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    MV Cable Sizing

    Preliminary Cu/Al XLPE sizing for directly buried and ducted 10–30 kV cables, with engineering assumptions and worked formulas.

    EstimatePreliminary result, not a design verification.
    Before you calculate

    Understanding MV cable sizing

    Select one XLPE single-core cable per phase for 10, 20 or 30 kV. Compare preliminary reference ampacity and indicative voltage drop under fixed, clearly stated buried reference conditions. Conductor short-circuit heating and metallic-screen checks need separate verification.

    Learn more — illustrated guide →
    Step 1 · Electrical inputs

    MV cable selection

    Choose the cable arrangement, enter the transformer nameplate kVA or feeder current, then verify the site-specific cable rating with the manufacturer.

    A transformer nameplate kVA gives rated full-load MV current. Switching units preserves the equivalent load.
    A design preference, not an IEC rule.
    10 kA is an illustrative 20 kV starting assumption, not a network-wide average. Replace with site-specific prospective RMS fault current from the network operator or IEC 60909 study. Clear it to leave fault verification pending.
    Default 1 s is a screening duration, not a predicted relay trip time. Enter total effective protection/clearing time (0.2–5 s for this model).
    Advanced electrical checks
    Illustrative default. Replace with actual flat-layout value.

    The phase-conductor check uses IEC-type adiabatic k factors (Cu XLPE: 143; Al XLPE: 94 A·√s/mm² at 90→250°C). Earth faults heat the copper screen differently: its size, fault share, bonding and clearing time are separate checks.

    Reference conditions · IEC 60502-2 Annex B and Prysmian MV catalogue20°C ground · 0.8 m burial · soil resistivity 1.5 K·m/W · 90°C XLPE conductor · 50 Hz · screens bonded both endsFlat horizontal, phase cables spaced by one cable outer diameter.The published Annex B ratings were calculated for 6/10 kV model cables. At 20 kV the actual 12/20 kV product rating must be verified. For different depths, soil conditions, ducts or bonding, this version does not claim a verified ampacity. Obtain the corrected rating using IEC 60287 or manufacturer data.
    Step 2 · Preliminary results

    Thermal cable screening

    MV full-load line current from apparent power46.19 A12/20 (24) kV cable class · Direct buried
    Not yet a verified MV cable recommendation

    For example, a 1,000 kVA transformer at 20 kV draws just 28.87 A at nominal full load. A 35 mm² conductor can exceed that current thermally, but that alone does not establish its suitability. For this 20 kV copper example, an illustrative 10 kA / 1 s duty raises the phase-conductor screening minimum to 70 mm² Cu. The final cross-section depends on the actual short-circuit current and clearing time; also check the copper screen, actual 12/20 kV cable rating, installation conditions and utility requirements. The conductor fault check is based on your entered or illustrative assumed duty; screen protection remains unverified.

    Fault thermal screening: 10 kA for 1 sMinimum calculated conductor area from I²t ≤ (kS)²: 69.93 mm² Cu. The selected standard size must also satisfy ampacity and the indicative voltage-drop target.Default values are only illustrative. Verify actual short-circuit duty, clearing time, screen/earth-fault requirements and manufacturer ratings for any real installation.
    Selected candidate3 × 1 × 70 mm² Cu XLPESingle-core cables, flat formation · Copper · XLPE 90°C
    Reference-condition ampacity246 ASingle-core cables, flat formation, direct buried
    Thermal headroom432.6%
    Conservative drop bound2.99 V0.015% of system voltage
    Lower-bound receiving voltage (approx.)19.997 kV
    Reference-condition ampacity ≥ loadReference pass
    Conservative drop bound ≤ selected targetBound pass
    Conductor short-circuit heatingAdiabatic pass
    Metallic screen / bonding / earth faultRequires separate study
    Engineering verification required

    Tabulated values are reference-condition screening, not a manufacturer-certified rating or a complete IEC 60287 study. The voltage-drop model shows a power-factor-independent conservative bound using hot DC resistance rather than full AC impedance. The metallic screen is NOT sized here: its copper screen area, earth-fault current contribution, bonding configuration and fault clearing time need a separate thermal and protection check. Site-specific soil and duct construction also remain unverified.

    1. Determine the design current

    Convert the known three-phase apparent power (kVA) to line current at the selected line-to-line voltage.

    IB=SkVA3 × ULL,kVI_B=\frac{S_{\mathrm{kVA}}}{\sqrt{3}\,\times\,U_{LL,\mathrm{kV}}}
    IB=16003 × 20≈46.19 AI_B=\frac{1600}{\sqrt{3}\,\times\,20}\approx46.19\,\mathrm{A}

    2. Compare load current with reference ampacity

    The chosen reference table assumes the exact buried geometry, ground temperature and resistivity stated above. No unverified ambient or soil correction has been applied.

