Voltage Drop Calculator Australia — AS/NZS 3000AS/NZS 3000
Circuit details
Measure the one-way cable route. The calculator doubles this for single-phase (live + neutral return).
Voltage drop
Circuit details
Result
Minimum compliant cable size
✓ Compliant — AS/NZS 3000 Clause 3.6.2
⚠ Non-compliant — AS/NZS 3000 Clause 3.6.2
This voltage drop calculator uses the formula specified in AS/NZS 3000:2018 (Australian/New Zealand Wiring Rules). Voltage drop is calculated as Vd = ρ × L × I / A, where ρ is the conductor resistivity at 75°C operating temperature (0.0225 Ω·mm²/m for copper, 0.0357 Ω·mm²/m for aluminium), L is the total conductor length (doubled for single-phase to account for the return path, or multiplied by √3 for three-phase), I is the design current in amps, and A is the conductor cross-sectional area in mm². The result is compared against the AS/NZS 3000 Clause 3.6.2 limit of 5% of the nominal supply voltage — 11.5 V for 230 V single-phase circuits.
Frequently Asked Questions
What is the maximum voltage drop allowed in Australia under AS/NZS 3000?
AS/NZS 3000:2018 Clause 3.6.2 sets the maximum allowable voltage drop at 5% of the nominal supply voltage from the point of supply to any point of utilisation. For a standard 230 V single-phase supply this is 11.5 V. For 400 V three-phase (line-to-line), it is 20 V. Many designers use 2.5% for sub-mains and 2.5% for final circuits to stay within the 5% total.
Why does voltage drop matter?
Excessive voltage drop causes equipment to run at reduced voltage, which can lead to overheating in motors, poor lighting output, nuisance tripping of sensitive electronics, and reduced efficiency. For motors, even a 5% drop in voltage can cause a 10% reduction in torque output and significant heating. For long cable runs to outbuildings, pumps, or sub-boards, voltage drop is often the limiting factor in cable sizing, not current-carrying capacity.
How do I reduce voltage drop on a long cable run?
The most effective ways are: (1) increase the cable cross-sectional area — doubling the CSA halves the voltage drop; (2) reduce the current by splitting the circuit; (3) increase the supply voltage where permitted; or (4) use three-phase instead of single-phase for long runs, as three-phase has a lower voltage drop per amp for the same CSA. Moving the switchboard closer to the load is also effective.
What conductor resistivity does this calculator use?
The calculator uses ρ = 0.0225 Ω·mm²/m for copper and 0.0357 Ω·mm²/m for aluminium at an assumed conductor operating temperature of 75°C, consistent with the values in AS/NZS 3008.1.1 (Selection of Cables). These figures are slightly higher than the 20°C DC resistance values to account for the increased resistivity at operating temperature.
Does voltage drop affect my circuit breaker sizing?
Voltage drop and circuit breaker (overcurrent protection) sizing are separate calculations in AS/NZS 3000. The circuit breaker is sized for overcurrent protection, not voltage drop. However, a cable sized purely for current-carrying capacity may still fail the voltage drop check on a long run, requiring an upgrade to a larger cable — with the breaker adjusted accordingly if the cable rating changes significantly.
How to use this calculator
- Select circuit type — single-phase (230 V) for most household and commercial final circuits; three-phase (400 V) for large motors, sub-mains, or three-phase equipment.
- Select conductor material — copper is standard for most Australian installations. Aluminium is common in large service cables and distribution mains.
- Choose cable size — select the cross-sectional area (CSA) in mm² you are checking, or start with your initial cable selection and adjust based on the result.
- Enter load current or power — enter amps directly if known, or enter the load in watts. For motors and air conditioning, use a power factor of 0.8; for resistive loads (heaters), use 1.0.
- Enter the one-way cable run length — measure the route the cable takes from the source to the load. The calculator doubles this automatically for single-phase to account for the live and neutral return path.
- Select the voltage drop limit — AS/NZS 3000 Clause 3.6.2 sets a 5% maximum for the total installation. Use 2.5% if checking a sub-main and you want headroom for the final circuit.
- Click "Calculate Voltage Drop" and review the compliance result.
Worked example: A 10 mm² copper single-phase circuit carries 32 A over a 45 m run. Vd = 0.0225 × 2 × 45 × 32 ÷ 10 = 6.48 V = 2.82% of 230 V — compliant within the 5% (11.5 V) limit. The minimum compliant cable for this circuit is 6 mm² copper (which gives 10.8 V = 4.70%).
