Free cable voltage drop calculator · No sign-up · Runs entirely in your browser
Estimate the voltage lost along a cable run for 12V, 24V, 230V and three-phase systems in copper or aluminium — and see how far you can run before it matters.
| Parameter | Value |
|---|
The AS/NZS 3008 standard governs cable selection in Australian electrical installations. Key factors: current-carrying capacity (ampacity), voltage drop, and fault current ratings. Maximum voltage drop for final subcircuits is 5% of nominal supply voltage.
Cable is not a perfect conductor. It has resistance, and pushing current through resistance costs voltage — so the appliance at the far end of a run always sees slightly less than the supply. That difference is the voltage drop.
It is Ohm's Law applied to the cable itself. The cable becomes an unintended resistor in series with your load, and the voltage it consumes is V = I × Rcable. Everything else on this page follows from that. For the underlying relationships, see our Ohm's Law calculator.
| Consequence | What actually happens |
|---|---|
| Motors | Run hotter and produce less torque. A 5% voltage deficit can cost roughly 10% of torque output |
| Lighting | Perceptible dimming, and shortened lamp life in some types |
| Electronics | Nuisance resets and dropouts, particularly in 12 V systems |
| Heating elements | Reduced output — power falls with the square of voltage |
| The cable itself | The lost voltage becomes heat in the conductor. It is wasted energy you pay for |
Using the caravan example below — 12 V, a 15 A fridge, 6 m of 6 mm² copper each way:
The cable is not a wire with a problem — it is behaving exactly as a resistor should. Those 7.9 watts are real energy leaving your battery and warming the conductor instead of running the fridge, and they scale with the square of current.
Enter the one-way length — the distance from source to load, not there and back. The formula already accounts for the return path.
| Copper | Aluminium | |
|---|---|---|
| Resistivity (Ω·mm²/m) | ~0.0175 | ~0.0282 |
| Drop for the same size | Baseline | About 61% more |
| Size needed for equal drop | Baseline | Roughly 1.6× the cross-section |
| Weight | Heavier | About one third the weight |
| Typical use | Almost all domestic and general wiring | Larger submains, overhead and utility runs |
| Terminations | Straightforward | Needs compatible lugs and correct preparation |
Aluminium is cheaper per amp delivered and much lighter, which is why it appears in large submains and overhead work. For the small runs most people are calculating — caravans, solar, sub-boards to a shed — copper is almost always the practical choice.
You need a current figure before you can calculate drop, and wattage is usually what is printed on the appliance. Divide watts by the system voltage.
| 230 V appliance | Typical power | Current |
|---|---|---|
| LED downlight | 8 W | 0.03 A |
| Laptop charger | 65 W | 0.28 A |
| Refrigerator (running) | 250 W | 1.09 A |
| Water pump | 750 W | 3.26 A |
| Microwave | 1,200 W | 5.22 A |
| Kettle | 2,200 W | 9.57 A |
| Split-system air conditioner | 2,500 W | 10.87 A |
| Electric oven | 3,000 W | 13.04 A |
| EV charger (single phase) | 7,400 W | 32.17 A |
| 12 V load | Typical power | Current |
|---|---|---|
| LED strip | 12 W | 1.00 A |
| Diesel heater | 30 W | 2.50 A |
| 12 V compressor fridge | 55 W | 4.58 A |
| Water pump | 60 W | 5.00 A |
| 600 W inverter at full output | ~700 W drawn | 58.33 A |
A 12 V fridge drawing 15 A, 6 m of cable from the battery, 6 mm² copper.
| Step | Working | Result |
|---|---|---|
| Cable resistance (one way) | 0.0175 × 6 ÷ 6 | 0.0175 Ω |
| Voltage drop | 2 × 15 × 0.0175 | 0.525 V |
| As a percentage | 0.525 ÷ 12 | 4.38% |
| Voltage at the fridge | 12 − 0.525 | 11.48 V |
| Upsized to 10 mm² | 2 × 15 × 0.0105 | 0.315 V (2.63%) |
A solar array delivering 20 A over a 15 m run to the charge controller, 6 mm² copper.
Well outside any sensible limit, and every one of those volts becomes heat in the cable rather than charge in the battery. Doubling to 16 mm² brings it to 0.66 V (2.73%). This is the classic solar mistake: sizing cable for current-carrying capacity alone, when on long runs it is voltage drop that governs.
