Cable selection algorithm · U ≤ 1 kV a.c.

LV cable selection algorithm

Every check a low-voltage cable has to pass, in the order a design review takes them: installation method, ampacity after correction, coordination with the protective device, voltage drop, short-circuit withstand, earth-fault loop impedance and the protective conductor. Fourteen steps, each cited to the clause it comes from.

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

A low-voltage cable is selected by taking a trial cross-section and then proving it against five independent criteria: current-carrying capacity after every correction factor (IEC 60364-5-52, 523.1), coordination with the protective device (IEC 60364-4-43, 433.1), voltage drop (IEC 60364-5-52, 525 and Annex G), short-circuit thermal withstand k²S² ≥ I²t (IEC 60364-4-43, 434.5.2) and earth-fault loop impedance against the required disconnection time (IEC 60364-4-41, 411.3.2).

The size that satisfies all five is the answer; the criterion that forced the last increase is the one worth recording, because it tells the next reviewer what the circuit is actually limited by. On short circuits it is nearly always ampacity; past about 60 m it is nearly always voltage drop.

The algorithm

Flowchart of the LV cable selection and verification algorithm to IEC 60364, from design current to the governing criterion LV CIRCUIT, U ≤ 1 kV a.c.Collect the design dataIEC 60364-5-52, 522; IEC 60364-1, Clause 13- design current I_B, and the harmonic spectrum if the load is non-linear- nominal voltage, frequency, system earthing (TN, TT, IT)- route length, and the length to each intermediate board- prospective short-circuit current at the origin and the let-through energyof the device- required disconnection time, and the earth-fault loop impedance available- external influences along the route: AA ambient, AD water, AE bodies, AFcorrosion, AG impact, AN solar1Choose the cable type and insulationIEC 60364-5-52, 521 and 522, Table 52.1; IEC 60332 series; IEC 60364-5-52, 527- maximum operating temperature: 70 °C thermoplastic (PVC), 90 °Cthermosetting (XLPE, EPR) — Table 52.1- conductor material and class (IEC 60228), armour, screen, sheath- reaction to fire and, for safety circuits, circuit integrity; sealing ofpenetrations per 5272Take a trial cross-section SIEC 60364-5-52, Table 52.2; IEC 60228- not below the minimum of Table 52.2 (1,5 mm² Cu for power circuits, 16 mm²Al)- from the standard sizes of IEC 602283Identify the installation methodIEC 60364-5-52, Annex A, Tables A.52.1 and A.52.3- match the real arrangement to a reference method A1, A2, B1, B2, C, D1,D2, E, F or G- record it for every distinct section of the route4Read the base current-carrying capacityIEC 60364-5-52, Annex B, Tables B.52.2 to B.52.13; Annex D- the tables are for 30 °C in air and 20 °C in ground, soil resistivity 2,5K·m/W- Annex D gives the same capacities as formulae I = a·S^m − b·S^n forinterpolation5Apply every correction factorIEC 60364-5-52, 523.5 to 523.7, Tables B.52.14 to B.52.21; Annex E- ambient air B.52.14 (base 30 °C), ambient ground B.52.15 (base 20 °C)- soil thermal resistivity B.52.16 (base 2,5 K·m/W)- grouping B.52.17 to B.52.21, by arrangement, not by cable count alone- number of loaded conductors 523.6; conductors in parallel 523.7- harmonics: Annex E is normative — a third-harmonic-rich neutral deratesthe circuit- I_z is I_t multiplied by the product of the factors6Is the route one single installation condition?IEC 60364-5-52, 523.87yesIs I_z at least I_B ?IEC 60364-5-52, 523.18yesI_B ≤ I_n ≤ I_z and I₂ ≤ 1,45·I_z ?IEC 60364-4-43, 433.1, conditions (1) and (2)9yesIs the voltage drop within the limit?IEC 60364-5-52, 525 and Annex G, Table G.52.110yesIs k²S² at least the I²t let through?IEC 60364-4-43, 434.5.2 and Table 43A11yesDoes an earth fault at the far end clear in time?IEC 60364-4-41, 411.3.2 and Table 41.1; 411.4 to 411.6 by earthing system12yesSize the protective conductorIEC 60364-5-54, 543.1.1 to 543.1.3, Table 54.2- either the adiabatic calculation of 543.1.2, or the S / 16 / S÷2 rule ofTable 54.2- 543.1.3 sets the minimum for a conductor not in the same cable as the lineconductors13Check the mechanical and erection constraintsIEC 60364-5-52, 522.6 to 522.8, 526, 528; cable maker's data- bending radius and pulling tension from the manufacturer's data- support and fixing, 522.8; connections, 526- proximity to other services, 52814RECORD THE GOVERNING CRITERIONRate the whole circuit on its worst section523.8 — the section with the lowest capacity governs thewhole route; a length of 0,35 m or less through a wallis disregardednoIncrease S, change the method, or split intocables in parallel523.7 for conductors in parallel; re-check the groupingfactor after splittingnoChange the device rating, or increase S433.3 lists the cases where overload protection may beomitted; 433.4 covers conductors in parallelnoIncrease S — voltage drop is what usuallygoverns a long circuitu = b(ρ₁·L/S·cos φ + λ·L·sin φ)·I_B with ρ₁ = 1,25·ρ₂₀;3 % lighting and 5 % other from the origin of a publicsupply, 6 % and 8 % for a private supply, plus 0,005 %/mbeyond 100 m capped at 0,5 %noIncrease S, or use a device that limits thelet-through energyk = 115 for Cu/PVC up to 300 mm² and 143 for Cu/XLPE, 76and 94 for the aluminium equivalents (Table 43A); below0,1 s use the manufacturer's I²tnoIncrease S or the protective conductor,shorten the run, or add an RCDthe loop impedance has to let enough current flow tooperate the device within the tabulated time; the samecheck decides whether an RCD is requiredno
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 design dataNothing downstream is decidable without I_B, the earthing system, the fault level and the external influences.IEC 60364-5-52, 522; IEC 60364-1, Clause 13
2Choose the cable type and insulationInsulation class fixes the permitted operating temperature and therefore every ampacity table used later.IEC 60364-5-52, 521 and 522, Table 52.1; IEC 60332 series; IEC 60364-5-52, 527
3Take a trial cross-section SS at or above the minimum of Table 52.2, chosen from the standard IEC 60228 sizes.IEC 60364-5-52, Table 52.2; IEC 60228
4Identify the installation methodReference method A1 to G, recorded per section of the route.IEC 60364-5-52, Annex A, Tables A.52.1 and A.52.3
5Read the base current-carrying capacityI_t from the Annex B table for that method, insulation and number of loaded conductors.IEC 60364-5-52, Annex B, Tables B.52.2 to B.52.13; Annex D
6Apply every correction factorI_z = I_t times k_ambient, k_soil, k_grouping and k_harmonic, each read for the actual arrangement.IEC 60364-5-52, 523.5 to 523.7, Tables B.52.14 to B.52.21; Annex E
7Is the route one single installation condition523.8 — the section with the lowest capacity governs the whole route; a length of 0,35 m or less through a wall is disregardedIEC 60364-5-52, 523.8
8Is I_z at least I_B523.7 for conductors in parallel; re-check the grouping factor after splittingIEC 60364-5-52, 523.1
9I_B ≤ I_n ≤ I_z and I₂ ≤ 1,45·I_z433.3 lists the cases where overload protection may be omitted; 433.4 covers conductors in parallelIEC 60364-4-43, 433.1, conditions (1) and (2)
10Is the voltage drop within the limitu = b(ρ₁·L/S·cos φ + λ·L·sin φ)·I_B with ρ₁ = 1,25·ρ₂₀; 3 % lighting and 5 % other from the origin of a public supply, 6 % and 8 % for a private supply, plus 0,005 %/m beyond 100 m capped at 0,5 %IEC 60364-5-52, 525 and Annex G, Table G.52.1
11Is k²S² at least the I²t let throughk = 115 for Cu/PVC up to 300 mm² and 143 for Cu/XLPE, 76 and 94 for the aluminium equivalents (Table 43A); below 0,1 s use the manufacturer's I²tIEC 60364-4-43, 434.5.2 and Table 43A
12Does an earth fault at the far end clear in timethe loop impedance has to let enough current flow to operate the device within the tabulated time; the same check decides whether an RCD is requiredIEC 60364-4-41, 411.3.2 and Table 41.1; 411.4 to 411.6 by earthing system
13Size the protective conductorS_PE from Table 54.2 or from the adiabatic equation of 543.1.2, with 543.1.3 as the floor.IEC 60364-5-54, 543.1.1 to 543.1.3, Table 54.2
14Check the mechanical and erection constraintsBending radius, pulling tension, support spacing and segregation all satisfied as installed.IEC 60364-5-52, 522.6 to 522.8, 526, 528; cable maker's data

