Cable voltage drop: units, route length and distributed loads
Voltage drop calculations become unreliable when the route length, current or resistance basis is unclear. Start by identifying the circuit arrangement, conductor material, load distribution and operating temperature. A cable can meet a voltage drop check and still fail thermal, fault or installation requirements.
Use one unit system
For a balanced three-phase circuit, ΔV = √3 × I × L × (R cosφ + X sinφ). Use line current in amperes and one-way route length in kilometres when resistance and reactance are in ohms per kilometre. Percentage drop is 100 × ΔV / line-to-line supply voltage.
For a two-wire DC circuit with equal outgoing and return conductors, ΔV = 2 × I × R × L. The factor of two includes the return path. Do not double the length again. Single-phase AC also needs the reactance and power factor terms where material.
Worked example: three-phase feeder
Take 100 A, 400 V, a 60 m route, 0.90 power factor, R = 0.40 Ω/km and X = 0.08 Ω/km. These impedances are hypothetical values, not a cable-size lookup. With sinφ = √(1 − 0.90²) = 0.436, ΔV = 1.732 × 100 × 0.060 × (0.40 × 0.90 + 0.08 × 0.436) = 4.10 V, or 1.03%.
For this fixed-current example, doubling the route length doubles the voltage drop. Increasing cable size changes resistance, but the actual reactance, grouping and installation method must come from the appropriate cable data.
Distributed loads: why the model matters
Imagine ten identical DC fittings at equal intervals. The first cable segment carries current for ten fittings; the last carries current for one. Calculate the drop in each segment from its downstream load, then add the segment drops. A lumped-end calculation instead uses the total current over the full length. It is a different model and should be labelled accordingly.
Complete the selection
- Check current-carrying capacity with ambient, grouping and installation corrections.
- Check fault withstand and the protective device's operating behaviour.
- Check motor starting where applicable, terminal temperature ratings and parallel conductor arrangements.
- Compare total installation drop against the applicable project and jurisdiction requirements.
The example deliberately supplies no universal percentage limit. Apply the limit for the actual circuit and governing requirements, and include upstream drop rather than treating every feeder in isolation.
Further reference
Schneider Electric: steady-load voltage drop equations. External guidance supports the topic; the illustrative inputs and worked examples above are prepared for this guide and are not manufacturer product selections.
Verify assumptions and results before project use. These worked examples are educational and do not provide authority approval. Report a correction.