All common cable sizes for your circuit. Blue row = minimum recommended size.
| Cable | VD (V) | VD (%) | Ampacity (A) | Status |
|---|
House circuit: 2.4kW air conditioner, 25m from switchboard
Scenario: A 2,400W split-system air conditioner is installed 25 metres from the main switchboard. Supply is 230V single phase. Cable is 2.5mm² copper TPS in conduit. Power factor 0.95.
Step 1 — Find current: I = P / (V × PF) = 2,400 / (230 × 0.95) = 10.98 A
Step 2 — Apply formula: VD = 2 × L × I × ρ / A = 2 × 25 × 10.98 × 0.0175 / 2.5
Step 3 — Calculate: VD = 2 × 25 × 10.98 × 0.007 = 3.84 V
Step 4 — Check %: VD% = 3.84 / 230 × 100 = 1.67%
✅ PASS — 1.67% is well within the AS/NZS 3000 limit of 5%. 2.5mm² is adequate.
Factory sub-board: 22kW motor, 80m cable run
Scenario: A 22kW three-phase motor is supplied from a sub-board 80 metres away. Supply is 400V three phase. Cable is 6mm² copper. Full load current is 40A. Power factor 0.86.
Step 1 — Formula for 3-phase: VD = √3 × L × I × ρ / A
Step 2 — Calculate: VD = 1.732 × 80 × 40 × 0.0175 / 6 = 16.13 V
Step 3 — Check %: VD% = 16.13 / 400 × 100 = 4.03%
⚠️ BORDERLINE — 4.03% is within 5% but tight. Consider upgrading to 10mm² (VD = 2.42%) for any future load growth.
Solar battery system: 48V DC, 60A, 12m cable run
Scenario: A 48V DC solar battery system feeds an inverter 12 metres away. The battery cable carries 60A and is 16mm² copper. Maximum allowable voltage drop is 3%.
Step 1 — DC uses same formula as single phase: VD = 2 × L × I × ρ / A
Step 2 — Calculate: VD = 2 × 12 × 60 × 0.0175 / 16 = 1.58 V
Step 3 — Check %: VD% = 1.58 / 48 × 100 = 3.28%
⚠️ SLIGHTLY OVER 3% — Upgrade to 25mm² (VD = 1.01V, 2.10%) to meet the 3% design limit.
DOL motor start: 15kW, checking inrush voltage drop
Scenario: A 15kW DOL motor (full load current 28A) starts on a 400V three-phase circuit. Cable is 6mm² copper, 30m long. DOL starting current is 7× FLC = 196A. Max allowable starting VD is 15%.
Normal running VD: VD = 1.732 × 30 × 28 × 0.0175 / 6 = 4.24 V (1.06%)
Starting VD (196A): VD = 1.732 × 30 × 196 × 0.0175 / 6 = 29.7 V (7.42%)
Check ampacity: 6mm² copper rated approx. 38A — adequate for 28A FLC with 1.35× headroom.
✅ PASS — 7.42% starting VD is within the 15% limit. However, check that other sensitive equipment on the same supply is not affected by the momentary dip.
Three Phase AC: VD = √3 × L × I × ρ / A
Where: L = one-way cable length (metres), I = current (amps), ρ = resistivity (0.0175 Ω·mm²/m for copper, 0.028 for aluminium), A = cable cross-sectional area (mm²). The factor of 2 in single phase accounts for the active and neutral conductors — current travels down and back. In three phase, √3 (≈1.732) is used instead because the three phases partially cancel each other out.
What Is Voltage Drop? A Complete Guide for Electricians and Engineers
Voltage drop is one of the most fundamental concepts in electrical installation design. Whether you're wiring a house, sizing cables for an industrial motor, or designing a solar battery system, understanding and calculating voltage drop correctly is essential for safe and compliant electrical installations.
The Physics of Voltage Drop
Every conductor — whether copper or aluminium — has electrical resistance. When current flows through a resistance, energy is lost as heat, and the voltage at the far end of the cable is lower than at the source. This reduction in voltage is what we call voltage drop.
