Cable selection algorithm · 1 kV < U_m ≤ 36 kV

MV cable selection algorithm

Selecting a cable between 1 kV and 36 kV, in the order the decisions actually depend on each other: the system earth-fault category first, because it sets the rated voltage; then the construction, the rating, and the conductor and the screen checked separately against the fault current.

12 steps · clause and formula numbers cited at every node · no standard text reproduced

An MV cable is selected in two stages that a low-voltage designer does not meet. First the insulation level: the system is classified A, B or C by how long it may run with one phase earthed (IEC 60502-2, 4.1), and that classification — not the nominal voltage — fixes the rated voltage U₀ from Table 1. On a 12 kV system a category C classification means U₀ = 8,7 kV rather than 6 kV.

Then the conductor and the screen are checked separately. The conductor is sized on the continuous rating from IEC 60287 and its short-circuit withstand to IEC 60949 with a 250 °C limit for XLPE. The screen is sized on the earth-fault current, and its temperature limit comes from whatever it touches — 200 °C against a PVC oversheath, 150 °C against polyethylene (IEC 60986, Table 2). A screen sized on the insulation's 250 °C is a common and expensive error.

The algorithm

Flowchart of the MV cable selection algorithm for 1 kV to 36 kV to IEC 60502-2, IEC 60183, IEC 60287, IEC 60949 and IEC 60986 MV CIRCUIT, 1 kV < U_m ≤ 36 kVCollect the system and route dataIEC 60183, 4.2 a) to k) and 4.3- nominal system voltage, and the highest system voltage U_m- frequency; type of earthing and, where the neutral is not effectivelyearthed, the permitted earth-fault duration per occasion and per year- maximum current: continuous, cyclic and emergency, with the load curve- prospective short-circuit currents, phase-to-phase and to earth, and theirmaximum duration- route length and profile; trefoil or flat laying; the screen bondingintended- ground and air temperatures, soil thermal resistivity and whether measuredor assumed, depth of laying, other heat sources1Fix the insulation levelIEC 60502-2, 4.1, Table 1; IEC 60183, 5.2 to 5.4, Table 1- classify the system: category A clears an earth fault within 1 min,category B runs earthed for up to 1 h (8 h tolerated, 125 h a year),category C is everything else- read U₀ from Table 1: category C needs the next step up — on a 12 kVsystem U₀ = 8,7 kV instead of 6 kV- U_m not less than the highest system voltage (IEC 60183, 5.3); U_p notless than the impulse level from IEC 60071-1 (5.4)2Choose the constructionIEC 60502-2, 4.2, Tables 2 and 3, Clauses 6 to 13- insulating compound: XLPE, EPR or HEPR at 90 °C in normal operation, PVC/Bat 70 °C — Table 3- screen type and cross-section; the metal screen or sheath has to carry theearth-fault current- armour — non-magnetic for single-core cables; oversheath material and anywater barrier- three-core, or three single-core in trefoil or flat formation3Take a trial conductor sizeIEC 60183, Clause 6 a) to e); IEC 60228 class 2- a standard size from the cable construction standard, otherwise from IEC60228 class 2- the mechanical load during pulling and in service- the electric stress at the insulation surface — a small conductor diameterraises it- the economic optimum over the life of the circuit (IEC 