Cable Size Calculator
Calculate Smarter. Work Faster.
Free cable size calculator (also called a cable sizing calculator, mm²) — enter your load in kW, HP, or A, plus cable length, to calculate cable size in copper or aluminium, voltage drop %, and indicative protective-device rating.
Load & Cable Run Details
Enter your load, supply, and cable run details to get a recommended cable size.
Estimated resistive voltage-drop component at 20°C reference resistance — not a full AC impedance voltage-drop calculation; cable reactance and operating-temperature resistance effects are not included.
Enter your details and hit calculate
Calculate to compare ampacity and voltage drop across nearby cable sizes.
How to Calculate Cable Size
This cable size calculator provides a preliminary cable-size recommendation from load current, supply voltage, conductor material, and cable length, together with an estimated voltage drop and indicative protective-device rating. Cable size here refers to conductor cross-sectional area in mm², not the overall cable's outer construction. Cable size depends on load current, conductor material, installation conditions, allowable temperature rise, voltage-drop requirements, and cable length — this calculator simplifies the installation conditions into a preliminary reference-capacity model and separately checks voltage drop. In this calculator's simplified reference table, 2.5 mm² copper is assigned 16 A, 4 mm² 25 A, 6 mm² 32 A, and 10 mm² 40 A — these are illustrative reference values only, not tied to a specific installation method or table, so always check the voltage-drop-adjusted result below rather than the reference capacity alone.
Choosing the correct cable size is one of the most fundamental — and most frequently rushed — steps in any electrical installation. Undersize a cable and it can overheat, degrade its insulation, and become a fire risk. Unnecessary oversizing can increase material and installation cost, although a larger cable may be intentionally selected for voltage drop, derating, future expansion, or other design requirements.
This calculator estimates a preliminary cable cross-sectional area for a given electrical load by first finding the design current from voltage, power, and power factor, then matching that current against a representative preliminary reference-capacity table for copper or aluminium conductors. Cable selection must satisfy the applicable current-carrying capacity and voltage-drop requirements together; this calculator first uses reference capacity to establish a base size, then applies a preliminary voltage-drop screening step, stepping up the size if a longer run would otherwise produce excessive voltage drop.
The current is derived from the same single-phase and three-phase relationships used throughout electrical sizing: for single-phase, I = (kW × 1000) ÷ (V × PF), and for three-phase, I = (kW × 1000) ÷ (√3 × V × PF). Once current is known, it's checked against a preliminary reference-capacity table (sized differently for copper vs aluminium, since aluminium has lower conductivity and needs a larger cross-section for the same current). The calculator then checks the estimated voltage drop at your entered length and steps up to the next standard size — repeating the check — only if the drop exceeds the fixed 3% preliminary screening target, and a matching indicative protective-device rating is suggested based on the calculated load current.
Voltage drop estimate. Beyond the ampacity-based base size, this calculator also computes an approximate voltage drop percentage at the recommended cable size, using the conductor's illustrative reference resistance per kilometre. For single-phase runs, Vd = (2 × I × L × R) ÷ 1000, and for three-phase runs, Vd = (√3 × I × L × R) ÷ 1000, where R is the illustrative reference conductor resistance in ohms/km, L is the one-way cable length in metres, and I is the design current. The result is expressed as a percentage of supply voltage and checked against a 3% preliminary screening target — a quick sanity check on top of the ampacity-based size recommendation, so you can see whether the recommended size is comfortably within target or borderline. This is a resistive-only approximation (R only, no reactance) using illustrative reference resistance values around the standard 20°C reference region, not a specific manufacturer's published cable resistance; actual operating-temperature resistance — and therefore actual voltage drop — is typically higher. Power factor affects the calculated current used in this screening step; the voltage-drop estimate itself only accounts for the resistive component using that current, not the reactive/phase-angle effects a full AC impedance calculation would include. The dedicated Voltage Drop Calculator covers a more detailed R·cosφ + X·sinφ methodology.
The comparison table below the result shows ampacity and estimated voltage drop side by side for the recommended size and its immediate neighbours, and states the actual reason a smaller size falls short — insufficient ampacity, high voltage drop, or both — so you can see at a glance why a given size was or wasn't selected for your specific run length.
This tool is especially useful for electrical contractors and engineers planning a new circuit, panel feeder, or distribution run who need a starting-point cable size, voltage drop indication, and indicative protective-device rating before consulting a full derating and voltage-drop table for the final installation design.
