One cable, one current, one length, two supply types. A 6 mm² copper circuit carries 32 A over 50 m. On a three phase supply it passes with room to spare. On a single phase supply it sits at the edge of the limit. This page works both cases and names the two factors that separate them.
The inputs
- Conductor: 6 mm² copper, PVC insulated, one cable
- Design current: 32 A
- Route length: 50 m
- Power factor: 0.9
- Limit: 5 %
- Case A: 400 V three phase
- Case B: 230 V single phase
The clause and the tables
AS/NZS 3008.1.2:2017 Cl 4.5 combines the conductor resistance and reactance at the circuit power factor into one millivolt per amp metre figure. AS/NZS 3008.1.2:2017 Tables 30 to 35 hold the resistance and reactance the tool reads. AS/NZS 3000:2018 Cl 3.6.2 sets the drop allowed to any point of the installation.
The steps
Both cases start from the same conductor data.
- Resistance: 3.75 Ω/km
- Reactance: 0.097 Ω/km
Two things then change with the supply type. The first is the phase factor inside the millivolt per amp metre figure. The second is the voltage the drop is measured against.
The three phase result
- Drop per amp metre: 5.92 mV/A·m
- Voltage drop: 9.47 V
- Voltage drop: 2.37 %
- Maximum length at the limit: 105.6 m
The circuit passes. 2.37 % against a 5 % limit leaves most of the budget for the rest of the installation.
The single phase result
Change the supply to 230 V single phase and hold every other input.
- Drop per amp metre: 6.83 mV/A·m
- Voltage drop: 10.93 V
- Voltage drop: 4.75 %
- Maximum length at the limit: 52.58 m
The circuit passes, and the tool raises a warning that the drop is close to the limit. At 50 m of a 52.58 m allowance, any extra route length fails it. A single extra drop down a wall ends this circuit.
The two factors
The millivolt per amp metre figure moves from 5.92 to 6.83 for the same conductor. That is the phase factor. A single phase circuit carries current out and back through two conductors. A balanced three phase circuit uses the line to line relationship instead, and the factor is smaller.
The reference voltage moves from 400 V to 230 V. A drop of 9.47 V against 400 V is 2.37 %. A drop of 10.93 V against 230 V is 4.75 %. The volts changed a little. The percent nearly doubled, because the base halved.
The two effects push the same way. A long run that fails on single phase often passes on three phase, with no change to the cable. The maximum length figure shows the size of the move: 52.58 m becomes 105.6 m.
Using it
Switch the phases field in the tool and read the two results side by side before you order cable. When a submain sits near the limit on single phase, compare two options: the next conductor size, or the same conductor on a three phase supply. The tool gives both answers in two clicks.
Check capacity as well as drop. The cable size tool applies both checks and reports the size that passes each of them.
This page is a design aid. Verify every value against the current edition of the standard.
See also: Submain voltage drop estimate for a 160 A tender, Voltage drop with the mV/A·m method and Voltage drop basics.