Understanding your results
The voltage drop in volts is the actual voltage lost across the cable length at the stated design current and 75°C operating temperature. The percentage compares this against the nominal supply voltage (230 V for single-phase, 400 V line-to-line for three-phase). AS/NZS 3000 Clause 3.6.2 limits this to 5% from the point of supply to any point of utilisation.
The voltage at load end shows the voltage your equipment will actually receive. Sensitive electronics, variable-speed drives, and fluorescent luminaires are particularly affected by low voltage — many are designed for ±6% of nominal, giving a practical floor of about 216 V.
The minimum compliant cable size is the smallest standard Australian cable size that passes the selected limit for your circuit. If this is larger than your current-carrying capacity selection, you must upgrade to the larger size.
Common mistakes: Forgetting to double the run for single-phase return path (the most common error); using the cable's 20°C DC resistance rather than the 75°C operating temperature value; treating voltage drop as the only design criterion — current-carrying capacity, fault loop impedance, and protection device coordination must also be verified by a licensed electrician.
Voltage drop in Australian electrical installations
Voltage drop is one of the most frequently miscalculated aspects of electrical circuit design in Australia. AS/NZS 3000:2018 (Wiring Rules) sets the limit at 5% of the nominal supply voltage — 11.5 V for 230 V single-phase circuits, and 20 V for 400 V three-phase — measured from the point of supply (typically the utility meter or main switchboard) to the furthest point of utilisation.
Sub-mains versus final circuits
The 5% limit applies to the total cable path. A well-engineered installation typically allocates the budget as 2.5% for the sub-main (from main switchboard to distribution board) and 2.5% for the final circuit (from DB to the load). This leaves headroom for both sections and avoids non-compliance when loads are on the far end of a long circuit. For sub-mains feeding large buildings or outbuildings, this split may need to be revisited with a detailed load schedule.
Long cable runs — rural and outbuilding circuits
On Australian rural properties, cable runs to sheds, pump stations, or outbuildings can exceed 100–200 m. On these runs, voltage drop is almost always the governing design criterion — not current-carrying capacity. It is common to need cables 2–4 sizes larger than the minimum current rating would suggest. Three-phase supply is often more economical than single-phase for runs beyond 80–100 m, as the lower voltage drop factor reduces the required CSA.
Solar and battery inverter circuits
Grid-connected inverters (AS/NZS 4777.1) are sensitive to voltage — most will disconnect if voltage rises too high due to export current. Voltage rise (the reverse of voltage drop) from solar export must also be managed. Some DNSPs require the installer to demonstrate the cable size limits voltage rise to 2% on the circuit from the inverter to the switchboard. This calculator calculates drop but the principle and formula are the same — just reverse the direction of current flow for export conditions.
Aluminium wiring
Aluminium conductors have approximately 1.6× higher resistivity than copper (ρ = 0.0357 Ω·mm²/m vs 0.0225 for copper at 75°C). Aluminium is cost-effective for large sub-mains (35 mm² and above) where the weight and cost savings are significant. However, aluminium requires specialised connectors (lugs), anti-oxidant compound, and careful termination practice — using aluminium in final circuits is generally not recommended without specific design consideration.
Who can carry out electrical work?
All electrical installation work in Australia must be carried out by a licensed electrician — the specific licence category varies by state (electrical contractor licence, electrical worker licence, restricted licence). Voltage drop calculations are a design tool for licensed electricians and their clients. The result of this calculator does not constitute an electrical design — a licensed electrician must verify all design criteria before any installation work begins.
Australian standards and references
- AS/NZS 3000:2018 — Wiring Rules. Clause 3.6.2 sets the 5% maximum voltage drop limit for all Australian electrical installations.
- AS/NZS 3008.1.1:2017 — Selection of Cables (Polymeric Insulated — Up to and Including 0.6/1 kV). Provides conductor resistivity values, current-carrying capacity tables, and derating factors used in cable selection.
- State electrical safety regulations — Each state has its own electrical safety legislation: NSW (Electricity (Consumer Safety) Act 2004), VIC (Electricity Safety Act 1998), QLD (Electrical Safety Act 2002), WA (Electricity (Licensing) Regulations 1991), SA (Electricity Act 1996).
- AS/NZS 4777.1:2016 — Grid Connection of Energy Systems via Inverters. Relevant for voltage rise calculations on solar inverter circuits.
- National Electrical and Communications Association (NECA) — Industry body for Australian electrical contractors; publishes guidance on Wiring Rules interpretation and application.