Why Cable Size Matters: Current, Heat, and Voltage Drop
Every conductor has resistance, and current flowing through that resistance generates heat (I²R losses). If a cable is too thin for the current it carries, it heats up beyond the safe rating of its insulation, which accelerates insulation ageing and, in the worst case, leads to a fire. This is why every conductor size has a published current-carrying capacity (also called ampacity) for a given installation method, insulation type, and ambient temperature. At the same time, resistance also causes a voltage drop along the cable's length: the further the load is from the source, the more voltage is "lost" in the wire before it reaches the equipment. A motor or panel that receives a significantly lower voltage than it was designed for can overheat, underperform, or trip on undervoltage — which is why voltage drop, not just current rating, has to be checked separately for long cable runs.
Reference Standards and Engineering Methodology
In India, low-voltage XLPE insulated thermoplastic-sheathed power and control cables (working voltages up to and including 1100 V) are commonly manufactured to IS 7098 (Part 1) — check the current edition and any amendments in force, as standard revisions are periodically updated — which is primarily a cable construction and product specification rather than an ampacity or installation-method standard. Applicable IS 3961 current-rating tables (the part applicable to the cable construction) and IEC 60364 installation principles are used separately for derating and voltage-drop guidance. The calculator does not use an installation-specific IS/IEC ampacity table — its internal capacity values are simplified screening values created for preliminary comparison only, not extracted from any specific edition of these standards. Published current-carrying capacity tables vary by conductor size, insulation type, and installation method (free air, tray, conduit, buried), with correction factors for ambient temperature, grouping of multiple circuits, and soil resistivity for buried cables. The current-carrying capacity values and conductor resistance figures used in this calculator are representative reference values in the style of such tables — useful for quick, preliminary sizing, but not a substitute for the applicable cable manufacturer's data and the full derated table specific to your exact installation condition. The calculator does not reproduce the current-rating tables of IS 3961 or IEC 60364; it uses simplified illustrative reference values for preliminary screening.
Cable Sizing by Voltage and Region
Cable size depends on load current, cable length, conductor material, and the local supply voltage. This calculator accepts common low-voltage supply voltages used in many regions — India, the UK and Europe, the Middle East, Australia, South Africa, plus any custom voltage — while using a simplified metric mm² reference model rather than the installation-specific sizing rules of any particular jurisdiction. These regions commonly use metric conductor cross-sectional areas (mm²), but their installation standards, correction factors and allowable current tables differ (for example IS 7098 (Part 1) for cable product requirements alongside applicable Indian installation requirements, with IEC 60364 principles used where relevant; BS 7671 in the UK and other IEC-based jurisdictions in the rest of Europe; AS/NZS in Australia) — this calculator uses the shared mm² sizing convention throughout, not any single region's specific table.
The USA and Canada are the notable exception: they size conductors in AWG or kcmil under the NEC (NFPA 70) or CEC/CSA C22.1, using different reference ampacity tables (e.g. NEC Table 310.16) rather than a metric mm² chart. Because that's a genuinely different sizing system, not just a unit conversion, this calculator does not attempt to output an AWG size — use an AWG-based wire size calculator or the applicable NEC/CEC ampacity table for a USA or Canadian installation.
How to Use This Cable Size Calculator
Getting a preliminary recommendation only requires a few key inputs:
- Select the system type — single phase (230 V typical domestic supply) or three phase (415 V typical industrial/commercial supply).
- Enter the supply voltage in volts. Single-phase defaults to 230 V; switching to three-phase sets it to 415 V, and either can be edited to match your local supply, such as 240 V or 440 V.
- Enter the connected load in kW and the expected power factor. Use the measured/nameplate power factor where available — for a purely resistive load (such as a heater), PF may be close to 1.0; do not assume a generic PF for final design.
- Enter the one-way cable length in metres and choose the conductor material (copper or aluminium). Click Calculate Cable Size to get the recommended cross-section, design current, indicative protective-device rating, and estimated voltage drop.
Common Mistakes When Sizing Cables
- Sizing only for current, ignoring length. A cable that is technically rated for the current can still cause an unacceptable voltage drop on a long run — always check both.
- Using the wrong power factor. Assuming PF = 1 for motor or mixed loads underestimates current and can lead to undersized cable and breaker selection.
