Electrical
Voltage Drop Reference
Typical maximum drop limits by circuit type, copper/aluminium conductivity constants and the formulas behind single-phase, three-phase and DC voltage-drop planning. Use alongside the Voltage Drop Calculator.
Last updated: June 2026
Terms
- IECInternational Electrotechnical Commission
- The international body that writes electrical safety standards used across most of the world outside North America. When a chart cites an IEC number, it's pointing to the official rulebook behind a calculation.
- NECNational Electrical Code
- The set of electrical safety rules used in the United States — the American equivalent of IEC standards used elsewhere.
The Formulas
Three-phase: ΔU = (√3 × L × I × cos φ) ÷ (γ × A)
Single-phase / DC: ΔU = (2 × L × I × cos φ) ÷ (γ × A)
Where: L = one-way cable length (m) | I = load current (A) | cos φ = power factor | γ = conductor conductivity (m/(Ω·mm²)) | A = cross-section (mm²)
Percentage drop = ΔU ÷ system voltage × 100. DC circuits use the single-phase form with cos φ = 1.
Conductivity Constants (γ)
| Material | γ (m/(Ω·mm²)) | Relative to copper |
|---|---|---|
| Copper | 56 | 100 % (reference) |
| Aluminium | 35 | ~62 % — needs a larger CSA for the same drop |
These are standard planning-stage IEC values at normal operating temperature. Site-specific values can shift slightly with conductor temperature and exact alloy — use manufacturer datasheets for critical/long-run designs.
Typical Maximum Drop Limits
IEC 60364-5-52 and most national wiring regulations give recommended (not always mandatory) maximum voltage-drop guidance. Always confirm against the applicable local code.
| Circuit type | Typical max. drop | Notes |
|---|---|---|
| Lighting circuits (final) | 3 % | Tighter limit — visible flicker/dimming is more noticeable on lighting |
| Other final circuits (sockets, small power) | 5 % | Standard IEC guidance for final circuits other than lighting |
| Feeders / distribution circuits | 3 – 5 % | Often split between feeder and final-circuit allowance within an overall 5–8 % total |
| Combined feeder + final circuit (total) | 5 – 8 % | Total drop from origin to the furthest point of use, commonly capped around 5 % (lighting) to 8 % (other) |
| Motor starting / inrush | 10 – 15 % (transient) | Higher transient drop is often acceptable during motor start, distinct from steady-state limits |
Worked Example
Inputs: three-phase feeder, 32 A at 400 V, 50 m of 6 mm² copper, cos φ = 1.0.
Calculation: ΔU = (1.732 × 50 × 32) ÷ (56 × 6) ≈ 8.2 V
Result: 8.2 V ÷ 400 V ≈ 2.1 % — within the typical 3–5 % limit for final circuits.
Frequently Asked Questions
Why is the lighting limit tighter than other circuits?
Voltage drop on lighting circuits is visually noticeable — flicker, dimming or colour-temperature shift on sensitive fixtures — even at drop levels that wouldn't cause any functional problem for sockets or motor loads. That's why most codes recommend a tighter 3% limit specifically for lighting.
Is exceeding the voltage-drop limit dangerous?
Not typically a safety hazard by itself (that's what overcurrent protection is for), but it causes real operational problems: dimmer lights, motors that run hot and lose torque, equipment that browns out or behaves erratically, and energy wasted as heat in the conductor. It's a power-quality and equipment-longevity limit, not primarily a shock/fire-protection limit.
Why does aluminium need a bigger cross-section?
Aluminium's conductivity (γ ≈ 35) is roughly 62% of copper's (γ = 56) for the same cross-section. To achieve the same voltage drop, an aluminium conductor typically needs about 1.6× the cross-sectional area of an equivalent copper conductor — this is separate from (and in addition to) any ampacity-based sizing requirement.
How does this relate to the Cable Ampacity Calculator?
Ampacity (current-carrying capacity) and voltage drop are two independent checks on the same conductor — a cable must satisfy both. A conductor sized only for ampacity can still fail a voltage-drop check on long runs, and vice versa. Always check both; use the larger of the two resulting cross-sections. See the Cable Ampacity Calculator.