What the cable size calculator estimates
This cable size calculator provides an initial estimate of the conductor cross-sectional area required for an electrical line. The calculation uses line voltage, current, one-way cable length, allowable voltage drop and an optional current safety margin.
The results are shown separately for copper and aluminium conductors. For each material, the calculator displays the mathematical cross-sectional area and the next larger size from a typical metric range. This makes it easier to move from a calculated value, such as 5.4 mm², to a commonly available conductor size, such as 6 mm².
The calculator can be useful for early planning of battery cables, inverter connections, low-voltage lighting, LED strips, workshop equipment, extension leads and individual electrical runs. It also helps demonstrate why a long 12 V cable may require a much larger conductor than a short cable carrying the same current.
- Estimate load power from the entered voltage and current
- Calculate an initial copper conductor size
- Calculate an initial aluminium conductor size
- Select the next larger standard metric conductor size
- Account for one-way cable length
- Apply a maximum allowable voltage drop
- Compare continuous and peak current
- Add a current margin for preliminary sizing
Information required for the calculation
Enter the line type, operating voltage, one-way distance to the load, nominal current, optional peak current, conductor construction, maximum voltage drop and current safety margin. You do not need to choose copper or aluminium as an input because the calculator produces results for both materials.
Cable calculator input fields
| Input | What it means | What to check |
|---|---|---|
| Line type | A two-wire DC or single-phase AC circuit, or a three-phase AC circuit | Choose the option that matches the electrical system being estimated |
| Line voltage | The operating voltage of the system, such as 12 V, 24 V, 48 V, 120 V, 230 V or 400 V | Use the actual nominal voltage of the source and connected equipment |
| One-way cable length | The distance from the power source to the load | For a two-wire circuit, the formula accounts for the outgoing and return current paths |
| Nominal current | The current expected during normal continuous operation | Use manufacturer data or a properly measured value where available |
| Peak current | A temporary higher current during startup or a short load surge | This may be relevant for motors, pumps, compressors, inverters and power tools |
| Conductor type | Solid, stranded or flexible finely stranded conductor | Construction affects installation, termination and mechanical suitability |
| Allowed voltage drop | The maximum permitted reduction in voltage across the cable | Sensitive low-voltage devices may require a lower percentage |
| Current safety margin | An additional percentage applied to the nominal current | This prevents the preliminary calculation from being based exactly on the entered continuous load |
Why DC and single-phase AC use the two-wire option
For this simplified voltage-drop calculation, DC and single-phase AC are grouped as two-wire circuits. Current travels from the source to the load through one conductor and returns through another, so the resistance of both paths must be considered.
Two-wire circuit:
S = 2 × I × L × ρ / ΔU
Three-phase AC circuit:
S ≈ √3 × I × L × ρ / ΔUThe three-phase option uses a different multiplier because the voltage and current relationships in a balanced three-phase system differ from those in a two-wire circuit. This simplified approach does not model every electrical characteristic of an AC installation.
Calculated size and suggested standard size
The calculated cross-sectional area is the direct mathematical result of the voltage drop formula. It may produce a value that is not normally sold as a cable size, such as 3.7 mm² or 5.4 mm².
The suggested standard size is the next larger value from the metric size range used by the calculator. A result of 3.7 mm² is therefore moved up to 4 mm², while 5.4 mm² is moved up to 6 mm².
Example:
Calculated conductor area: 5.4 mm²
Next larger standard size: 6 mm²
Displayed result:
Copper calculated size: 5.4 mm²
Copper suggested standard size: 6 mm²Copper and aluminium conductor results
Copper and aluminium have different electrical resistivity. For the same current, length and allowable voltage drop, an aluminium conductor will generally require a larger cross-sectional area than a copper conductor. Aluminium is also spelled aluminum in US English.
How to read the material results
| Result | Meaning |
|---|---|
| Copper calculated size | The copper cross-sectional area produced by the voltage drop calculation |
| Copper suggested size | The next larger metric copper conductor size in the calculator's standard range |
| Aluminium calculated size | The aluminium cross-sectional area produced by the voltage drop calculation |
| Aluminium suggested size | The next larger metric aluminium conductor size in the calculator's standard range |
How estimated load power is calculated
The results include an approximate load power based on the entered voltage and nominal current. For the two-wire option, the calculator uses P ≈ U × I. For the three-phase option, it uses P ≈ √3 × U × I.
Two-wire circuit:
P ≈ U × I
Three-phase circuit:
P ≈ √3 × U × IThis is an indicative apparent-power-style estimate for planning. The active power of motors, transformers, compressors and other AC loads may differ because power factor and efficiency are not included in this simplified form.
Why cable length can change the required size
Every conductor has electrical resistance. As cable length or current increases, more voltage is lost along the conductor. A short run may therefore meet the selected voltage drop limit with a smaller conductor, while a longer run carrying the same current may require a much larger size.
Why can a 12 V circuit need a larger cable than a 230 V circuit?
Answer: A voltage loss of 0.5 V is more than 4% of a 12 V supply, but only a small fraction of a 230 V supply. Low-voltage systems therefore have much less room for voltage loss before connected equipment receives noticeably less voltage.
Explanation: The lower the system voltage, the larger the percentage represented by each volt lost in the cable.
Peak current and current margin
Nominal current represents the expected normal operating load. Some equipment can draw a much higher current briefly when it starts or when the load changes. Examples include pumps, motors, compressors, inverters and power tools.
For its design-current result, the calculator compares the nominal current after the selected margin with the entered peak current. It uses whichever value is higher. This avoids producing a result based only on the lower continuous-load figure.