- Not accounting for ambient temperature and grouping. Cables run in a hot switchroom or bundled tightly with several other loaded cables need to be derated below their reference capacity; this calculator uses simplified reference capacities as a baseline.
- Not accounting for the number of loaded conductors/cable cores. Ampacity tables vary by whether a cable is 2-core, 3-core, or 4-core and by how many current-carrying conductors are loaded — this calculator does not ask for cable core count and uses a single reference figure per conductor size regardless of it.
- Ignoring the material difference. Substituting aluminium for copper (or vice versa) without re-checking the size table can leave a circuit undersized.
Illustrative Screening Values — NOT Cable Ampacity
Illustrative screening capacity — NOT installation ampacity. These values are intentionally simplified screening values; actual allowable current must come from the applicable cable/current-rating table after installation-method and correction-factor selection.
| Size (sq.mm) | Illustrative Screening Capacity |
|---|---|
| 1.5 | 10 A |
| 2.5 | 16 A |
| 4 | 25 A |
| 6 | 32 A |
| 10 | 40 A |
| 16 | 63 A |
| 25 | 80 A |
Larger sizes (120–400 sq.mm) are also supported by the calculator above but omitted here for brevity — enter your full-load current above to see the full recommendation including protective-device rating and voltage drop.
Formulas Used
Illustrative calculator example: Three-phase load of 20 kW at 415 V, PF = 0.85, cable run = 60 m, copper conductor. Load current = (20 × 1000) ÷ (1.732 × 415 × 0.85) ≈ 32.7 A. With the +20% planning margin enabled, the planning sizing current is about 39.3 A. A 10 sq.mm copper reference capacity of 40 A satisfies the reference-capacity check, and at 60 m the estimated voltage drop is about 1.8% — comfortably under the 3% preliminary screening target. Within this calculator's simplified reference model, 10 mm² passes the preliminary capacity and voltage-drop screening checks, so it is not stepped up further. Since Ib = 39.3 A and the cable's reference capacity is 40 A, the simplified protective-device logic would return 40 A as the indicative rating.
Important: This calculator uses simplified preliminary reference capacities and does not automatically account for installation method, insulation rating, ambient temperature, grouping, buried conditions, number of loaded conductors, or other correction factors. Final cable selection must use the applicable current-carrying-capacity tables and manufacturer data for the actual installation.
Reference: The preliminary reference-capacity and conductor-resistance values used by this calculator are simplified figures commonly used in electrical engineering handbooks and reference tables. Cable product requirements may be covered by IS 7098, while current-carrying capacity and installation correction factors must be taken from the applicable current-rating/installation standard (such as IS 3961, IEC 60364, or the equivalent standard for your jurisdiction) and the cable manufacturer's data. This calculator is for preliminary, educational planning only and does not replace a full derating and voltage-drop calculation to the applicable local wiring code.
Ambient Temperature and Grouping Derating
A cable's rated ampacity is defined for a specific reference ambient temperature (commonly 30°C in air, a widely used baseline in IEC-family ampacity tables) and for a single cable installed alone. Real installations rarely match both conditions exactly, so two correction factors are applied to get the actual safe current for your installation. This section explains why installation conditions affect cable ampacity — the calculator above does not ask for insulation type and does not apply these correction factors automatically; for a size that accounts for temperature and grouping, use the dedicated Cable Derating Calculator.
Illustrative Example — Not for Final Cable Selection. Values shown are illustrative only and are not a complete IEC/IS correction-factor table.
| Ambient Temp | PVC Derating Factor | XLPE Derating Factor |
|---|---|---|
| 30°C (reference) | 1.00 | 1.00 |
| 35°C | 0.94 | 0.96 |
| 40°C | 0.87 | 0.91 |
| 45°C | 0.79 | 0.87 |
| 50°C | 0.71 | 0.82 |
Illustrative ambient-temperature correction factors for a specified reference condition; values vary with conductor insulation rating, installation method, and the applicable standard/table.
Notice XLPE (cross-linked polyethylene) insulation tolerates higher operating temperature than PVC and therefore derates less severely as ambient temperature climbs — this is one reason XLPE cables are preferred for hot plant rooms, direct sun exposure, or high-ambient industrial environments. A second, separate factor applies when multiple loaded cables are grouped together (in a tray, duct, or conduit), since each cable's own heat adds to the ambient temperature the neighboring cables experience. Grouping can significantly reduce allowable current; the applicable correction factor depends on the number of loaded circuits, spacing, and the installation arrangement in the applicable standard/table. Both factors multiply together and against the base rated ampacity, and the combined correction factors can substantially reduce the allowable current, sometimes requiring a larger standard cable size than the uncorrected reference calculation — always apply both derating factors, not just one.
