Voltage drop in Australian cable runs
How voltage drop is read in Australian 230/400 V cable work, including route length, conductor data and percent-drop checks.
What Voltage Drop Means
Voltage drop is the reduction in voltage along a cable run caused by current flowing through conductor impedance. In Australian low-voltage work it is usually read against the voltage basis for the run: 230 V a.c. active-to-neutral for many single-phase final circuits, or 400 V a.c. line-to-line for many balanced three-phase runs.
The result is normally shown as volts and percent. The percent is not meaningful unless the reference voltage is visible, because a 6 V drop is a different percentage on a 230 V run than on a 400 V run.
Inputs That Decide The Result
Voltage-drop review needs more than a cable size label. It needs the load current, route length, voltage basis, phase arrangement and conductor data used for that specific run. The same load can produce a different result when the route is longer, the conductor material changes, or the calculation moves from a final subcircuit to a submain.
Two common relationship styles appear in AUWiring tools and charts:
- With entered mV/A/m data:
voltage drop V = mV/A/m x current A x route length m / 1000. - With entered conductor impedance: single-phase uses a two-conductor factor; balanced three-phase uses
sqrt(3)with the line current and line-to-line voltage basis. - Percent drop is
voltage drop V / nominal voltage V x 100.
| Input | Typical Australian basis | Why it matters |
|---|---|---|
| Voltage basis | 230 V single-phase, 400 V three-phase or project-entered value. | Percent drop depends on the reference voltage. |
| Current basis | Known current, or current calculated from kW/kVA, phase and power factor. | Voltage drop rises with current. |
| Route length | Installed one-way route length in metres, entered consistently. | Longer routes increase the drop. |
| Conductor data | mV/A/m or resistance/reactance data for the actual cable. | Copper, aluminium, insulation and construction data are not interchangeable. |
| Target percent | Project review value entered by the user. | The calculator compares the result with the entered target, not a hidden universal rule. |
Worked Cable-Run Example
For a 230 V single-phase final subcircuit carrying 18 A over a 24 m route, entered cable data of 5.6 mV/A/m gives:
5.6 x 18 x 24 / 1000 = 2.42 V.
The percent drop is 2.42 / 230 x 100 = 1.05%. That is a voltage-drop result for that run only. It does not say the cable is acceptable for current-carrying capacity, protection or installation conditions.
| Run detail | Example entry | Review note |
|---|---|---|
| Supply context | 230 V final subcircuit, 50 Hz Australian low-voltage context. | Use the actual project voltage if it differs. |
| Load current | 18 A from the load schedule or circuit design note. | A different current changes the result. |
| Route length | 24 m one-way route length. | Keep length basis consistent across worksheets. |
| Cable data | 5.6 mV/A/m from the selected cable data source. | Do not borrow data from another material or cable family. |
Separate It From Current-Carrying Capacity
A cable can pass voltage drop and fail current-carrying capacity, or pass current-carrying capacity and fail voltage drop. Voltage drop is sensitive to route length and conductor impedance. Current-carrying capacity is sensitive to installation method, grouping, ambient temperature, enclosure, product data and derating source.
Keep both checks visible for final circuits, consumer mains, submains, distribution-board feeders and inverter AC runs. A candidate cable should not move forward just because one of the two checks looks comfortable.
Next checks
- Use the voltage-drop calculator when the current, route length, voltage basis and cable data are known.
- Use the voltage-drop formula chart when the relationship between mV/A/m, impedance, volts and percent needs to be checked.
- Use the cable-size workflow after voltage-drop review so current-carrying capacity, derating and protection remain separate.
Boundaries
- This page does not select a cable size or protective device.
- It does not reproduce AS/NZS 3008 cable tables or provide universal voltage-drop allowances.
- Project documents, current standards context, DNSP conditions where relevant, manufacturer data, installation conditions and competent review can override a general worked example.