In Australia, the UK, New Zealand and most of Canada, fixed electrical wiring is licensed work and DIY installation is illegal. Use this to understand a design, check a quote or plan a run — then have the work designed and installed by a licensed electrician who can verify it against the current standard and your actual installation conditions.
One-way length, not there and back.
1.0 resistive, 0.8 motors.
AS/NZS 3008 allows 5% total.
Derating factors
Cables in ceiling insulation run hot.
Upstream submains, if any.
This cable size calculator solves both constraints that govern a cable — current-carrying capacity and voltage drop — because on any run of length it’s usually the second one that decides, and most calculators only check the first.
The short answer
Two things size a cable: whether it can carry the current without overheating, and whether enough voltage arrives at the far end. Work out both and use the larger.
A 32 A load over 40 m needs only 4 mm² for current — but 6 mm² to stay inside a 5% voltage drop. The second number governs.
In Australia, New Zealand, the UK and most of Canada, installing or altering fixed wiring is illegal without a licence. Even where DIY is permitted, incorrect cable sizing causes fires and equipment damage, and unlicensed work will void insurance.
Use this to understand a design, sanity-check a quote or plan a route. Then have a licensed electrician specify and install it against the current standard and your actual conditions.
On this page
How to use this cable size calculator
Three ways in:
- Known current — you have the design current in amps
- From wattage — appliance load in watts, converted using voltage and power factor
- 12 V / 24 V DC — solar, caravan, marine and battery systems
Set the supply, the voltage drop limit and — importantly — the derating factors under the advanced panel. Ambient temperature, grouped circuits and thermal insulation all reduce what a cable can carry, and they multiply together.
The result shows both required sizes side by side, so you can see which constraint is doing the work.
The two constraints
Constraint one
Current capacity
Can the cable carry the current without its insulation overheating? A safety limit. Depends on conductor size, insulation type and how the cable is installed.
Constraint two
Voltage drop
Does enough voltage reach the far end? A performance limit. Depends on current, length and conductor resistance — and grows directly with distance.
The rule
Larger wins
Both must be satisfied. Solve each independently and take the bigger cable. They’re not alternatives.
Current capacity doesn’t care how long the run is — a 2.5 mm² cable carries the same amps at 5 m or 100 m. Voltage drop cares about nothing else. That’s why the governing constraint switches over as runs get longer.
Where voltage drop takes over
Same 32 A load, 230 V single phase, 5% voltage drop limit, varying only the run length:
| Run | By current | By voltage drop | Use | Drop at 4 mm² |
|---|---|---|---|---|
| 10 m | 4.0 mm² | 1.5 mm² | 4.0 mm² | 1.28% |
| 20 m | 4.0 mm² | 2.5 mm² | 4.0 mm² | 2.57% |
| 30 m | 4.0 mm² | 4.0 mm² | 4.0 mm² | 3.85% |
| 40 m | 4.0 mm² | 6.0 mm² | 6.0 mm² | 5.13% |
| 50 m | 4.0 mm² | 6.0 mm² | 6.0 mm² | 6.41% |
| 80 m | 4.0 mm² | 10 mm² | 10 mm² | 10.26% |
| 120 m | 4.0 mm² | 16 mm² | 16 mm² | 15.39% |
The “by current” column never changes — 4 mm² carries 32 A whatever the distance. But past about 35–40 m, voltage drop starts demanding more, and by 120 m it needs four sizes up.
At 120 m, that 4 mm² cable would carry 32 A without overheating, pass a thermal check, and drop 15.4% of the supply voltage. The appliance sees around 195 V instead of 230.
Motors then draw more current to compensate and run hot, resistive heaters deliver noticeably less output, and electronics behave unpredictably. None of that trips a breaker — it just quietly costs you equipment life.
The voltage drop formula
Single phase: Vd = 2 × I × L × R ÷ 1,000
Three phase: Vd = √3 × I × L × R ÷ 1,000
I = amps · L = one-way length in metres · R = mΩ per metre
The 2 is because current travels out and back
Worked example — 32 A, 40 m, 6 mm² copper, 230 V
The ×2 catches people out. Current flows to the load and returns through the neutral, so it passes through twice the run length of conductor. Halving that gives an answer that looks fine and isn’t.