Phase Arrangement, Neutral Sizing & Cable Laying Methods
The calculator above sizes a single conductor per phase for a straightforward run. Larger installations often split each phase across two or more parallel single-core cables instead of one large conductor, and route it using a specific laying method — both of which introduce considerations this calculator does not cover. This section explains those concepts at a general level; the calculator itself does not model parallel-cable arrangement, harmonic-adjusted neutral sizing, or a specific laying method.
Optimal Arrangement for Parallel Single-Core Cables
When a single conductor would be too large to handle, manufacture, or physically pull into a duct, each phase is instead run as two or more single-core cables in parallel, sharing the total current between them. In principle each parallel cable should carry an equal share of the current, but in practice the physical arrangement of the cables relative to each other changes the mutual inductance between phases and between parallel cables of the same phase — and an uneven arrangement can leave some cables carrying meaningfully more current than others, while also increasing localized heating between closely-grouped cables. Two broad arrangement families address this: a tight triangular ("trefoil") grouping of the three phases for each parallel circuit, and a flat, evenly-spaced arrangement across all circuits — each with different current-sharing and derating behavior.
Because the current-sharing imbalance gets progressively harder to manage as more parallel cables are added per phase, installation standards (such as AS/NZS 3008.1 for arrangement guidance, alongside IEC 60364-5-52 and equivalent national installation codes) publish specific recommended physical layouts for up to typically four parallel circuits per phase, rather than leaving the arrangement to installer preference. The general pattern across these arrangements is summarized below at a concept level — always use the specific diagram and cable-transposition sequence given in the applicable standard for the actual number of parallel cables in your design, since the exact layout (not just the general idea) is what keeps current-sharing balanced.
Illustrative example arrangements
Illustrative example layouts only, showing the general concept of grouped ("trefoil") vs flat conductor arrangement across parallel circuits — not a reproduction of any specific standard's published figure. The applicable installation standard (e.g. AS/NZS 3008.1, IEC 60364-5-52) gives the complete, specific set of recommended and alternative arrangements, including additional symmetric options and neutral-conductor placement, for the actual number of parallel conductors in your design.
Summary by conductor count
| Parallel Cables per Phase | General Arrangement Concept | Why It Matters |
|---|---|---|
| 1 (single cable per phase) | No arrangement choice needed | Only one path for current, so sharing isn't a concern |
| 2 | Cables of the same phase kept close together, evenly positioned relative to the other phases | Keeps the mutual inductance each parallel cable experiences roughly similar |
| 3 | Grouped-trefoil per circuit, circuits positioned symmetrically, or a defined flat sequence | Imbalance risk grows with more parallel paths, so the specific sequence matters more |
| 4 | A defined repeating/transposed sequence across the full group, per the applicable standard's diagram | At this count, an arbitrary arrangement can produce a materially uneven current split between cables |
General concept summary only, not a substitute for the specific arrangement diagrams published in the applicable installation standard (e.g. AS/NZS 3008.1, IEC 60364-5-52, or the equivalent standard for your jurisdiction).
Neutral Conductor Sizing
In a perfectly balanced three-phase system, the neutral carries little to no current, since the three phase currents cancel out at the neutral point. Historically, this let designers size the neutral the same as, or occasionally smaller than, the phase conductors for predominantly balanced multi-phase loads. Two things complicate that default: genuine load imbalance between phases, which the neutral must carry directly, and harmonic currents from nonlinear loads — in particular, a substantial population of nonlinear single-phase loads on a three-phase, four-wire system (such as electronic lighting drivers, switched-mode power supplies, and other IT/data equipment) generates triplen harmonics (3rd, 9th, 15th, and so on) that do not cancel between phases the way the fundamental current does, and can add together in the neutral instead, sometimes pushing neutral current above any individual phase's current. Three-phase nonlinear loads such as VFDs generate harmonics too, but this specific neutral-accumulation effect is most strongly associated with nonlinear single-phase loads.
