The formula
ΔU = (2·L·I·(R·cosφ + X·sinφ)) / 1000 for single phase. For 3-phase the 2 becomes √3. Per IEC 60364-5-52, the drop for lighting is up to 3%, for others up to 5%.
Case: garden line 80 m
80 m of 3×2.5 mm² at 16 A: R≈7.4 Ω/km, ΔU ≈ 2·80·16·0.0074 ≈ 18.9 V → 8.6%. Too much. Solution: 3×6 mm² → ΔU ≈ 2·80·16·0.0037 ≈ 9.5 V → 4.3% — fits 5%.
Case: cottage with sauna
Sauna 9 kW 3-phase at 40 m: I=13 A per phase. 5×4 mm²: ΔU ≈ √3·40·13·0.0055 ≈ 5.0 V → 2.2% — fine. GorkyCAD recommends 5×6 if the line is in the ground.
Case: office floor
Server room 20 A at 25 m: 3×4 mm²: ΔU ≈ 2·25·20·0.0055 ≈ 5.5 V → 2.5% — acceptable for sockets; for the UPS input, take 3×6 mm².
Why GorkyCAD counts it
The program calculates ΔU for every line and marks failures in red — before installation. You do not guess: 70+ checks cover cable, breaker and drop together.
Practical rule
Up to 20 m — standard sections work. Over 30 m — check every line. Over 50 m — almost always increase the section by one step.