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Cable size MDB to SDB-1 (60 kW) and SDB-2 (75 kW)

Short answer

SDB-1 (60 kW, 80 m): 3½ core 35 mm² Cu (drop 6.10 V). SDB-2 (75 kW, 125 m): 3½ core 50 mm² Cu (drop8.34 V). Both within2.5% limit.

Determine the size of cable used from MDB to SDB-1 and SDB-2 for the following conditions. The total power on the SDB-1 and SDB-2 are 60 kW and 75 kW respectively. The distance between MDB to SDB-1 and SDB-2 are 80m and 125m respectively. The supply system is 3-phase, 4 wire with 400V, 50 Hz. The current ratings of Cu conductor laid in ground are given as follow.

Conductor area (sq.mm)1.52.54610162535507095120150
3.5 core cable current rating (A)21273145607799120145175210240270

Each feeder is sized by current rating from the given table and checked for voltage drop.

Assumptions: pf = 0.8 (not given), allowed drop 2.5 % of 400 V = 10 V, ρ (Cu) = 0.0178 Ω·mm²/m.

I  = P / (√3 × V_L × cosφ)
ΔV = √3 × I × ρ × L × cosφ / A

MDB to SDB-1 (60 kW, 80 m)

I = 60000 / (√3 × 400 × 0.8) = 108.25 A
By current: 35 mm² (120 A)
ΔV = √3 × 108.25 × 0.0178 × 80 × 0.8 / 35
   = 6.10 V (1.53 %)  OK

MDB to SDB-2 (75 kW, 125 m)

I = 75000 / (√3 × 400 × 0.8) = 135.32 A
By current: 50 mm² (145 A)
ΔV = √3 × 135.32 × 0.0178 × 125 × 0.8 / 50
   = 8.34 V (2.09 %)  OK (< 2.5 %)
FeederI (A)CableRating (A)ΔV
MDB–SDB-1108.2535 mm²1206.10 V (1.53 %)
MDB–SDB-2135.3250 mm²1458.34 V (2.09 %)

Answer: MDB–SDB-1: 3½ core 35 mm² Cu; MDB–SDB-2: 3½ core 50 mm² Cu. If a stricter 2 % limit is used, SDB-2 needs 70 mm² (drop 5.96 V = 1.49 %).

✓ Answer checked by Claude Opus 5.5 (high effort)

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