Cable and MCB size for two motors (η = pf)

Here 5 kW and 10 kW are motor output ratings, so input power = output / η. A safety factor of 1.25 on full-load current is used for MCB selection, and the cable is chosen so that its rating is not less than the MCB rating (Ib ≤ In ≤ Iz).

Input power  P_in = P_out / η
Line current I    = P_in / (√3 × V × pf)

Motor (i): 5 kW, η = pf = 0.8

P_in = 5000 / 0.8 = 6250 W
I1   = 6250 / (√3 × 415 × 0.8) = 10.87 A
1.25 × I1 = 13.59 A → MCB 16 A
Cable: rating ≥ 16 A → 1.5 mm² (21 A)

Motor (ii): 10 kW, η = pf = 0.85

P_in = 10000 / 0.85 = 11764.71 W
I2   = 11764.71 / (√3 × 415 × 0.85) = 19.26 A
1.25 × I2 = 24.07 A → MCB 32 A
Cable: rating ≥ 32 A → 4 mm² (36 A)

MDB to SDB

I_total = I1 + I2 = 10.87 + 19.26 = 30.12 A
1.25 × 30.12 = 37.66 A → MCB 40 A
Cable: rating ≥ 40 A → 6 mm² (45 A)

(Arithmetic sum of currents is taken, which is slightly conservative since the power factors differ.)

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

All drops are well below 2.5 %, so the current rating decides the size.

Answer

Use D-curve (motor) MCBs so that starting current does not cause nuisance tripping; overload protection of the motor is given by a thermal overload relay in the starter.