The formula
I_z = I_table × k_ambient × k_grouping ; V_drop = √3 × I × ρL/A [three phase]
- I_table
- published rating for the cable type and installation method
- k_ambient
- derating for ambient temperature (1.0 at 40 °C, 0.82 at 50 °C)
- k_grouping
- derating for circuits bunched together (0.80 for two, 0.70 for three)
- ρ
- resistivity at the 70 °C the conductor actually runs at — 0.0206 Ω·mm²/m for copper
- L
- one-way run length in metres
- A
- conductor cross-section in mm²
Worked example
32 A three-phase load on 415 V, 40 m of copper in conduit at 40 °C, one circuit, 3 % drop allowed.
- 6 mm² is rated 29 A for three loaded conductors — below 32 A, so too small
- 10 mm² is rated 40 A; no derating applies at 40 °C with one circuit, so I_z = 40 A ✓
- Voltage drop = 1.732 × 32 × (0.0206 × 40 / 10) = 4.57 V
- As a percentage: 4.57 / 415 = 1.10 % ✓
10 mm² copper. Capacity 40 A, drop 1.10 % — inside the 3 % limit.
Where you'll use it
Wiring a sub-circuit, a motor feeder or a submersible pump. Ratings quoted here are typical published values for PVC-insulated conductors in conduit at 40 °C, of the kind given in IS 732; the derating and voltage-drop arithmetic is exact, but check the final choice against the cable maker's own table before installing.