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3.5 core cable for 60 A over 200 m

Short answer

For 60 A, 10 mm² meets current rating but voltage drop requires 46.25 mm². Use 3½ core 50 mm² Cu cable; it has 145 A rating and 7.4 V drop (1.85% < 2%).

Determine the size of 3.5 core copper cables for MDB to SDB required to carry the maximum current of 60A. It is given that length of cable is 200m. The permissible voltage drop is 2% of supply voltage. The supply system is 3-phase, 400V, 50 Hz. Current ratings of copper conductor is given in table below.

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

The cable is selected by current rating and then checked for voltage drop; the larger size governs. Power factor is not given, so cosφ = 1 is assumed (largest resistive drop, conservative). ρ (Cu) = 0.0178 Ω·mm²/m.

By current rating

I = 60 A → from the table, 10 mm² carries 60 A (exactly), so 10 mm² is the minimum by current.

By voltage drop

Allowed drop = 2 % of 400 V = 8 V
ΔV = √3 × I × ρ × L / A
For 10 mm²: ΔV = √3 × 60 × 0.0178 × 200 / 10
              = 36.99 V (9.25 %)  → NOT OK
A_min = √3 × 60 × 0.0178 × 200 / 8
      = 46.25 mm² → next standard size 50 mm²
Check 50 mm²: ΔV = 369.97 / 50 = 7.40 V (1.85 %) → OK

Answer: use 3½ core, 50 mm² copper cable (rating 145 A, voltage drop 7.40 V = 1.85 % < 2 %). The long length makes voltage drop, not current, the deciding factor. (With pf 0.8 the minimum area would be 37 mm², still giving 50 mm².)

✓ Answer checked by Claude Opus 5.5 (high effort)

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