Voltage drop is the one cable check that is easy to do in your head and easy to get wrong. The millivolt per amp metre method is the short form used in New Zealand, and it hides two things worth understanding: the power factor and the reactance.
The method in one line
Every conductor has a resistance and a reactance per kilometre. Combine them at the circuit power factor and you get one number: the drop in millivolts for each amp carried over each metre of route. Multiply that by the current and the length, and you have the drop in volts. Cl 4.5 of AS/NZS 3008.1.2:2017 sets out the combination.
The combination is not a simple sum. Resistance acts in phase with the current and reactance acts at ninety degrees to it. At a power factor of 1 the reactance contributes almost nothing. At 0.8 it starts to matter. On large conductors, where reactance is a bigger share of the impedance, it matters more again.
A worked run
Take the same workshop circuit: 230 V single phase, 32 A, 25 m, power factor 0.9, on 6 mm² multicore copper with PVC insulation. One cable, no parallel runs. The limit is 5 %.
The tool reads the conductor data first.
- Resistance: 3.75 Ω/km
- Reactance: 0.097 Ω/km
It combines those at a power factor of 0.9 and gets 6.834 mV/A·m.
From there the arithmetic is short. 6.834 mV multiplied by 32 A and by 25 m gives 5.47 V. As a share of 230 V that is 2.38 %.
Reading the result
2.38 % against a 5 % limit is a comfortable circuit. The more useful number the tool gives is the maximum length: 52.58 m. That is how far this cable can run at 32 A before the drop reaches the limit. It answers the question that actually comes up on site, which is not "does this pass" but "how much further can I go".
If the run has to reach 70 m, the 6 mm² cable will not do. The next size up roughly halves the millivolt figure, and the maximum length roughly doubles with it.
Where the limit comes from
Cl 3.6.2 of AS/NZS 3000:2018 sets the drop allowed from the point of supply to any point of the installation. The default in the tool is 5 %. Some designs split that budget: a share for the consumer mains, a share for the submains, and the rest for the final subcircuit. If you are sizing one leg of a longer chain, set the limit to that leg's share rather than the whole 5 %, or the first cable will eat the whole allowance.
Equipment sets its own floor too. Long runs to motors, LED drivers and control gear sometimes need a tighter limit than the standard asks for, because the equipment will not start or will not hold its output at the bottom of the range.
Three habits worth keeping
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. Guessing high flatters the result.
Use the route length, not the straight line. The drop follows the cable, including every drop down a wall and every run around an obstacle.
Check drop and capacity together. A cable can be big enough to carry the current and still fail on drop, and on long runs it usually does. The cable size tool applies both checks at once and reports the size that passes each of them.
The result here is a design aid. Verify it against the current standard before you commit a design.