Australia has one 5% limit. America has two numbers and a different formula.
This is the one most free calculators get wrong for Australia, because the tools that rank well are written for the American NEC.
The NEC recommends roughly 3% on a branch circuit and 5% on a feeder, and its calculators are built on conductor resistivity in ohms per thousand feet with a factor of 2 for single phase and 1.732 for three phase. AS/NZS 3000 does it differently in both respects. It sets a single 5% limit measured from the point of supply to any point of use, and the arithmetic runs off a published mV/A/m figure per cable size from AS/NZS 3008.1:
Volts dropped = mV/A/m × amps × metres ÷ 1000
Feed an Australian job into an American calculator and you can get told a cable is fine at 3% when it has already eaten most of a 5% budget shared with a submain, or told it fails at 3% when it is comfortably legal here. Neither error is obvious on the day.
A worked example
A 20 A load, 40 metres away, 230 V single phase, 5% limit:
- Budget: 230 × 5 ÷ 100 = 11.5 V
- 1.0 mm² at 44.0 mV/A/m: 44.0 × 20 × 40 ÷ 1000 = 35.2 V, which is 15.3% — nowhere near
- 2.5 mm² at 18.1: 18.1 × 20 × 40 ÷ 1000 = 14.48 V = 6.3% — still over
- 4 mm² at 11.2: 11.2 × 20 × 40 ÷ 1000 = 8.96 V = 3.9% — under
- 6 mm² at 7.4: 5.92 V = 2.57% — comfortable, and worth the extra if the run might grow
So 4 mm² is the smallest that comes in. The 1.0 mm² row is the one worth sitting with: at 40 metres it is not marginally over, it is three times over. Drop is linear in distance — double the run, double the drop.
Indicative mV/A/m, and how far you can go at 20 A
| Cable | mV/A/m (indicative) | Max run at 20 A, 230 V, 5% |
|---|---|---|
| 1.0 mm² | 44.0 | 13.1 m |
| 1.5 mm² | 29.4 | 19.6 m |
| 2.5 mm² | 18.1 | 31.8 m |
| 4 mm² | 11.2 | 51.3 m |
| 6 mm² | 7.4 | 77.7 m |
| 10 mm² | 4.4 | 130.7 m |
| 16 mm² | 2.8 | 205.4 m |
| 25 mm² | 1.75 | 328.6 m |
These are indicative values for V75 copper and nothing more. The real figure changes with installation method, how many circuits are bunched together, ambient temperature and the operating temperature of the conductor — which is exactly what AS/NZS 3008.1 tabulates and why the standard is a book rather than a line. Use the table above to get in the right area, then confirm the actual figure.
The 5% is a total, not a per-circuit allowance
The limit is measured from the point of supply to any point of use. If a submain to a shed has already spent 2% getting there, every final subcircuit in that shed is working with 3% left, not a fresh 5%.
This is the mistake that hurts on rural properties, farm sheds, granny flats and anything with a long driveway. Each leg looks fine on its own. Added up, the appliance at the far end is sitting at 210 V, the motor runs hot and the customer rings about a fridge that keeps failing. Work the submain first, subtract what it used, and put the remainder into the drop-limit field above for the final circuits.
Voltage drop is one of three checks
Sizing a cable on drop alone is not sizing a cable. All three of these have to be satisfied and the largest answer wins:
- Current-carrying capacity. Can the cable carry the load continuously in the installation method used, after derating for grouping, insulation and ambient temperature? A cable in a bundle in a hot roof space carries far less than the same cable in free air.
- Voltage drop. This calculator. Almost always the one that decides it on a long run, and almost never the one that decides it on a short one.
- Earth fault loop impedance. Will the protective device actually disconnect in time on a fault at the far end? On very long runs this can demand a bigger active or a bigger earth than either of the other two.
Take the largest cable the three checks produce. That is the size.
Things that quietly make it worse
- Route length, not straight-line distance. Up the wall, along the roof, around the tank stand. Measure the cable, not the map.
- Motor starting. Starting current is a multiple of running current, and the dip it causes can drop out contactors elsewhere on the board.
- Load growth. A shed gets a welder. A carport gets an EV charger. Going one size up now is cheap; re-pulling a 60 metre run later is not.
- Aluminium. Different figures entirely — do not read a copper table and apply it to aluminium.
- Supply already low. If the incoming voltage sits under nominal, your real margin at the appliance is smaller than the percentage suggests.
For licensed electricians
Electrical installation work in Australia must be done by a licensed electrician, and the certificate of compliance is theirs to issue. This is an estimating aid for that trade — it gets you to a cable size fast on site so you can price the job and order the drum.
Sized the cable. Now price the job.
Yamate turns metres of cable, accessories and hours into a quote the customer accepts on their phone, then an invoice, then a payment — with GST handled and the follow-ups chased for you.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberVoltage drop & cable size FAQ
What is the maximum voltage drop allowed in Australia?
5% total, measured from the point of supply to any point of use, under AS/NZS 3000. It is a single figure covering the whole path — not the American split of 3% for a branch circuit and 5% for a feeder.
How do you calculate voltage drop in Australia?
Drop in volts equals mV/A/m multiplied by the current in amps, multiplied by the route length in metres, divided by 1000. The mV/A/m figure for the cable comes from AS/NZS 3008.1 and depends on the installation conditions.
Why is an American voltage drop calculator wrong for Australia?
Two reasons. The limits are different — a flat 5% here versus a 3% branch and 5% feeder recommendation there. And the formula is different, because Australia works from published mV/A/m values per cable size rather than ohms per thousand feet.
What size cable do I need for 20 amps over 40 metres?
On the indicative figures, 4 mm² gives 8.96 V of drop, which is 3.9% of 230 V and under the limit. 2.5 mm² comes out at 6.3% and is over. 6 mm² gives 2.57% and leaves room if the load grows.
Does the 5% apply to each circuit separately?
No. It is the total from the point of supply. If a submain has already used 2%, the final subcircuits fed from it only have 3% left. This is what catches people out on sheds and granny flats down a long driveway.
Is voltage drop the only thing that decides cable size?
No. Current-carrying capacity after derating, and earth fault loop impedance, are the other two checks. Work out all three and use the largest cable any of them demands.
What does mV/A/m mean?
Millivolts dropped per amp of current per metre of run, for a given cable. It is the Australian way of tabulating cable impedance, and AS/NZS 3008.1 publishes the values for each size, installation method and operating temperature.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. The 5% limit follows published Australian guidance on the AS/NZS 3000 voltage drop provisions. The mV/A/m values used here are indicative figures for V75 copper only — real values depend on installation method, grouping, ambient and conductor operating temperature and must be confirmed against AS/NZS 3008.1 for the actual installation. This tool does not check current-carrying capacity or earth fault loop impedance, both of which can demand a larger cable. Electrical installation work in Australia must be carried out by a licensed electrician; this is an estimating aid, not a design or a verification.