MINIMUM CROSS-SECTION

  S_min  =  factor × ρ × L × I × cos φ / (U × ΔU%_max / 100)

  Variables :
    factor      2  (DC or AC single-phase)  or  √3  (AC three-phase)
    ρ           resistivity (Ω·mm²/m), Cu = 0.0175  /  Al = 0.029
    L           cable length (m)
    I           load current (A)
    cos φ       power factor
    U           nominal voltage (V)
    ΔU%_max     maximum allowed voltage drop (%)

  Rationale : this is the inverse of the voltage-drop formula. Instead
  of computing ΔU for a given cross-section, we determine the minimum
  cross-section that satisfies ΔU% ≤ target.


STANDARD ROUNDING

  Standard cross-sections (mm²) :
    1.5  →  2.5  →  4  →  6  →  10  →  16  →  25  →  35  →  50
    →  70  →  95  →  120  →  150  →  185  →  240  →  300

  Rationale : the computed S_min must always be rounded UP to the next
  standard cross-section — these match the cables actually available
  on the market (IEC 60228 class 2 conductors).


THERMAL CAPACITY — INDEPENDENT CHECK

  Iz  ≥  I  (continuous current capacity of the cable ≥ load current)

  Rationale : this calculator sizes ONLY for voltage drop. You must
  ALWAYS also verify the thermal capacity Iz, which depends on the
  installation method, ambient temperature and grouping with other
  cables (IEC 60364-5-52 tables). On short runs Iz often drives the
  sizing, not ΔU.