Why 12 V systems need absurd-looking cable
The percentage limit is the same. The voltage it’s a percentage of is not.
| System | 5% budget |
|---|---|
| 230 V AC | 11.5 V |
| 24 V DC | 1.2 V |
| 12 V DC | 0.6 V |
And because power = volts × amps, a low-voltage system draws far more current for the same load. High current plus a tiny voltage budget is a brutal combination:
| 20 A at 12 V | By current | By voltage drop | Oversized by |
|---|---|---|---|
| 3 m run | 2.5 mm² | 4.0 mm² | 2× |
| 5 m run | 2.5 mm² | 10 mm² | 4× |
| 10 m run | 2.5 mm² | 16 mm² | 6× |
| 20 m run | 2.5 mm² | 25 mm² | 10× |
This is why solar, caravan and marine wiring looks wildly oversized to anyone used to mains work. It isn’t over-engineering — it’s the only way to get useful voltage to the other end. Many 12 V installations use a 3% limit rather than 5%, which pushes sizes higher still.
Derating factors
Published current capacities assume favourable conditions. Real installations rarely have them, and the corrections multiply:
| Condition | Factor |
|---|---|
| Ambient 25 °C or below | ×1.00 |
| Ambient 35 °C | ×0.94 |
| Ambient 45 °C | ×0.87 |
| 2 circuits grouped | ×0.80 |
| 3 circuits grouped | ×0.70 |
| Touching thermal insulation one side | ×0.75 |
| Fully surrounded by insulation | ×0.50 |
A cable in a 35 °C roof space, bundled with two others, touching ceiling insulation on one side:
0.94 × 0.70 × 0.75 = ×0.493
The cable has lost half its capacity. A 32 A load now needs a cable rated 64.8 A before derating — which takes you from 4 mm² to 16 mm², four sizes up, on current capacity alone.
None of those three conditions looks alarming individually. That’s exactly why they get missed.
Three phase
Three-phase runs use √3 (1.732) in place of the single-phase 2 — so for the same current, length and cable, voltage drop is 13.4% lower.
The larger saving is upstream of that. A 7,000 W load draws 30.4 A at 230 V single phase but only 10.1 A at 400 V three phase, because the power is split across three conductors at a higher voltage. Lower current means dramatically lower voltage drop, which is why long runs and large loads go three phase where it’s available.
Measuring the run
Cable length means the actual route — up walls, along joists, around obstacles, into and out of the switchboard — not the straight-line distance between the two points.
Runs are commonly 30–50% longer than a first estimate, and voltage drop is directly proportional to length. A 40 m estimate that turns out to be 55 m on site takes the example above from 3.43% to 4.7%, which eats the margin entirely.
Measure the route you’ll actually pull, then add a margin for the diversion you haven’t discovered yet.
Standards by country
| Region | Standard | Voltage drop limit |
|---|---|---|
| Australia / NZ | AS/NZS 3008.1 | 5% total |
| UK / Europe | BS 7671 / IEC 60364 | 3% lighting, 5% other |
| United States | NEC Article 310 | 3% branch, 5% total (advisory) |
The 5% is a total from the point of supply to the point of use. If a submain has already consumed 2%, the final subcircuit has only 3% left — which is why the calculator has a field for voltage drop already used upstream.
US practice uses AWG rather than mm² and the NEC voltage drop figures are recommendations rather than requirements, though ampacity in Article 310 certainly is not.
Cable sizing mistakes to avoid
- Checking current capacity only. Voltage drop governs past about 40 m.
- Forgetting the ×2. Current goes out and comes back.
- Using straight-line distance. Measure the actual route.
- Ignoring derating. Three mild conditions can halve capacity.
- Applying only the worst derating factor. They all multiply together.
- Applying 5% to a subcircuit. It’s a total budget from the supply point.
- Sizing 12 V like mains. The budget is 0.6 V, not 11.5.
- Assuming aluminium equals copper. 1.6× the resistance for the same size.
- Doing the work yourself. It’s licensed in most countries.
Frequently asked questions
How do I calculate cable size?
Work out two sizes and use the larger. First, the size whose current-carrying capacity — after derating — exceeds your design current. Second, the size that keeps voltage drop inside your limit over the run length. A 32 A load over 40 m needs 4 mm² for current but 6 mm² for voltage drop, so 6 mm² governs.
How do I calculate voltage drop?
Single phase: 2 × amps × length in metres × resistance in mΩ/m ÷ 1,000. Three phase uses √3 instead of 2. For 32 A over 40 m in 6 mm² copper at 3.08 mΩ/m, that’s 7.88 V — 3.43% of 230 V.
What is the maximum allowable voltage drop?
5% total from the point of supply to the point of use under AS/NZS 3008. The UK’s BS 7671 allows 3% for lighting and 5% for other uses. It’s a total budget, so if a submain has used 2% the final subcircuit only has 3% remaining.