The relationship is described by Ohm's Law: V = I × R, where V is the voltage drop in volts, I is the current in amps, and R is the total resistance of the cable in ohms. For a two-wire circuit (active and neutral, or positive and negative), the total resistance is twice the resistance of a single conductor, which is why the factor of 2 appears in the standard formula.
Three Phase AC: VD = √3 × L × I × ρ / A
Resistivity Values
| Material | Resistivity ρ (Ω·mm²/m) | Relative to Copper |
|---|---|---|
| Copper | 0.0175 | 1.0× |
| Aluminium | 0.028 | 1.6× |
| Gold | 0.022 | 1.26× |
| Silver | 0.016 | 0.91× (better, but too expensive) |
Australian Standards for Voltage Drop
In Australia, voltage drop limits are governed by AS/NZS 3000:2018 (Wiring Rules), which specifies that the voltage drop from the point of supply to any point in the installation shall not exceed 5% of the nominal supply voltage.
- 230V single phase: maximum 11.5V drop
- 400V three phase: maximum 20V drop
- Extra-low voltage (ELV) circuits (e.g. 12V, 24V): often limited to 5% which is just 0.6V or 1.2V — making cable sizing critical
Many designers work to a tighter 3% limit to allow for future load growth and to account for the fact that supply voltage itself may vary ±6% from nominal. Some sensitive equipment such as variable frequency drives (VFDs) and medical equipment specify even tighter voltage tolerance.
Factors Affecting Voltage Drop
Four variables determine voltage drop: current (I), length (L), cable cross-section (A), and resistivity (ρ). Of these, the designer typically controls cable size and routing. To reduce voltage drop, you can:
- Increase cable size — doubling the cross-section halves the voltage drop
- Reduce cable length — move the distribution board closer to the load where possible
- Reduce load current — improve power factor or reduce the load
- Use copper instead of aluminium — copper has 37.5% lower resistivity
- Run parallel cables — two cables in parallel halve the effective resistance
Voltage Drop vs. Ampacity — Two Separate Checks
A common mistake is to think that a cable sized for ampacity is automatically adequate for voltage drop. This is not always the case. For long cable runs — particularly in industrial, agricultural and rural settings — voltage drop is often the more limiting factor than ampacity. It is essential to check both:
- Ampacity check: Can the cable carry the load current without overheating?
- Voltage drop check: Is the voltage at the load end within the allowable limit?
Take the larger cable size from both checks.
Motor Starting Voltage Drop
Electric motors draw a large inrush current when they start — typically 5 to 7 times their full load current for direct-on-line (DOL) starters. This inrush lasts for 2–10 seconds and causes a momentary voltage dip on the supply. The voltage drop during starting must be checked separately from running voltage drop, as the currents involved are much larger.
Starting methods and their typical current multipliers:
| Starting Method | Starting Current (× FLC) | Notes |
|---|---|---|
| Direct-on-line (DOL) | 6–7× | Highest inrush, simplest control |
| Star-Delta | 2–3× | Reduces starting torque too |
| Soft Starter | 2–4× | Programmable ramp time |
| Variable Frequency Drive (VFD) | 1–1.5× | Best control, highest cost |
| Autotransformer | 2–4× | Used for large motors |
Voltage Drop in DC and ELV Systems
DC systems — including solar battery installations, EV charging systems, 12V/24V lighting, and telecommunications — can be particularly susceptible to voltage drop because the system voltage is low. A 1.2V drop on a 230V circuit is just 0.5%, but on a 12V circuit it is 10% — significant enough to cause LED lights to flicker, solar charge controllers to malfunction, or battery charging to be incomplete.
For 12V and 24V systems, it is common to design for a maximum of 2–3% voltage drop, requiring substantial cable cross-sections for high-current loads over any meaningful distance.
International Standards Summary
| Region | Standard | Max VD (typical) |
|---|---|---|
| Australia / New Zealand | AS/NZS 3000 | 5% |
| United States | NEC (informational note) | 3% branch, 5% total |
| India | IS 732 | 5% (lighting 3%) |
| South Africa | SANS 10142 | 5% |
| United Kingdom | BS 7671 | 3% (lighting), 5% (other) |
| Europe | IEC 60364-5-52 | 3% (lighting), 5% (other) |