Because of this, most wiring standards (including AS/NZS 3000 and IEC 60364-5-52-style guidance) converge on a similar overall approach even though the exact clause wording differs by jurisdiction: size the neutral the same as the phase conductors by default, allow a reduced neutral only for predominantly balanced multi-phase loads with low harmonic content (and even then, sized to carry at least the expected out-of-balance current), and require a neutral rated equal to or larger than the phase conductors whenever a substantial population of triplen-harmonic-generating nonlinear single-phase loads is expected. The table below summarizes this general pattern.
| Circuit Type | Typical Neutral Sizing Guidance |
|---|---|
| General/default multiphase circuit (mains, submains, final subcircuits) | Neutral sized the same as the phase conductors |
| Predominantly balanced multiphase loads, low harmonic content | May be reduced, but must still carry the expected out-of-balance (and any harmonic) current |
| Substantial nonlinear single-phase loads expected (electronic drivers, SMPS, IT/data loads, etc.) | Neutral sized equal to, or larger than, the phase conductors |
General pattern summary only — the specific sizing rule, exceptions, and any required calculation method are set by the applicable wiring standard for your jurisdiction (e.g. AS/NZS 3000, IEC 60364-5-52, IS installation practice, or BS 7671), not by this calculator.
Common Cable Laying / Installation Methods
The same conductor size can have a meaningfully different current rating depending on how it's installed, because each method affects how easily the cable sheds heat. The table below is a general orientation to commonly used laying methods, not an exhaustive or standard-specific installation-method reference table.
| Laying Method | Typical Application | Key Consideration |
|---|---|---|
| Direct burial in ground | Underground distribution runs between buildings or substations | Soil thermal resistivity and burial depth strongly affect rating |
| Buried duct/pipe | Underground runs needing future re-pulling or multiple cables in one route | Trapped air around the cable typically lowers the rating versus direct burial |
| Perforated cable tray | Indoor switchrooms, panel rooms, and general industrial routing | Good airflow; rating depends on single-layer vs multi-layer/touching arrangement |
| Cable ladder | Large power cables in industrial plant areas | Open support with good ventilation; simplifies future circuit additions |
| Conduit (surface or embedded) | Control wiring and shorter power runs, walls/floors/ceilings | Restricted airflow versus open tray; number of cables per conduit affects derating |
| Cleated direct on a surface | Short exposed runs, utility or temporary wiring | No mechanical protection from conduit or tray; spacing from the surface matters |
| Aerial/overhead (messenger wire) | Service runs to remote or separate structures where trenching isn't practical | Needs adequate mechanical support and ground clearance; weather exposure |
| Free air, spaced from surface | Where the highest ampacity for a given size is needed | Requires sufficient spacing/support to actually achieve free-air airflow |
General orientation only — the applicable current-rating table always specifies ratings per installation method precisely, and the exact terminology and permitted methods vary by standard and jurisdiction.
This section is conceptual background, not a design tool — it does not calculate a parallel-cable arrangement or an installation-method-specific ampacity, and it does not adjust the neutral or earth size shown in the calculator above for load imbalance or harmonic content (that calculator shows only the simplified same-as-active neutral default and a rule-of-thumb earth size). For an actual design, use the specific arrangement diagrams, neutral-sizing clauses, and installation-method rating tables in the standard applicable to your installation (such as AS/NZS 3008.1/3000, IEC 60364-5-52, IS installation practice, or BS 7671), and verify with a qualified electrical engineer.
Frequently Asked Questions
Content last reviewed: August 2026 · Reference framework: IS 7098 (Part 1) (product), IEC 60364 (installation)
Why does cable length affect the recommended size? +
Longer cable runs have more resistance, which causes a larger voltage drop for the same current. Even if a smaller cable can technically carry the current safely, a long run may cause the voltage at the load end to drop too much. This calculator increases cable size only when the estimated voltage drop exceeds its fixed 3% preliminary screening target, and shows the voltage drop percentage that supports that choice.
What does the estimated voltage drop % actually tell me? +
It's an approximate percentage of your supply voltage that is "lost" to cable resistance over the given run length, calculated using a typical resistance-per-km value for the chosen conductor and size. This calculator uses 3% as a conservative preliminary screening target — applicable local codes and equipment requirements may permit different limits, so treat it as a useful sanity check rather than a universal regulatory value. Actual drop can also vary with cable construction, temperature, and installation method.
Why does aluminium need a larger cable size than copper for the same load? +
Aluminium has substantially lower electrical conductivity than copper for the same cross-sectional area, so a larger conductor is generally required for comparable current-carrying and voltage-drop performance — which is why the two materials use separate sizing tables here.
