An inverter pushes current towards the point of supply. The cable between the inverter and the switchboard carries that current. The voltage at the inverter therefore sits above the voltage at the board. That difference is the voltage rise.
The inputs
A 5 kW single-phase inverter in a detached garage.
- Phases: 1, supply 230 V
- Inverter output: 5 kW
- Power factor: 1
- Run length: 40 m
- Conductor: 10 mm² copper, PVC, one cable
- Rise limit: 2 %
The clause and the tables
Cl 3.6.2 Note 3 of AS/NZS 3000:2018 refers voltage rise to AS/NZS 4777.1. Cl 3.3.3 of AS/NZS 4777.1:2016 sets the limit at 2 % of the nominal voltage at the point of supply, and it requires the calculation to use the rated current of the inverter energy system. The 2 % default in the tool is that limit. Confirm the edition that applies to the installation.
The impedance data is the same data the voltage drop tool uses. Reactance comes from Table 30 of AS/NZS 3008.1.2:2017 and a.c. resistance from Tables 34 and 35. Cl 4.5 sets how the two combine into one millivolt per amp metre figure.
The steps
The tool first converts the inverter rating into a current. 5 kW at 230 V and a power factor of 1 gives an output current of 21.74 A.
It then reads the conductor data. The resistance of 10 mm² copper is 2.23 Ω/km. Combined at a power factor of 1, the cable gives 4.46 mV/A·m.
4.46 mV multiplied by 21.74 A and by 40 m gives a rise of 3.88 V. Against 230 V that is 1.69 %.
The result
- Voltage rise: 1.69 %
- Inverter current: 21.74 A
- Rise in volts: 3.88 V
- Maximum length at the limit: 47.44 m
The result sits under the 2 % input, and the tool marks it a pass. It also raises a warning, because 1.69 % is close to the limit. The maximum length figure explains why. This cable reaches 47.44 m before the rise meets 2 %, and the run is already 40 m.
That leaves little margin for a route change. A deviation around a wall or a longer drop into the board adds route length. A later move of the inverter can take the run past 47.44 m.
The next size up answers that. On 16 mm² the same run gives a rise of 1.06 % and 2.43 V. The maximum length goes to 75.57 m.
Power factor changes the answer too. This example uses 1, which is the usual inverter case. An inverter set to absorb or supply reactive power draws more current for the same kilowatts.
At a power factor of 0.9 the same inverter draws 24.15 A. The rise becomes 1.72 % and the maximum length falls to 46.53 m.
The network voltage sits underneath
The rise adds to whatever the network already delivers at the point of supply. A site near the top of the supply range has less room for rise than this calculation shows. Measure the voltage at the board before committing to a long inverter run.
This page is a design aid. Verify every value against the current edition of the standard.