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    LV Cable Sizing Explained

    How do engineers choose a cable? Understand ampacity, installation methods, correction factors, multiple parallel runs, voltage drop and protection — without getting lost in tables.

    THE CORE IDEA

    Cable sizing is not simply picking a conductor that can carry the load current. The cable must also remain within its temperature limit, deliver adequate voltage, survive faults and work correctly with its protective device.

    We'll build the idea one step at a time. The KiloVolt calculator is a screening assistant: the quick mode recommends a preliminary minimum from a limited set of IEC-based reference ratings, while the advanced mode lets you verify a specific cable using manufacturer data.

    Try LV Cable Sizing →12 sections · original diagrams · worked example
    00 · Getting a recommendation

    Minimum cable, recommended cable and parallel alternatives

    Start with the Quick Cable Selection. Enter your load current (up to 1,500 A), cable length, supply voltage, insulation, material and actual installation arrangement. Parallel runs are selected automatically unless you specify a fixed number to inspect.

    The engine checks each available conductor's IEC-based reference ampacity, ambient-temperature correction, grouping factors and voltage drop for the specified method. A preliminary candidate must pass both checks:

    IZ=n×IZ,ref×kT×kG≥IBI_Z=n\times I_{Z,\mathrm{ref}}\times k_T\times k_G\geq I_B
    ΔU%≤selected voltage-drop limit\Delta U_{\%}\leq\text{selected voltage-drop limit}

    Minimum IEC-based cable identifies a passing preliminary arrangement with no additional voluntary thermal headroom. To avoid the impractical mathematical solution of dozens of very small cables, this searches the fewest feasible run count and at most one additional parallel set. It is a screening minimum within these practical limits, not a guaranteed globally minimal or IEC-certified design.

    Recommended cable requires your selected thermal headroom (15% by default) and prefers fewer parallel runs. Recommended Option 2 only appears when a valid arrangement using a limited number of extra parallel runs genuinely reduces conductor cross-section per phase. Limiting the number of additional runs and the selectable thermal headroom are KiloVolt design choices, not IEC requirements.

    When one cable cannot meet the specified load and voltage drop, the search automatically considers parallel conductors without pretending the installation method has changed. Up to 20 bunched/enclosed B2 runs or up to nine single-layer C/E runs are screened, following the limited correction-factor data. Method F can compare up to three perforated trays automatically, clearly reporting the number of separate trays assumed; you may instead restrict the calculation to a fixed tray count. This is a proposed layout, not confirmation that the required trays exist at the site. This is an extension of feeder-current selection to 1,500 A, not a fictitious 1,500 A ampacity for one cable.

    These comparisons are not cost estimates. The comparison counts PHASE-conductor metal area only. It ignores neutral/PE conductors, insulation, installation labour, glands, lugs, switchgear terminations, cable support and loss costs. More than nine grouped C/E runs are not automatically assigned a grouping factor in this version; they require a separate layout analysis. At very high currents also compare busbar trunking and verify thermal interaction, electromagnetic forces and equal current sharing.

    Minimum or recommended does not mean design approved. The quick tool selects phase conductors only (maximum 300 mm² per cable). It does not size neutral/PE conductors or check actual protective-device settings, short-circuit withstand, earth-fault disconnection, harmonic neutral current, AC skin/proximity effects or manufacturer-defined installation limits. Use Advanced Verification and authoritative data for those studies.
    01 · Fundamentals

    What does cable sizing actually mean?

    Think of a cable as a pipe carrying electrical current. Passing current through the conductor produces heat. How much heat the cable can safely release depends on the conductor size, insulation, installation method, ambient conditions and nearby cables.

    A large conductor in open air may carry more current than that same cable packed tightly among many other loaded cables. That's why selecting only by cross-sectional area (mm²) is not enough.

    I

    Ib

    Design current

    How much current does the load draw?

    Iz

    Iz

    Corrected ampacity

    How much current can this installed cable carry?

    In

    In

    Protective rating

    What is the rating or long-time setting of its protection?

    02 · The complete picture

    Four checks for one cable

    Four separate calculations answer different questions. For a fully verified design, none should be skipped merely because another has passed. The quick KiloVolt recommendation deliberately limits its outputs to phase conductor sections, not neutral/PE design.

