Load, run length and cable size in, drop in volts and percent out, checked against the 5% limit and the 3% subcircuit guide. Built for quoting on the phone, free, no email.
10.5V (4.5%)
2.5mm² Cu over 30m at 20A, single phase
Over the 3% subcircuit guide (6.9V) but inside the 5% total limit (11.5V), fine only if the rest of the installation drops almost nothing.
Smallest size passing the 3% guide at these numbers: 4mm² (6.5V, 2.8%).
| Size | Drop | % | Verdict at your numbers |
|---|---|---|---|
| 1.5mm² | 17.0V | 7.4% | Undersized for 20A |
| 2.5mm² | 10.5V | 4.5% | Inside 5% total only |
| 4mm² | 6.5V | 2.8% | Passes 3% guide |
| 6mm² | 4.4V | 1.9% | Passes 3% guide |
| 10mm² | 2.6V | 1.1% | Passes 3% guide |
| 16mm² | 1.6V | 0.7% | Passes 3% guide |
| 25mm² | 1.0V | 0.5% | Passes 3% guide |
| 35mm² | 0.7V | 0.3% | Passes 3% guide |
| 50mm² | 0.6V | 0.2% | Passes 3% guide |
| 70mm² | 0.4V | 0.2% | Passes 3% guide |
| 95mm² | 0.3V | 0.1% | Passes 3% guide |
Guide only. Values are typical copper/PVC mV/A·m figures of the kind tabulated in AS/NZS 3008.1.1; your actual installation method, insulation, ambient temperature, grouping and derating change the answer. Final cable selection is the licensed electrician's call against the standard, not this page.
The classic quoting trap: the shed sub-board is 45 metres from the house, the load is only 20A, and 2.5mm² carries 20A all day. Except at 45 metres the run itself drops nearly 8%, the compressor at the far end starts hard and runs hot, and the fix after handover costs more than the whole original job. Quote the 6mm² up front and the problem never exists.
Slide the numbers around and watch where each size gives out. The table shows every common size at your load and length, so the “smallest passing” answer is one glance, on site, before the price leaves your mouth.
AS/NZS 3000 allows a total drop of 5% from the point of supply to the furthest point of the installation (11.5V on 230V single phase, 20V on 400V three phase). A common working split is to keep the final subcircuit under about 3% so the mains and submains have room. This calculator shows both lines so you can see where your run sits.
Vd = Vc × L × I ÷ 1000, where Vc is the millivolt-per-amp-metre value for the cable, L is the route length one way in metres, and I is the load current. The Vc values used are typical published copper/PVC figures of the kind tabulated in AS/NZS 3008.1.1, with single phase using the three-phase value times 1.155.
No, and it says so under every result. Installation method, insulation type, ambient temperature, grouping and derating all change real cable selection, and the standard's tables are the reference. This tool is for the ten-second sanity check on the phone: is 2.5mm² ever going to survive a 45m shed run at 20A, or should you quote 6mm² before you price the job? (It won't, and you should.)
Current capacity and voltage drop are separate limits. A 2.5mm² circuit happily carries 20A, but at 45 metres the resistance of the run itself eats more than 5% of the supply voltage, so gear at the far end runs hot and undervoltaged. Long runs are usually sized by drop, not by current. That's why shed, granny flat and pump runs jump two sizes.
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