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Cable size MDB to SDB1 (70 kW) and SDB2 (90 kW)

Short answer

For SDB1 (70 kW, 30 m), select 3½ core 50 mm² Cu (drop 1.87 V). For SDB2 (90 kW, 120 m), select 3½ core 70 mm² Cu (drop 6.87 V). Both within 2.5% limit.

Determine the size of cable used from MDB to SDB1 and SDB2 for following conditions. The total power on the SDB1 and SDB2 are 70 kW and 90 kW respectively. The distance between MDB to SDB1 and SDB2 are 30 m and 120 m respectively. The supply system is 3 phase 4 wire system with 400 volts. Current rating of Copper conductor for laid in ground is given:

Conductor area (sq.mm)1.52.54610162535507095120150
3.5 core cable current rating (A)21273645607799120145175210240270

Each feeder is sized by current rating from the 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 × 400 × 0.8)
ΔV = √3 × I × ρ × L × cosφ / A

MDB to SDB1 (70 kW, 30 m)

I = 70000 / (√3 × 400 × 0.8) = 126.30 A
By current: 50 mm² (145 A)
ΔV = √3 × 126.30 × 0.0178 × 30 × 0.8 / 50
   = 1.87 V (0.47 %)  OK

MDB to SDB2 (90 kW, 120 m)

I = 90000 / (√3 × 400 × 0.8) = 162.38 A
By current: 70 mm² (175 A)
ΔV = √3 × 162.38 × 0.0178 × 120 × 0.8 / 70
   = 6.87 V (1.72 %)  OK
FeederI (A)CableRating (A)ΔV
MDB–SDB1126.3050 mm²1451.87 V (0.47 %)
MDB–SDB2162.3870 mm²1756.87 V (1.72 %)

Answer: MDB–SDB1: 3½ core 50 mm² Cu; MDB–SDB2: 3½ core 70 mm² Cu.

✓ Answer checked by Claude Opus 5.5 (high effort)

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