    Four different questions — one feederAll checks matter
    1Load & installationIb + Iz,ref + derating2Voltage dropR, X, L, cos φ3ProtectionIb ≤ In ≤ Iz, I₂4Fault heatingIk, t, k × SA voltage-drop PASS does not imply safe current-carrying capacity or correct protection.

    There are further installation-specific checks too, including disconnection times, neutral/PE conductors, thermal environment, fire performance and manufacturer installation instructions.

    03 · Current-carrying capacity

    Why cable installation method matters

    The reference current rating, usually denoted Iz,ref, is the rated current for a particular cable and reference installation condition. For example, a multicore cable in a conduit will not necessarily have the same rating as single-core cables installed on cable ladder.

    Conduit / trunking

    Enclosures can restrict heat escape; use the appropriate installation reference method.

    Clipped direct

    More heat can escape than from a tightly enclosed cable, but the actual surface and spacing matter.

    Perforated tray / ladder

    Open-air installation helps cooling, but closely spaced parallel circuits may need substantial grouping corrections.

    Buried cable / duct

    Soil thermal resistivity, depth, spacing and soil temperature must be accounted for.

    In Quick Selection, the calculator uses a curated IEC-based table subset for multicore method B2 (common conduit/trunking, including bunched-group correction when multiple runs are declared), multicore method C (clipped direct), multicore method E (perforated tray) and three-phase single-core method F (flat formation on a perforated tray). These are not interchangeable: choose the actual cable construction and method. B2 is preselected for convenience, not because it always describes Greek installations. C may allow 6 mm² at 40 A with no thermal margin; B2 requires 10 mm² under common PVC/copper/three-loaded reference conditions. A2 for thermally insulated walls may require 16 mm² and is not yet available in quick mode. For buried cables, other installation arrangements, or a specific product, use Advanced Verification with data from the applicable standards or manufacturer.
    04 · Derating factors

    Why do we multiply the cable rating by correction factors?

    Reference ampacity is usually stated for specific reference conditions. A higher ambient temperature or a group of nearby loaded cables can reduce the permissible current. Appropriate correction factors account for those differences.

    IZ=n×IZ,ref×kT×kG×kOI_Z = n\times I_{Z,\mathrm{ref}}\times k_T\times k_G\times k_O

    Here n is the number of parallel cable paths, kT represents ambient/soil temperature, kG represents grouping and kO any additional applicable factor. Each factor must match the same reference ampacity basis.

    If a manufacturer already publishes a rating for the actual grouped installation, don't apply the same grouping reduction again. Double-derating is also a mistake.

    05 · High-current feeders

    Using multiple parallel cables per phase

    Imagine an LV board with a 900 A design current. Instead of one enormous conductor, an engineer may consider 3 × 240 mm² copper per phase. In an ideal, perfectly balanced arrangement, each of the three parallel cables carries one third of the total current.

    Three identical parallel paths per phase (simplified view of one phase)3 × 240 mm² Cu / phase
    SOURCEPhase L1900 ALOADPhase L1900 ARun 1 · 240 mm²300 A (ideal sharing)Run 2 · 240 mm²300 A (ideal sharing)Run 3 · 240 mm²300 A (ideal sharing)Actual three-phase feeder requires equivalent balanced arrangements for L1, L2 and L3.
    Three identical runs divide total current ideally as 900 / 3 = 300 A per run. AC electromagnetic coupling, installation conditions and unequal impedances can cause different sharing in practice.
    Irun=IBn=9003=300 AI_{\mathrm{run}}=\frac{I_B}{n}=\frac{900}{3}=300\ \mathrm{A}

    But three runs do not automatically give triple ampacity. The cables exchange heat, and AC single-core cable geometry influences impedance. They need compatible materials, cross-sections, lengths, routing and terminations. Symmetric cable placement helps equalize AC impedance; the protection scheme must also be compatible.

    The Quick Selection result (phase conductors only, up to 300 mm² each) automatically applies a grouping factor only for its stated idealized arrangement and remains PRELIMINARY. At high current, the number of runs may become impractically high for a real tray or trunking: verify physical feasibility and consider busbar trunking. In Advanced Verification, ampacity stays PENDING until you confirm a suitable grouping factor for the real installation.
    Note · Reading the checks

    What Pass, Review and Not assessed mean

    The verification checks use the same badges as every KiloVolt calculator. Pass means the check was calculated and met. Review means a limit was exceeded and the design must be changed or justified. Not assessed means a required input, such as the reference ampacity or the protective-device rating, was not entered, so the check could not be made; it is never a pass. Messages that always apply to the method are kept in the collapsed “Scope and assumptions” panel. See how to read the results.