60287-3-2)- above 1 600 mm² the skin and proximity effects need explicit treatment4Calculate the continuous ratingIEC 60287-1-1 and IEC 60287-2-1; IEC 60502-2, Annex B for a first pass- Annex B of IEC 60502-2 is tabulated for 6/10 kV constructions at 30 °C inair, 20 °C in ground, 1,5 K·m/W soil and 0,8 m depth- correct with Tables B.10 air and B.11 ground and the thermal-resistivityfactors, or calculate the real case with IEC 60287- grouping and mutual heating from the actual trench or tray section5Is the rating enough, continuous and cyclic?IEC 60287 series steady state; IEC 60853-2 cyclic and emergency6yesIs the voltage regulation acceptable?no IEC limit at MV — the limit comes from the grid code or the projectspecification7yesDoes the conductor withstand the short-circuit?IEC 60949, Clauses 2 and 3; final temperature from IEC 60986, Table 18yesDo the screen and armour withstand the earth fault?IEC 60949, Clause 6; IEC 60986, Tables 2 and 39yesSettle the screen bondingIEC 60183, 4.2 f); IEC 60287-1-1, 2.3- solid bonding is simplest, but circulating screen currents cut the ratingof single-core circuits- single-point bonding and cross-bonding remove that loss, at the price ofsheath voltage limiters and an earth continuity conductor- the choice changes the loss factor and therefore the rating — go back andrecalculate10Specify the accessoriesIEC 60183, 7.1 to 7.3; IEC 60502-4; IEC TS 60815-1- joints and terminations for U_m up to 36 kV are tested to the IEC 60502series- terminations: creepage for the pollution class, and increased clearancesabove 1 000 m altitude- accessories have to take the electrodynamic force of the short-circuit andthe thermo-mechanical movement of the conductor11Check the installation requirementsIEC 61936-1, 6.2.9.1 to 6.2.9.5- maximum permitted temperature respected in normal operation, in agreedspecial conditions and on short-circuit (6.2.9.1)- relieve the thermo-mechanical stress by snaking, flexible connections orexpansion terminations (6.2.9.2)- crossings and proximity to pipes, telecom and other cables: clearance,induced overvoltage, mutual heating (6.2.9.4)- laying: stone-free bedding, cover slabs or warning grid, single-coreforces, vertical cleating (6.2.9.5)12RECORD THE GOVERNING CRITERIONIncrease the size, space the cables, improvethe backfill, or use a cyclic ratinga cyclic rating to IEC 60853-2 is often what makes asize work — but only with a load curve to support itnoIncrease the size, or move the sourceΔU = √3·I·L·(R·cos φ + X·sin φ); at MV the reactance isnot negligible, so past a point a larger conductor stopshelpingnoIncrease the conductor size, or reduce theclearing timeI = ε·I_AD with I_AD²·t = K²S²·ln((θ_f+β)/(θ_i+β)); θ_iis the maximum operating temperature and θ_f is 250 °Cfor XLPE and EPR, 160 or 140 °C for PVC/BnoIncrease the screen cross-section, or add anearth continuity conductorthe limit is set by whatever the screen touches — 200 °Cagainst a PVC oversheath, 150 °C against polyethylene —not by the insulation; split the fault current betweenscreen, sheath and armour in inverse proportion to theirresistancesno
Spine down the left is the path a compliant design takes. Every branch to the right is a failed check and the change it forces, with the dashed arrow showing where the algorithm restarts. The table below carries the same content in text.

