Cable and MCB size for two motors

Efficiency is not given, so the 5 kW and 10 kW ratings are taken as the electrical input power of the motors (η = 1). MCBs are chosen with a 1.25 safety factor on full-load current, and cables so that their rating is not less than the MCB rating (Ib ≤ In ≤ Iz).

I = P / (√3 × V_L × pf)

Motor (a): 5 kW, pf 0.8

Ia = 5000 / (√3 × 415 × 0.8) = 8.70 A
1.25 × 8.70 = 10.87 A → MCB 16 A
Cable rating ≥ 16 A → 1.5 mm² (21 A)

Motor (b): 10 kW, pf 0.85

Ib = 10000 / (√3 × 415 × 0.85) = 16.37 A
1.25 × 16.37 = 20.46 A → MCB 32 A
Cable rating ≥ 32 A → 4 mm² (36 A)

MDB to SDB

I = 8.70 + 16.37 = 25.06 A
1.25 × 25.06 = 31.33 A → 32 A would be enough,
but for discrimination with the 32 A motor MCB
the next size is taken → MCB 40 A
Cable rating ≥ 40 A → 6 mm² (45 A)

Voltage drop check (10 m, Cu, ρ = 0.0178 Ω·mm²/m)

ΔV = √3 × I × ρ × L × cosφ / A
Motor (a): √3 × 8.70 × 0.0178 × 10 × 0.8 / 1.5
         = 1.43 V (0.34 %)
Motor (b): √3 × 16.37 × 0.0178 × 10 × 0.85 / 4
         = 1.07 V (0.26 %)
MDB–SDB : about 1.1 V (0.26 %)

All are far below 2.5 %, so the current rating governs.

Answer

If the 5 kW and 10 kW are output ratings, the currents rise by the efficiency factor; with a typical η of about 0.85 the same MCB and cable sizes still apply.