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
Short runs, high currentAmpacity after grouping and ambient correction. Correction factors multiply: four circuits bunched on a tray at 45 °C can take a 90 °C cable down to about 60 % of its tabulated capacity, and 523.8 makes the worst section govern the whole route.
Long runs, modest currentVoltage drop. Annex G's 3 % / 5 % (public supply) or 6 % / 8 % (private supply) limits are reached long before the thermal limit, and the resistivity to use is ρ₁ = 1,25 ρ₂₀ — the conductor at operating temperature, not at 20 °C.
Final circuits in TT and long TN circuitsEarth-fault loop impedance. A cable that is thermally and electrically fine can still fail to bring an earth fault to the disconnection time of Table 41.1, and the fix is a bigger protective conductor, a shorter run or an RCD.
Circuits behind a large fault levelShort-circuit withstand. With k = 143 for copper XLPE and k = 115 for copper PVC, the same fault energy permits very different sizes, and below 0,1 s the manufacturer's let-through energy has to be used instead of a curve reading.
Non-linear loadsHarmonics. Annex E is normative: with a third-harmonic-rich neutral the circuit is derated on the neutral current, and a 4-core cable sized on the phase current alone is undersized.

Standards this algorithm is built from

StandardWhat it supplies here
IEC 60364-5-52:2009Wiring systems — installation methods, ampacity tables and correction factors, voltage drop
IEC 60364-4-43:2008Protection against overcurrent — coordination 433.1, short-circuit 434.5, k values Table 43A
IEC 60364-4-41:2017Protection against electric shock — disconnection times, loop impedance
IEC 60364-5-54:2011Earthing arrangements and protective conductors — 543.1, Table 54.2
IEC 60228:2004Conductors of insulated cables — standard sizes and classes
IEC 60287 seriesCurrent rating by calculation, where the tabulated cases do not fit

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.

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