How it works
Cable manufacturers publish a mV/A·m figure for each conductor size — it captures the resistance per metre at a standard temperature. Multiply by current and length to get the drop in millivolts, then divide by 1,000 for volts. Compare the result to your maximum allowable drop (AS/NZS 3008 allows up to 5% for most sub-mains).
Worked example
20 A × 30 m × 7.41 mV/A·m = 4,446 mV ÷ 1,000 = 4.45 V. As a percentage: 4.45 ÷ 230 × 100 = 1.93%.
Common cable mV/A·m values (copper, single-phase)
1.5 mm²: 27.5 | 2.5 mm²: 16.6 | 4 mm²: 10.4 | 6 mm²: 7.41 | 10 mm²: 4.40 | 16 mm²: 2.78 | 25 mm²: 1.78. These are approximate for thermoplastic (PVC) at 75 °C — check the cable manufacturer's data or AS/NZS 3008 Table 42 for your exact installation.
Assumptions & limitations
Uses the cable's published mV/A·m at rated temperature. Real drop varies with ambient temperature, installation method and power factor. This gives the single-phase drop; for three-phase, multiply by √3 ÷ 2 (≈ 0.866).