    06 · Voltage at the load

    Why cable size also affects voltage drop

    A cable with sufficient current capacity can still have excessive voltage drop if it's long. Higher resistance and reactance produce larger voltage changes at the receiving end.

    ΔU3φ≈3×IBn×L×(Rcos⁡φ+Xsin⁡φ)\Delta U_{3\varphi}\approx\sqrt{3}\times\frac{I_B}{n}\times L\times\left(R\cos\varphi+X\sin\varphi\right)

    Here L is the one-way length in kilometres, R and X are per-run impedance terms in Ω/km, and the displayed expression assumes an inductive load. The basic two-wire AC circuit uses a factor of 2 instead of √3; a two-wire DC circuit uses the resistive term only.

    Remember to add upstream feeder contributions and apply project-specific voltage-drop criteria. KiloVolt labels the percent limit as your selected target, not a universal limit.

    Open the separate Voltage Drop Calculator →
    07 · Circuit breakers

    How the cable and its protective device must work together

    When overload protection is required, the device must carry normal load without nuisance trips and also protect the installed cable from overheating. The standard conditions are commonly expressed as:

    IB≤In≤IZI_B\leq I_n\leq I_Z
    I2≤1.45×IZI_2\leq1.45\times I_Z

    I₂ is the current that ensures operation within the device's specified conventional time. It depends on the protective device and standard; it is not always 1.45 × In. For adjustable breakers, the relevant long-time current setting is often the meaningful In input.

    A cable may pass its voltage-drop check but fail breaker coordination. Treat the results as independent checks.

    08 · Fault current heating

    Why faults require another cable check

    During a short circuit, thousands of amperes may flow for a short time. The copper or aluminium conductor heats rapidly. Under the correct assumptions, the adiabatic equation provides a preliminary withstand check:

    Smin⁡=IktkS_{\min}=\frac{I_k\sqrt{t}}{k}

    S is cross-sectional area in mm², Ik is fault current in amperes, t is clearing time in seconds, and k depends on conductor material and permissible temperature rise. With equal fault-current division across n identical paths, the per-run required area becomes:

    Smin⁡,run=Iktk×nS_{\min,\mathrm{run}}=\frac{I_k\sqrt{t}}{k\times n}
    Cu / PVC: k ≈ 115Al / PVC: k ≈ 76Cu / XLPE: k ≈ 143Al / XLPE: k ≈ 94

    For PVC conductors larger than 300 mm² the permitted short-circuit final temperature is lower (140 °C instead of 160 °C), so the calculator uses k ≈ 103 for copper and k ≈ 68 for aluminium; XLPE values do not change. These are commonly used approximate insulated phase-conductor k factors for conventional temperature assumptions. They aren't universal for all constructions or starting temperatures. The equation is not a substitute for actual device let-through I²t curves, fault-current division analysis or protection verification.

    09 · The fourth conductor

    The neutral conductor: how big should it be?

    Everything so far sized the phase conductors. In a three-phase four-wire circuit there is also a neutral, and it is chosen after the phase size is known. It is not simply “the same as the phase” and not automatically “half the phase”: it depends on what the loads do.

    Why the neutral is different

    With three equal, purely linear loads the three phase currents cancel in the neutral and it carries almost nothing. Real circuits are not like that. Single-phase loads are never shared perfectly between the phases, and electronic loads such as computers and LED drivers draw current in short pulses that are rich in triplen harmonics (the 3rd, 9th, 15th…). These are single-phase, line-to-neutral loads. A conventional three-phase, three-wire variable-speed drive has no neutral connection: it injects characteristic line harmonics (5th, 7th, 11th…) into the phase conductors, which matter for heating and for the supply, but it does not load a neutral with triplen current.