How is the indicative protective-device rating chosen? +
The calculator picks a predefined indicative protective-device rating that sits at or above your planning sizing current and at or below the selected cable's own reference capacity — a simplified Ib ≤ In ≤ reference capacity check — from this calculator's predefined indicative rating list, and labels it "indicative" rather than a final selection. Individual ratings in this list may or may not be available across MCB, MCCB, and RCBO product families depending on manufacturer and jurisdiction. Proper protective-device selection (MCB, MCCB, RCBO, or fuse) requires verification against the actual derated cable capacity, fault level, breaking capacity, trip characteristics and coordination with upstream/downstream protection, and any short-circuit or coordination study required for the installation.
Does this replace a full cable sizing and voltage-drop study? +
No — this is a preliminary, educational estimate using standard formulas and typical resistance values. A final installation design should also account for ambient temperature derating, grouping of multiple cables, installation method (conduit, tray, buried, etc.), harmonic loading, and the specific requirements of your local electrical code, ideally reviewed by a qualified electrical engineer.
What size cable do I need for a given current? +
As a rough guide, using representative illustrative reference values with the metric mm² sizing convention commonly used across India, the UK and Europe, the Middle East, Australia, and South Africa, in this calculator's simplified reference table, 4 sq.mm copper is assigned a preliminary reference capacity of around 25 A, 6 sq.mm around 32 A, and 10 sq.mm around 40 A. The exact figure depends on insulation type, installation method, ambient temperature, and grouping with other cables, which is why this calculator also runs a voltage-drop check at your entered length rather than relying on ampacity alone. The USA and Canada use AWG-based sizing instead — see the reference chart above for the mm² range covered here.
Can I use this calculator for domestic house wiring? +
Yes, for a preliminary estimate, but not as a substitute for the applicable domestic wiring rules, protective-device requirements, installation method and final circuit design. Select single phase, enter your supply voltage (typically 230 V), your connected load in kW, and the cable run length from the distribution board to the point of use. As a preliminary screening approach, keeping the estimated voltage drop under the 3% preliminary screening target and choosing the next standard cable size above the calculated minimum is useful groundwork — but always have the final wiring plan checked against your local wiring code.
Does this calculator apply ambient temperature or grouping derating automatically? +
No — this calculator's reference capacity values are simplified baseline figures for preliminary screening, and do not represent an installation-specific 30°C IEC/IS ampacity table. If your installation runs at higher ambient temperature or has multiple loaded cables grouped together, apply the applicable correction factors to the reference capacity shown above and select the next standard size that satisfies the resulting derated condition. Do not assume that moving up one or two standard sizes automatically compensates for derating.
How should parallel single-core cables be arranged for balanced phase currents? +
Cables of the same phase should be grouped and positioned according to the arrangement diagram in the applicable installation standard (such as AS/NZS 3008.1 or IEC 60364-5-52) for the number of parallel cables per phase in your design, rather than an arbitrary layout — mutual inductance between an uneven arrangement of parallel cables and phases can otherwise leave some cables carrying meaningfully more current than others, and this calculator does not model or check parallel-cable arrangement.
What size should the neutral conductor be? +
By default, size the neutral the same as the phase conductors — the calculator's result panel shows this default value alongside the active conductor size. For predominantly balanced multiphase loads with low harmonic content, a reduced neutral may be permitted under the applicable standard, but it must still carry the expected out-of-balance current; the calculator does not check for or apply that reduction. Where a substantial population of nonlinear single-phase loads is expected (such as electronic lighting drivers, switched-mode power supplies, and IT/data equipment on a three-phase four-wire system), the neutral may need to be sized equal to or larger than the phase conductors, since the triplen harmonics from these loads can add together in the neutral rather than cancelling — the calculator does not detect harmonic loading, so increase the neutral size manually if this applies to your installation.
How is the preliminary earth (CPC) conductor size calculated? +
The calculator uses a simplified, widely-cited rule of thumb: the earth/CPC conductor is sized the same as the active conductor up to 16 sq.mm, held at 16 sq.mm for active sizes from 16 to 35 sq.mm, and sized at half the active conductor's cross-section (rounded up to the next standard size) above 35 sq.mm. This is a preliminary approximation only, not the adiabatic calculation (based on fault current and disconnection time) that most wiring standards require for a final design — use the dedicated Earthing Conductor Size Calculator for a properly calculated value.
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