Example:
Nominal current: 20 A
Current margin: 20%
Nominal current with margin: 24 A
Entered peak current: 30 A
Design current used by the calculator: 30 ASolid, stranded and flexible conductors
Conductor construction does not make the same true cross-sectional area several times more or less conductive. Its main importance is mechanical behaviour, installation method and termination. Solid conductors are commonly used where the cable remains fixed, while flexible finely stranded cable is more suitable for equipment that moves, vibrates or requires repeated bending.
Typical uses for different conductor constructions
| Conductor type | Typical applications | Installation considerations |
|---|---|---|
| Solid conductor | Fixed wiring, lighting circuits and permanently installed cable | Relatively rigid and not intended for repeated movement or tight flexing |
| Stranded conductor | Larger cables, panels, feeders and semi-fixed connections | More flexible than solid wire and must be terminated with compatible equipment |
| Flexible finely stranded conductor | Battery leads, inverter cables, vehicles, portable equipment and extension leads | May require ferrules, lugs or crimped terminals approved for finely stranded conductors |
Choose conductor construction for the installation
Fixed wiring normally uses cable designed and approved for permanent installation in the relevant building environment.
Battery, inverter, vehicle and portable-equipment connections often benefit from flexible copper cable that can tolerate movement and vibration.
Stranded conductors must be connected using suitable terminals, lugs, ferrules and crimping methods. Placing loose fine strands directly into an unsuitable terminal can lead to poor contact and heating.
Low-voltage cable sizing example
A 12 V load draws 20 A and is 5 m from the source. Why might the calculated conductor be larger than expected?
Answer: In a two-wire circuit, the current travels through the outgoing and return conductors, so the voltage drop calculation represents 10 m of conductor path. With a 3% limit, the maximum permitted loss is only 0.36 V. This strict voltage limit can require a much larger conductor than a simple current-only estimate suggests.
Explanation: Low-voltage circuits allow only a small voltage loss in volts before the percentage drop becomes significant.
Three-phase calculation example
How does the three-phase option change the calculation?
Answer: The calculator uses the √3 multiplier in the simplified three-phase voltage drop formula. It also estimates power using √3 × voltage × current. The result can therefore differ from the two-wire option even when the numerical values entered for voltage, current and length appear similar.
Explanation: Balanced three-phase systems use different relationships between line voltage, current, power and conductor voltage drop.
Common cable sizing mistakes
- Selecting a conductor only by current and ignoring cable length
- Confusing conductor diameter with cross-sectional area in square millimetres
- Ignoring voltage drop in 12 V, 24 V or 48 V systems
- Assuming copper and aluminium of the same size perform identically
- Ignoring startup or surge current from motors, pumps, compressors and inverters
- Using flexible fine-stranded wire in terminals not designed for it
- Selecting cable without coordinating the fuse, circuit breaker or other protective device
- Ignoring temperature, insulation, grouping and installation conditions
- Treating an online result as a finished electrical design
Metric conductor sizes used by the calculator
When the mathematical result falls between two values, the calculator selects the next larger size from its metric conductor range. Availability and permitted sizes can vary by cable type, manufacturer and country.
Metric sizes used by the calculator:
0.5 / 0.75 / 1 / 1.5 / 2.5 / 4 / 6 / 10 / 16 / 25 / 35 / 50 / 70 / 95 / 120 / 150 / 185 / 240 mm²When a qualified electrician should check the project
- A consumer unit, distribution board, panel, breaker or residual-current protection must be changed
- The circuit will supply a cooker, water heater, boiler, heat pump, generator or high-power inverter
- Existing aluminium wiring is present or the condition of the installation is unknown
- Cable will pass through insulation, timber or other locations where heat dissipation and fire safety are important
- Three-phase equipment or a three-phase supply is involved
- There is uncertainty about earthing, bonding, neutral conductors or fault protection
- The cable will carry high current or operate in a hot, enclosed, grouped or otherwise demanding environment
- The installation requires inspection, certification, permits or compliance with local electrical regulations
Frequently asked questions about cable size
Why is conductor material not an input?
The calculator automatically provides separate copper and aluminium results. This keeps the form shorter while allowing you to compare the effect of conductor resistivity on the required cross-sectional area.
Why are DC and single-phase AC combined?
For this simplified resistance-based voltage drop estimate, both are treated as two-wire circuits with an outgoing and return current path. More advanced AC calculations may also consider reactance, power factor and other system characteristics.
Why is the suggested size larger than the calculated size?
A formula may produce a non-standard value such as 2.9 mm². Since that exact conductor size may not be available, the calculator moves to the next larger value in its metric range, which would be 4 mm² in this example.
What voltage drop percentage should I enter?
The appropriate limit depends on the equipment, installation and local requirements. Values around 3% to 5% are commonly used as preliminary planning targets for many circuits, while sensitive electronics, LED lighting and low-voltage equipment may require a smaller drop. A higher permitted percentage can reduce the calculated conductor size but also lowers the voltage available at the load.
Does this calculator confirm the cable ampacity?
No. The main calculation is based on conductor resistance and the selected voltage drop. A real installation must also be checked against current-carrying-capacity tables and correction factors for the exact cable type, insulation, ambient temperature, grouping and installation method.
Summary
The cable size calculator uses line type, voltage, current, one-way length, peak current, safety margin and allowable voltage drop to estimate copper and aluminium conductor sizes. It is useful for comparing preliminary options and understanding how current, distance and system voltage affect conductor requirements. Before installation, the result should be checked against cable ampacity, protective devices, termination requirements, local regulations and the actual conditions of the circuit.