A 16 A load 35 m from the switchboard on 2.5 mm² copper.
| Step | Working | Result |
|---|---|---|
| Voltage drop | 2 × 16 × 0.0175 × 35 ÷ 2.5 | 7.84 V |
| As a percentage | 7.84 ÷ 230 | 3.41% |
| Upsized to 4 mm² | 4.90 V (2.13%) |
The AS/NZS 3000 Wiring Rules set a maximum voltage drop of 5% of nominal supply voltage, measured from the point of supply to any point of utilisation. The critical word is cumulative. It is a budget for the entire path, shared between every segment.
| Nominal voltage | 5% budget | Comment |
|---|---|---|
| 230 V single phase | 11.5 V | Shared across mains, submains and final subcircuit |
| 400 V three phase | 20.0 V | Line-to-line. Older equipment is often still labelled 415 V |
| 24 V DC | 1.2 V | Not covered by the Wiring Rules, but a useful design target |
| 12 V DC | 0.6 V | Very tight — most designers work to 3% instead |
Because the budget is shared, designers allocate it in advance. A common domestic approach is roughly 1.5% for consumer mains, 1.5% for submains and 2% for final subcircuits; another is a simple 2.5% / 2.5% split between submain and final circuit. The allocation is a design decision, not a fixed rule.
Two different things get conflated here, so it is worth separating them. The regulated figure is a cumulative limit for a whole installation. The design target is what experienced installers aim for on a single run so the cumulative total stays inside it.
| System | Regulated limit | Practical target per run | Why |
|---|---|---|---|
| 12 V DC | Not covered by the Wiring Rules | ≤3% (0.36 V) | 5% is only 0.6 V; many 12 V devices misbehave below ~11.5 V |
| 24 V DC | Not covered | ≤3% (0.72 V) | Same reasoning, with twice the headroom in volts |
| 48 V DC | Not covered | ≤3% (1.44 V) | Common in larger off-grid and telecoms systems |
| 230 V single phase | 5% cumulative (11.5 V) | ≤2–2.5% per segment | Leaves budget for mains and submains upstream |
| 400 V three phase | 5% cumulative (20 V) | ≤2–2.5% per segment | Same cumulative budget, shared across the path |
Maximum one-way run for a 3% drop on 12 V copper. This is the table caravan, 4WD and small solar builders need most.
| Conductor | 5 A | 10 A | 15 A | 20 A | 30 A |
|---|---|---|---|---|---|
| 1.5 mm² | 3.1 m | 1.5 m | 1.0 m | 0.8 m | 0.5 m |
| 2.5 mm² | 5.1 m | 2.6 m | 1.7 m | 1.3 m | 0.9 m |
| 4 mm² | 8.2 m | 4.1 m | 2.7 m | 2.1 m | 1.4 m |
| 6 mm² | 12.3 m | 6.2 m | 4.1 m | 3.1 m | 2.1 m |
| 10 mm² | 20.6 m | 10.3 m | 6.9 m | 5.1 m | 3.4 m |
| 16 mm² | 32.9 m | 16.5 m | 11.0 m | 8.2 m | 5.5 m |
| 25 mm² | 51.4 m | 25.7 m | 17.1 m | 12.9 m | 8.6 m |
Copper at about 20 °C, 3% of 12 V. Halve these lengths for a 1.5% target; roughly double them for a 24 V system at the same power.
These are the sizes commonly encountered in Australian installations. Read this as what you are likely to find, not as a selection guide — actual cable selection depends on current, run length, installation method, ambient temperature, grouping with other cables and the protective device.
| Size | Commonly seen on | Typical circuit |
|---|---|---|
| 1.5 mm² | Lighting circuits | 10 A |
| 2.5 mm² | General power outlets | 16–20 A |
| 4 mm² | Larger appliance circuits, some air conditioning | 20–25 A |
| 6 mm² | Ovens, single-phase EV chargers, long outlet runs | 32 A |
| 10 mm² | Submains to sheds and granny flats | 40–50 A |
| 16 mm² | Consumer mains, larger submains | 63 A+ |
| 25 mm² | Consumer mains on larger installations | 80 A+ |
| Approach | Effect | Practicality |
|---|---|---|
| Increase conductor size | Drop is inversely proportional — double the mm², halve the drop | Usually the first and easiest fix |
| Shorten the run | Directly proportional — half the length, half the drop | Relocating a battery or sub-board is often cheaper than heavy cable |
| Raise system voltage | Halves current, so halves drop and quarters power loss | 12 V → 24 V is transformative for off-grid |
| Split the load | Less current per cable | Two circuits instead of one long shared run |
| Copper instead of aluminium | About 38% less drop for the same size | Usually already the case in small installations |
What is voltage drop?
