A cable is a conductor with resistance. Current through resistance produces a voltage difference between the two ends. The equipment at the far end receives less than the supply voltage, and that shortfall is the voltage drop.
The circuit is longer than the route
Current has to return. A single-phase circuit carries the current out along the active and back along the neutral, so the conductor length is twice the route length. A three-phase balanced circuit returns through the other phases instead, and its drop works out lower for the same current and the same route.
The tool holds that difference in one factor, so a three-phase result is not a single-phase result divided by three.
The mV/A·m shortcut
Working from ohms per kilometre is slow. AS/NZS 3008.1.2:2017 Cl 4.5 combines the conductor resistance and reactance at the circuit power factor into one figure: the drop in millivolts for each amp carried over each metre of route.
Reactance matters more than most people expect. Resistance acts in phase with the current. Reactance acts at ninety degrees to it. At a power factor of 1 the reactance contributes almost nothing. At 0.8 it adds a real share. On large conductors reactance is a larger part of the impedance, so it matters more again.
Tables 30 to 35 of AS/NZS 3008.1.2:2017 hold the resistance and reactance the combination uses.
A worked run
Take a 230 V single-phase circuit at 25 A over 35 m. The power factor is 0.9. The cable is 4 mm² multicore copper with PVC insulation. The limit is 5 %.
The tool reads the conductor data first.
- Resistance: 5.61 Ω/km
- Reactance: 0.102 Ω/km
It combines those at a power factor of 0.9 and gets 10.19 mV/A·m. Multiply by 25 A and by 35 m and the drop is 8.91 V. Against 230 V that is 3.88 %.
The maximum length is the answer to the real question
The result also gives a maximum length of 45.16 m. That is how far this cable runs at 25 A before the drop reaches 5 %. On site the question is rarely whether a design passes. The question is how much further the cable can go, or how much bigger it has to be.
Maximum length scales with conductor size. The next size up roughly halves the mV/A·m figure and roughly doubles the distance.
Where the limit comes from
Cl 3.6.2 of AS/NZS 3000:2018 sets the drop allowed between the point of supply and any point in the installation. The tool defaults to 5 %.
That allowance covers the whole chain. Consumer mains, submains and final subcircuits share it. A design that spends the full allowance on the first leg leaves nothing for the rest. When you size one leg, set the limit to that leg's share.
Extra low voltage is a separate case. Cl 7.5.7 of AS/NZS 3000:2018 governs ELV circuits, and a 24 V DC instrument loop tolerates a much larger percentage than a 230 V circuit. The volts lost are small; the share of 24 V is not.
Equipment sets its own floor as well. Motors, LED drivers and control gear state a minimum supply voltage. A circuit can meet Cl 3.6.2 and still sit below what the equipment needs to start.
Voltage rise is voltage drop backwards
An inverter pushes current toward the point of supply instead of away from it. The same impedance produces the same volts, but the far end now sits above the supply voltage rather than below it. The arithmetic is identical and the sign changes. The voltage rise tool applies it to a solar inverter cable and reports the rise as a percentage.
Drop is the second check, not the first
Capacity comes first: the derated cable has to carry the design current. Drop is the check that often takes over on long runs, because capacity depends on the conductor and drop depends on the conductor and the distance together.
The cable size tool applies both checks at once and reports the size that satisfies each of them. On short runs capacity usually decides. As the run lengthens, drop takes over.
Three habits worth keeping
Use the route length. The drop follows the cable, including every drop down a wall and every detour around a beam. A straight line on a plan underestimates it.
Use the real power factor. A resistive load at 1.0 and a motor circuit at 0.8 give different answers on the same cable.
Check the leg, not the whole allowance. A submain that passes at 5 % may leave the final subcircuit no room at all.
This page is a design aid. Verify every value against the current edition of the standard.