    Triplen harmonics: why many neutrals stay full size

    The fundamental currents of the three phases are 120° apart, so they cancel in the neutral. A third-harmonic current is different: in all three phases it reaches its peak at the same instant, so the three contributions are in step and add. For identical phases the neutral carries about three times the triplen current of one phase:

    IN≈3 I3(balanced fundamental cancels)I_{N}\approx 3\,I_{3}\qquad(\text{balanced fundamental cancels})

    Example A: why a reduced neutral cannot be assumed acceptable

    Each phase carries 100 A of fundamental current plus 20 A of third harmonic (20 % third-harmonic content). The fundamental cancels in the neutral, but the third harmonic does not:

    IN≈3×20=60 A=60 % of the 100 A phase currentI_N\approx 3\times 20=60\ \mathrm{A}=60\,\%\ \text{of the 100 A phase current}

    In this illustrative balanced case the neutral carries 60 A of third-harmonic current even though the fundamental currents cancel. Whether a reduced neutral is acceptable depends on the neutral's temperature-corrected current rating and the harmonic assessment under IEC 60364-5-52 clause 524.3 / Annex E. This example alone cannot establish a compliant neutral size.

    When the neutral must be the same size as the phase

    • Single-phase circuits, where the neutral carries the whole load current.
    • Three-phase circuits whose phase conductors are up to 16 mm² copper or 25 mm² aluminium.
    • Circuits for which the neutral current, thermal conditions or harmonic assessment does not justify reduction; a full-size or larger neutral may be necessary.

    When a smaller neutral may be considered

    Only for larger phase conductors, and only if all of these are true (IEC 60364-5-52 clause 524, as understood here; check the clause and the national annex):

    • the phase conductors are larger than 16 mm² Cu or 25 mm² Al;
    • the load is practically balanced between the phases;
    • the neutral current, including harmonics, stays within the current rating of the smaller neutral in your installation;
    • applicable neutral overcurrent protection and disconnection rules are met;
    • the neutral is at least 16 mm² Cu / 25 mm² Al AND at least half the phase section.

    Summaries of IEC 60364-5-52:2009/AMD1:2024 (CENELEC HD 60364-5-52:2011/A1:2025) give two mandatory minimum-area conditions when a reduced neutral is permitted: at least 16 mm² Cu / 25 mm² Al, and not less than 50% of the phase area. These are necessary, not sufficient, conditions. For triplen harmonics, the standard refers to its normative Annex E with harmonic-dependent reduction factors and thermal requirements. Do not treat 15% and 33% as universal design cutoffs. Earlier editions (IEC 60364-5-52:2009 and HD 60364-5-52:2011) required only the 16 mm² Cu / 25 mm² Al floor, so a project designed under those editions may not require the half-phase minimum; the licensed text and the national adoption (ELOT) decide.

    Example B: a 95 mm² copper phase

    Phase conductors 95 mm² Cu, balanced load, 5 % third harmonic, neutral protected.

    SN,IEC min=16 mm2,SN,half phase=952=47.5 mm2 → 50 mm2S_{N,\mathrm{IEC\ min}}=16\ \mathrm{mm^2},\qquad S_{N,\mathrm{half\ phase}}=\frac{95}{2}=47.5\ \mathrm{mm^2}\ \rightarrow\ 50\ \mathrm{mm^2}

    The 2024 IEC minimum from the combined area rules is the next standard size, 50 mm². In practice you would select an available construction, such as 3 × 95 + 50 mm², only after verifying the neutral current, harmonic derating, thermal conditions and protective requirements. Example A illustrates why harmonic assessment is necessary; it does not determine a unique section.

    What the calculator shows

    Under a three-phase recommendation, LV Cable Sizing shows a collapsed “Neutral conductor” block that applies these rules to the phase size you selected. It asks for nothing and reports no pass or fail: it only tells you whether the neutral must equal the phase and, if not, the combined 2024 IEC minimum section (material floor and half-phase limit). It does not cover TN-C systems (PEN conductor) or several cables per phase, where the neutral arrangement is a design decision.

    Check before relying on this chapter. Clause 524.2 is described here according to IEC 60364-5-52:2009/AMD1:2024. Clauses 524.3 and Annex E require project-specific harmonic/thermal assessment. Verify the nationally applicable edition, including ELOT/HD adoption and any additional rules.
    10 · Worked example

    Example: 900 A feeder with 3 × 240 mm² copper per phase

    Consider a balanced 400 V feeder, 25 m long, with an XLPE-insulated copper conductor arrangement. The values 500 A reference ampacity, 0.91 ambient factor and 0.75 grouping factor are hypothetical illustrative inputs, not manufacturer ratings for a real cable. With three cables per phase the neutral arrangement is a design decision, so this example sizes the phase conductors only (see chapter 09 for the neutral).