Voltage drop is the voltage lost along a cable because the conductor has resistance. The appliance at the far end receives less than the supply voltage. It is Ohm's Law applied to the cable itself: the drop equals the current multiplied by the cable's resistance.
How do you calculate voltage drop?
For DC and single-phase circuits: voltage drop = 2 × current × resistivity × one-way length ÷ conductor cross-section. Copper resistivity is about 0.0175 Ω·mm²/m. The 2 accounts for current travelling out and back. Three-phase circuits use √3 in place of 2.
How much voltage drop is allowed in Australia?
The AS/NZS 3000 Wiring Rules limit total voltage drop to 5% of nominal supply voltage from the point of supply to any point of utilisation — 11.5 V on a 230 V supply, 20 V on 400 V three phase. Critically, this is cumulative across consumer mains, submains and final subcircuits, not a per-cable allowance.
Does this calculator tell me if my installation complies?
No. It estimates the drop in one cable run at around 20 °C using resistance only. Compliance depends on the cumulative drop across the entire path, and formal assessment under AS/NZS 3008 uses tabulated values at the conductor's operating temperature that also account for reactance. Treat this as a design aid and have a licensed electrician verify any installation.
Does cable length affect voltage drop?
Directly and proportionally. Double the length and you double the drop. This is why long runs to sheds, pumps and outbuildings so often need cable far larger than the current alone would suggest — and why moving the source closer to the load can be cheaper than upsizing.
Does wire size affect voltage drop?
Inversely and proportionally. Doubling the conductor cross-section halves the drop, so moving from 4 mm² to 6 mm² reduces it by about a third. Increasing cable size is usually the simplest and most effective fix.
What cable should I use for a 12 V system?
Larger than most people expect, because 3% of 12 V is only 0.36 V. A 20 A load on 6 mm² copper exceeds 3% past roughly three metres one-way. Use the table above as a starting point, and consider whether 24 V or relocating the battery would serve you better than very heavy cable.
Why is voltage drop worse on 12 V than 230 V?
Two reasons compound. Delivering the same power at 12 V needs roughly nineteen times the current of a 230 V circuit, and drop is proportional to current. Then the percentage is measured against a much smaller number — a 0.5 V loss is 0.2% at 230 V but 4.2% at 12 V.
What is the difference between copper and aluminium for voltage drop?
Aluminium has roughly 61% higher resistivity, so for the same cross-section it drops about 61% more voltage. To match copper's performance you need roughly 1.6 times the cross-sectional area. Aluminium is lighter and cheaper per amp delivered, which is why it appears in large submains and overhead runs.
Does temperature affect voltage drop?
Yes. Copper's resistance rises with temperature, so a cable carrying its rated current runs hot and drops more than a 20 °C calculation suggests. Formal cable selection uses figures at the conductor's maximum operating temperature for exactly this reason, which is why tabulated values come out higher than a simple resistivity calculation.
How do I reduce voltage drop?
In rough order of effectiveness: increase the conductor size, shorten the run, raise the system voltage, or split the load across more than one circuit. Doubling conductor size halves the drop; doubling system voltage halves the current and therefore the drop as well, while quartering power loss in the cable.
Why does voltage drop waste energy?
The lost voltage becomes heat in the conductor. Power wasted equals current squared times cable resistance, so losses rise with the square of current — doubling current quadruples the waste. In a solar system, that is energy leaving the panels and never reaching the battery.
On short runs, current-carrying capacity decides cable size. On long ones — and on almost every 12 V system — voltage drop decides it first, usually by a wide margin. Checking both is the difference between a system that works and one that browns out under load.
From here, the Ohm's Law calculator covers the underlying relationships, the resistor calculator decodes component values, and the electricity cost calculator prices what those losses are costing you.