    IB≤IZ,refI_B\leq I_{Z,\mathrm{ref}}
    46.19 A  ≤  246 A46.19\,\mathrm{A}\;\leq\;246\,\mathrm{A}

    Practical headroom setting: 15% above design current (optional design preference).

    3. Estimate hot-conductor resistance

    The cable's DC resistance is adjusted to the reference maximum XLPE conductor temperature. AC skin, proximity and screen effects are not yet included.

    R90=R20[1+α × (90−20)]R_{90}=R_{20}\left[1+\alpha\,\times\,(90-20)\right]
    R90≈0.3417 Ω/kmR_{90}\approx0.3417\,\Omega/\mathrm{km}

    4. Estimate voltage drop

    Without an entered power factor, we use a conservative first-order bound based on the magnitude of R + jX. It is not the predicted voltage drop at the actual operating power factor. Manufacturer AC impedance is required for detailed design.

    ∣ΔU∣bound≈3 × IB × L × R902+X2|\Delta U|_{\mathrm{bound}}\approx\sqrt{3}\,\times\,I_B\,\times\,L\,\times\,\sqrt{R_{90}^{2}+X^{2}}
    ∣ΔU∣bound≈3 × 46.19 × 0.1 × 0.34172+0.152|\Delta U|_{\mathrm{bound}}\approx\sqrt{3}\,\times\,46.19\,\times\,0.1\,\times\,\sqrt{0.3417^{2}+0.15^{2}}
    ∣ΔU∣bound≈2.986 V(0.015%)|\Delta U|_{\mathrm{bound}}\approx2.986\,\mathrm{V}\quad(0.015\%)

    5. Check conductor thermal short-circuit withstand

    An indicative adiabatic check only, applicable within its assumptions. The metallic screen has a different cross-sectional area and separate fault duty.

    Smin⁡,fault=Ik × tkS_{\min,\mathrm{fault}}=\frac{I_k\,\times\,\sqrt{t}}{k}
    Smin⁡,fault=10000 × 1143≈69.93 mm2S_{\min,\mathrm{fault}}=\frac{10000\,\times\,\sqrt{1}}{143}\approx 69.93\,\mathrm{mm}^2
    Ik,max⁡=k × StI_{k,\max}=\frac{k\,\times\,S}{\sqrt{t}}
    Ik,max⁡≈10.01 kA(1 s)I_{k,\max}\approx10.01\,\mathrm{kA}\quad(1\,\mathrm{s})

    The mathematical explanation rounds displayed values. All calculations use unrounded values internally. This is not a product-specific IEC 60287 ampacity calculation. The voltage-drop figure is a conservative first-order bound, not a measured or predicted drop at an actual power factor.

    Check before relying on this result
    • Note for 12/20 and 18/30 kV: the reference ratings are calculated for 6/10 kV cables. A Prysmian catalogue lists the same aluminium ratings for 6/10, 12/20 and 18/30 kV but states they were calculated for 6/10 kV, so confirm the rating of the actual product on its manufacturer datasheet.
    Scope and assumptions of this screening (6)
    • Preliminary screening only: the reference ampacities are for one balanced 50 Hz circuit at 20°C soil temperature, 1.5 K·m/W soil resistivity, 0.8 m burial depth, both-end-bonded screens and the specified arrangement.
    • Direct-buried flat reference assumes one cable outside diameter of spacing between the three phase cables.
    • Voltage-drop value is a conservative, power-factor-independent first-order bound using hot DC resistance, not the predicted voltage drop at a known power factor. AC impedance and actual operating power factor must be checked for the completed design.
    • The prospective three-phase fault current and total heating duration must come from the site/network study and actual protection behavior. A software preset is NOT an average for the Greek grid or a verified fault level.
    • Metallic-screen earth-fault thermal withstand, bonding losses, short-circuit protection/disconnection and installation-specific derating require independent verification.
    • The IEC 60502-2:2014 reference table values represent model cables, not a certified ampacity for any specific VOP product. Verify against manufacturer data and the current standard edition.

    References & assumptions

    The reference-condition ampacity subset is attributed to public technical presentations of IEC 60502-2:2014 Annex B (model cable ratings calculated for 6/10 kV), with physical cable examples from VOP. The trefoil and three-core arrangements come from the Prysmian medium-voltage catalogue (a distributor-hosted copy; its edition is not stated); its single-core flat values agree with Annex B. For 20 kV, verify the actual 12/20 kV cable. Commercial deployment requires independent standards/data licensing review and technical validation.