The steps in words

The same algorithm as a table: what is checked at each step, the criterion, and the clause it comes from.
#Check or actionCriterionReference
1Collect the system and route dataThe earthing arrangement and the earth-fault duration matter as much as the load current — they set the insulation level.IEC 60183, 4.2 a) to k) and 4.3
2Fix the insulation levelU₀/U(U_m) from the system category, with U_m at or above the highest system voltage and U_p at or above the LIWL of IEC 60071-1.IEC 60502-2, 4.1, Table 1; IEC 60183, 5.2 to 5.4, Table 1
3Choose the constructionCompound gives 90 °C for XLPE/EPR and 70 °C for PVC/B; screen and armour chosen to carry the earth-fault current.IEC 60502-2, 4.2, Tables 2 and 3, Clauses 6 to 13
4Take a trial conductor sizeA standard IEC 60228 class 2 size, checked against stress, mechanical load and the economic optimum.IEC 60183, Clause 6 a) to e); IEC 60228 class 2
5Calculate the continuous ratingI_z at the real ambient, soil resistivity, depth and grouping — Annex B only for the first pass.IEC 60287-1-1 and IEC 60287-2-1; IEC 60502-2, Annex B for a first pass
6Is the rating enough, continuous and cyclica cyclic rating to IEC 60853-2 is often what makes a size work — but only with a load curve to support itIEC 60287 series steady state; IEC 60853-2 cyclic and emergency
7Is the voltage regulation acceptableΔU = √3·I·L·(R·cos φ + X·sin φ); at MV the reactance is not negligible, so past a point a larger conductor stops helpingno IEC limit at MV — the limit comes from the grid code or the project specification
8Does the conductor withstand the short-circuitI = ε·I_AD with I_AD²·t = K²S²·ln((θ_f+β)/(θ_i+β)); θ_i is the maximum operating temperature and θ_f is 250 °C for XLPE and EPR, 160 or 140 °C for PVC/BIEC 60949, Clauses 2 and 3; final temperature from IEC 60986, Table 1
9Do the screen and armour withstand the earth faultthe limit is set by whatever the screen touches — 200 °C against a PVC oversheath, 150 °C against polyethylene — not by the insulation; split the fault current between screen, sheath and armour in inverse proportion to their resistancesIEC 60949, Clause 6; IEC 60986, Tables 2 and 3
10Settle the screen bondingBonding chosen, and the rating recalculated with the screen loss factor that belongs to it.IEC 60183, 4.2 f); IEC 60287-1-1, 2.3
11Specify the accessoriesAccessories type-tested to IEC 60502-4, with the creepage and altitude corrections applied.IEC 60183, 7.1 to 7.3; IEC 60502-4; IEC TS 60815-1
12Check the installation requirementsTemperature, thermo-mechanical stress, crossings and laying all verified against IEC 61936-1, 6.2.9.IEC 61936-1, 6.2.9.1 to 6.2.9.5

What actually governs the size

Running the algorithm is mechanical. Knowing which check will bind before you start is what makes it quick — and what tells you whether a schedule someone else produced was ever checked at all.

CaseWhat binds
Feeders in a trenchThe continuous rating, and the soil. The tabulated ratings of IEC 60502-2 Annex B assume 20 °C ground, 1,5 K·m/W and 0,8 m depth; a real trench at 35 °C in 2,5 K·m/W soil bears little relation to them.
Circuits with a long earth-fault timeThe rated voltage. A category C classification pushes the whole cable specification up one voltage step, changes the insulation thickness and changes the accessory range.
Circuits behind a large fault levelThe screen, not the conductor. The earth-fault current divides between screen, sheath and armour in inverse proportion to their resistances, and the permitted final temperature is set by the material in contact with each.
Single-core circuitsThe bonding. Solid bonding puts circulating currents in the screens, which is a real reduction in rating; single-point and cross-bonding remove it but add sheath voltage limiters and an earth continuity conductor to the scope.
Cyclic loadsThe load curve. A cyclic rating to IEC 60853-2 can justify a smaller conductor — but only with a documented load curve; without one it is an assumption dressed as a calculation.

Standards this algorithm is built from

StandardWhat it supplies here
IEC 60502-2:2014MV cables 6 kV to 30 kV — rated voltages, compounds and temperatures, construction, Annex B ratings
IEC 60183:2015Guidance for the selection of high-voltage a.c. cable systems — service conditions, insulation level, conductor size
IEC 60287 seriesCurrent rating by calculation, and the loss factors for each bonding arrangement
IEC 60853-2Cyclic and emergency ratings
IEC 60949:1988+A1:2008Thermally permissible short-circuit currents, including the non-adiabatic effect
IEC 60986:2000+A1:2008Short-circuit temperature limits for cables from 6 kV to 30 kV
IEC 61936-1:2010+A1:2014Power installations above 1 kV a.c. — cable installation requirements, 6.2.9

The clause and formula numbers above were read from the standards themselves. What is not reproduced anywhere on this site is the text of a standard: if you calculate for a living, buy the document from IEC. How each engine here is checked against the standards' own published values is on the validation page.

Calculators that carry out these steps