are rated at 16 A, and there are two groups of them, each protected by a 20 A breaker with 30 mA RCD.
Since 2.5 mm2 wires are much stiffer and hence difficult to run through conduits (and it's not a common practice to use stranded wires for wall installations in Europe), I am inclined to run 1.5 mm2 wires into my garden.
However, as the existing installation and googling show, it is a common practice to use 2.5 mm2 or thicker wires for everything but lighting, and 1.5 mm2 for lighting. I wonder why? Aren't these standards outdated?
My logic is as follows. A 1.5 mm2 wire is rated for 16 A (I used this tool, and entered 30 °C, PVC insulation, 3 elements in confined bundle). At 230 V, this gives the maximum power capacity of 3680 W. The highest power device I was able to think of for a garden is Kärcher K7, which is rated at 2.8 kW. Thus, 1.5 mm2 wire should be more than adequate.
Obviously for modern LED lighting, even 1 mm2 is ridiculously thick, based on such calculation.
Another aspect to consider is the voltage drop. Copper's resistivity is 17 nΩ·m. Hence a 1.5 mm2 20 m cable has a return-path resistance of R = rho × L / A = 0.45 Ω. It will cause, at full current, a voltage drop of 7.2 V, which is probably perfectly fine for all modern devices. A connected LED bulb might be more picky, but it consumes only 10 W, so the voltage drop will be much less, 10 / 230 × 0.45 = 0.02 V.
Where am I wrong in my consideration?