Why does voltage drop matter if the cable is safe?
Because safety and performance are different questions. A cable can carry the current without overheating while still delivering too little voltage. Motors then draw more current and run hot, heaters underperform, and electronics misbehave — none of which trips a breaker.
At what length does voltage drop start to matter?
Around 35 to 40 m for a typical 32 A circuit at 230 V. Below that, current capacity governs. Beyond it, voltage drop demands progressively larger cable — at 120 m the same load needs 16 mm² rather than 4 mm², four sizes up.
Why do 12V systems need such thick cable?
Because 5% of 12 V is only 0.6 V, against 11.5 V at 230 V, and low voltage means high current for the same power. A 20 A load over 10 m needs 2.5 mm² for current but 16 mm² for voltage drop — six sizes up. It’s why solar and caravan wiring looks oversized.
What is cable derating?
Reducing published current capacity for real installation conditions. Ambient heat, grouping with other circuits and contact with thermal insulation each reduce what a cable can carry, and the factors multiply. A cable at 35 °C, grouped with two others and touching insulation runs at ×0.49 — about half capacity.
Do derating factors add or multiply?
They multiply. The common error isn’t the arithmetic — it’s applying only the worst factor and ignoring the rest. Taking grouping alone gives ×0.70; applying all three gives ×0.49, which is a whole cable size or more. Every applicable factor counts, not just the largest.
Why multiply by 2 in the voltage drop formula?
Because current flows out to the load through the active conductor and back through the neutral, so it passes through twice the run length. Three-phase circuits use √3 instead, since the return path is shared across phases.
Is aluminium cable the same as copper?
No. Aluminium has about 1.6 times the resistance of copper for the same cross-section, so it needs roughly two sizes larger for the same voltage drop, and carries around 78% of the current. It’s lighter and cheaper, which is why it’s used for large submains and service cables.
Does three phase reduce voltage drop?
Yes, twice over. The formula uses √3 rather than 2, which is 13.4% lower for the same current. More importantly, a 7,000 W load draws 30.4 A at 230 V single phase but only 10.1 A at 400 V three phase — and voltage drop is proportional to current.
Can I install cable myself?
In Australia, New Zealand, the UK and most of Canada, no — fixed wiring is licensed work and DIY installation is illegal, will void insurance, and creates real fire risk. Some US states allow homeowner work on their own residence with a permit and inspection. Check your jurisdiction, and use this calculator to understand a design rather than to execute one.
Standards and sources
- AS/NZS 3008.1 — cable selection, current-carrying capacity and voltage drop for Australia and New Zealand
- BS 7671 and IEC 60364 — the equivalent framework in the UK and Europe
- NEC Article 310 — US conductor ampacity; NEC voltage drop figures are advisory rather than mandatory
- Resistance values are typical for copper at 75 °C conductor temperature; aluminium taken at 1.6× copper resistance and 78% of its current capacity
- Current-carrying capacities shown are for thermoplastic-insulated cable in common installation methods — your cable type, installation method and protective device all change them
- Derating factors are representative. Actual values come from the tables in your governing standard
Related calculators
Solar Panel CalculatorSystem size, output and savings
Insulation CalculatorCable in insulation needs derating
Pool Volume CalculatorPump circuits are a common long run
Roof Pitch CalculatorTilt for solar panels
Square Footage CalculatorPlan a route and area
How this cable size calculator works out its numbers
Two independent searches run down the standard size range. The first finds the smallest conductor whose current-carrying capacity, after all derating factors are applied, still exceeds the design current. The second finds the smallest conductor whose voltage drop over your run length stays inside the available budget. The larger of the two is reported, along with which constraint governed.
Derating factors multiply rather than adding, and the required raw capacity is the design current divided by the combined factor — so a 32 A load at ×0.49 needs a cable rated 64.8 A, not 32 A minus something.
Voltage drop uses 2 × I × L × R ÷ 1,000 for single phase and √3 in place of 2 for three phase, with resistance in milliohms per metre at 75 °C conductor temperature. The comparison table shows the sizes either side of the answer with both checks marked, so you can see the margin rather than just the result.
This is a design aid and nothing more. Real cable selection depends on the specific cable construction, installation method, conductor operating temperature, protective device characteristics, fault loop impedance and disconnection times — none of which this calculator addresses. The governing standard differs by country and is periodically revised. In most jurisdictions the work is licensed, and for good reason: undersized cable is a fire risk that gives no warning before it becomes one. Have a licensed electrician verify any design before installation.