    Load current900 AParallel runs3Area per run240 mm² CuProtection setting950 A (illustrative)Fault assumption25 kA for 0.2 sInsulationXLPE

    Current per run

    Irun=9003=300 AI_{\mathrm{run}}=\frac{900}{3}=300\ \mathrm{A}

    Corrected total current capacity

    IZ=3×500×0.91×0.75=1023.75 AI_Z=3\times500\times0.91\times0.75=1023.75\ \mathrm{A}

    Because 900 ≤ 950 ≤ 1,023.75 A, the illustrated long-time setting satisfies the first overload condition.

    Estimated voltage drop

    ΔU≈1.52 V=0.38%\Delta U\approx1.52\ \mathrm{V}=0.38\%

    The voltage-drop estimate is small for this illustrative 25 m run. Actual cable AC impedance must still be checked.

    Thermal minimum area per run

    Smin⁡,run=25,000×0.2143×3S_{\min,\mathrm{run}}=\frac{25{,}000\times\sqrt{0.2}}{143\times3}
    Smin⁡,run≈26.06 mm2S_{\min,\mathrm{run}}\approx26.06\ \mathrm{mm}^2

    Passing this short-duration thermal estimate alone does not establish that the installation is safe.

    11 · Practical lessons

    Common mistakes to avoid

    Three parallel runs means exactly three times Iz?

    No. Grouping and ambient conditions affect the corrected current capacity.

    Enter total three-cable ampacity as one cable rating?

    No. The reference ampacity input is per run; KiloVolt multiplies by the number of parallel paths.

    Pick cable size based only on the load current?

    Also check drop, protection, fault withstand and any relevant installation-specific requirements.

    Assume a breaker is safe because In is above Ib?

    In must also coordinate with corrected Iz, and I₂ must be checked where required.

    Assume a balanced three-phase circuit has no neutral current?

    Only for linear, balanced loads. Triplen harmonics from electronic loads add in the neutral and can make it carry as much as, or more than, a phase conductor.

    Halve the neutral because the phase is large?

    Size alone does not allow a reduction. The load must be balanced, harmonics low and the neutral protected, and the neutral may not be below 16 mm² Cu or 25 mm² Al.

    Treat power factor correction as harmonic mitigation?

    No. A capacitor bank corrects reactive power; it does not remove harmonic currents from the neutral.

    Copy one grouping factor to every project?

    Spacing, arrangement, burial conditions, circuit count and reference installation method matter.

    12 · References & scope

    Standards, references and limitations

    The calculation workflow is informed by the following publicly accessible engineering references. Use the applicable purchased IEC/EN/ELOT publications and manufacturer data for detailed design.

    KiloVolt currently includes a limited curated subset of published IEC-based reference ratings for specific above-ground configurations. Its largest recommended individual conductor is 300 mm². Its reference ratings were compared, size by size, with two published sources that reproduce IEC 60364-5-52 (Schneider Electric's Electrical Installation Guide and a manufacturer's technical data), covering installation methods B2, C, E and F up to 300 mm². Where the two differ, the lower value is used, and aluminium 2.5 to 10 mm² has one source only. Neither source is the standard text, so verify the ratings against IEC 60364-5-52 before relying on them. It does not provide comprehensive standards tables, manufacturers’ verified product ratings, installation certification, or complete earthing and protection studies. A preliminary recommendation checks only current-carrying capacity and voltage drop under the selected assumptions.
    NOW TRY IT YOURSELF

    Calculate your next LV feeder

    Enter load current and length, then select the installation method and insulation. Receive a preliminary minimum conductor size and inspect the calculations or continue with advanced verification.

    Open Cable Sizing Calculator →

    If the voltage drop of the chosen cable reaches the supply voltage, the advanced verification shows “Not feasible” instead of a percentage and a receiving voltage (they would be meaningless), the check fails, and the minimum standard area for voltage drop is still given when one exists.