Cable size MDB to SDB1 (80 kW) and SDB2 (10 kW)

The cable is first chosen by current rating from the given table, then checked for voltage drop; the larger of the two sizes is selected.

Assumptions: power factor 0.8 (not given), allowed voltage drop 2.5 % of 400 V = 10 V for the MDB–SDB feeder.

Formulas and assumptions

Line current   I  = P / (√3 · V_L · cosφ)
Voltage drop   ΔV = √3 · I · (ρ·L/A) · cosφ
               (reactance neglected)
Min. area      A  = √3 · I · ρ · L · cosφ / ΔV_allowed
ρ (copper)     = 0.0178 Ω·mm²/m

MDB to SDB1 (80 kW, 10 m)

I  = 80 000 / (√3 × 400 × 0.8) = 144.34 A
From table: 50 mm² → 145 A ≥ 144.34 A
ΔV = √3 × 144.34 × 0.0178 × 10 × 0.8 / 50
   = 0.71 V  (0.18 %)  < 10 V  OK

50 mm² is just enough by current; since it runs at almost 100 % of its rating, 70 mm² (175 A) is the safer practical choice.

MDB to SDB2 (10 kW, 100 m)

I  = 10 000 / (√3 × 400 × 0.8) = 18.04 A
From table: 1.5 mm² → 21 A ≥ 18.04 A
ΔV (1.5 mm²) = √3 × 18.04 × 0.0178 × 100 × 0.8 / 1.5
             = 29.67 V (7.42 %)  > 10 V  NOT OK
A_min = √3 × 18.04 × 0.0178 × 100 × 0.8 / 10
      = 4.45 mm²  → next size 6 mm² (45 A)
ΔV (6 mm²) = 7.42 V (1.85 %)  OK

Answer: MDB–SDB1: 3½ core 50 mm² Cu (70 mm² recommended for margin); MDB–SDB2: 3½ core 6 mm² Cu. The short heavy feeder is decided by current rating; the long light feeder is decided by voltage drop.