Every cable has resistance, so some voltage is lost along its length. On short runs it doesn't matter. On long runs to a garage, workshop, pump, backyard office or EV charger, it can mean dim lights, motors that struggle to start, chargers that throttle back, and an installation that fails inspection. Voltage drop is often what decides the cable size on these jobs.
Why it happens
Current flowing through a conductor's resistance loses voltage: V = I × R. The resistance grows with length and shrinks with conductor size. So a long run carrying a big current in a small cable loses the most. Copper has lower resistance than aluminium, so an aluminium conductor needs a bigger size for the same drop.
The formula
For single-phase: VD = 2 × ρ × L × I ÷ A, where ρ is about 0.0225 Ω·mm²/m for copper at working temperature (0.036 for aluminium), L is the one-way length in metres, I is the current in amps, and A is the cross-section in mm². For three-phase, use √3 (1.732) instead of 2. Designers usually use the mV/A/m values in AS/NZS 3008.1.1, which give the same answer.
Divide the drop by the supply voltage and multiply by 100 to get the percentage.
Typical limits
AS/NZS 3000 (the Wiring Rules) generally limits the voltage drop from the point of supply to any point in the installation to 5% of the nominal voltage, which is 11.5 V on a 230 V supply (7% is allowed where the supply comes from a substation on the premises). That 5% has to cover the consumer mains and any sub-mains as well as the final circuit.
A worked example
A 30 A load on a 230 V single-phase circuit, 30 m from the switchboard, in 6 mm² copper: VD = 2 × 0.0225 × 30 × 30 ÷ 6 = 6.75 V, or 2.9%. That's within the 5% limit on its own, but if the consumer mains already lose 2–3%, the total passes 5%, so a 10 mm² cable may be needed.
Where it bites
- Outbuildings and backyard offices, often 20–50 metres from the house.
- EV chargers on long runs to a driveway or parking space.
- Pumps and motors, which draw high current when starting.
- Low-voltage lighting, where a few volts lost is a large percentage.
- Solar and battery systems, where losses cost energy every day.
Voltage drop is only one check
Cable sizing also depends on current-carrying capacity, installation method, grouping with other cables, ambient temperature, the protective device, and fault protection. Voltage drop often decides the size on long runs, but it's never the only check. Underground runs also need the right cable type, depth and protection.
This work must be designed and installed by a licensed electrician to AS/NZS 3000, with the electrical safety certificate your state requires. Use the calculator to sense-check a quote or to understand why a bigger cable is specified, not to replace the design.
Reducing voltage drop
- Use a larger conductor.
- Shorten the route where possible.
- Use a higher voltage or three-phase for big loads, where available.
- Split loads across separate circuits.
- For distant buildings, run a sub-main to a local sub-board instead of several long final circuits.
The voltage drop calculator gives the drop for any run and the smallest conductor that passes, and the Ohm's law calculator converts between watts, amps, volts and resistance.
Common questions
Is voltage drop the same as a loose connection?
No, although both cause lost voltage. Voltage drop is the normal, predictable loss along a cable. A loose or corroded connection adds extra resistance at one point, which can overheat and is a fire risk. If voltage is far lower than the calculation predicts, check the connections.
Does voltage drop waste energy?
Yes. The lost voltage becomes heat in the cable. On circuits that run for many hours, such as heaters, pumps and EV chargers, a bigger cable pays for itself in lower losses over its life.
How do I measure it on an existing circuit?
An electrician measures the voltage at the supply and at the load with the load running, or uses the loop impedance test results. The difference is the drop.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.