IEC 60364-5-52 — Cable Sizing and Voltage Drop
IEC 60364-5-52 is the part of the low-voltage installation standard that governs wiring systems: how a cable is selected so its insulation temperature is never exceeded, how the tabulated rating is corrected for the conditions it is actually installed in, and how much voltage drop the installation may have.
What is IEC 60364-5-52?
IEC 60364-5-52 is the standard used to select and install cables in low-voltage installations. Its central requirement, clause 523.1, is that the current a conductor carries for sustained periods must not take the insulation above its temperature limit — and it provides the tables and correction factors that make that requirement checkable.
It is the document behind the phrase “cable sized to IEC” in a low-voltage design. It is not the same calculation as IEC 60287: this standard works from tabulated ratings for reference installation methods, while IEC 60287 computes the rating from the cable's own thermal model. For an unusual installation the table cannot answer, and the thermal model can.
The temperature limit that governs everything
| Type of insulation | Temperature limit | Measured at |
|---|---|---|
| Thermoplastic (PVC) | 70 °C | conductor |
| Thermosetting (XLPE or EPR rubber) | 90 °C | conductor |
| Mineral, PVC covered or bare and exposed to touch | 70 °C | sheath |
| Mineral, bare, not exposed to touch and not in contact with combustible material | 105 °C | sheath |
Clause 523.1 then gives two routes to a compliant current: take it from the tables per 523.2, or determine it by calculation per 523.3. Everything else in the clause is about making the table value fit reality.
Reference installation methods (Annex A)
A tabulated rating belongs to an installation method, not to a cable. Annex A lists the methods and maps a real installation onto the reference arrangement whose table applies — A1 and A2 inside a thermally insulated wall, B1 and B2 in conduit on a wall, C clipped direct, D1 and D2 buried in duct or direct in ground, E, F and G in free air.
Getting this mapping wrong is the most common error in cable sizing, because each method has its own table and the ratings differ substantially: the same cable in free air and buried in a duct are two different current ratings.
Current-carrying capacity tables (Annex B)
Annex B carries the tabulated ratings for each combination of insulation, conductor material, number of loaded conductors and installation method. Annex D gives the formulae the smooth curves behind those tables follow — I = a Sm − b Sn, with coefficients per cable and method — and Annex C shows how the tables may be simplified.
Annex D is about current-carrying capacity, not voltage drop — a distinction worth keeping, because the voltage-drop formula lives in Annex G (see below).
Correction factors
The tabulated value assumes a reference ambient and a single circuit. Real conditions are corrected for:
- Ambient temperature — clause 522.1 with the correction factors of Annex B; air and ground have separate base temperatures and separate tables;
- Grouping — clause 523.5 for more than one circuit, with reduction factors that depend on the arrangement, and separate tables for cables in air, buried directly and in buried ducts;
- Number of loaded conductors — clause 523.6;
- Soil thermal resistivity for buried routes, with a correction against the 2,5 K·m/W base of the buried tables;
- Harmonic currents — Annex E, which is where the reduction factor for triplen harmonics in the neutral comes from;
- Conductors in parallel — clause 523.7 and Annex H.
where It is the tabulated rating for the reference method, Ca the ambient factor, Cg the grouping factor, Cs the soil-resistivity factor, and IB the design current of the circuit.
A route whose conditions change along the way
Clause 523.8 decides this: where installation conditions vary along a route, the current-carrying capacity is that of the most onerous section. The clause makes one allowance — a section passing through a wall shorter than 0,35 m may be disregarded.
In practice this is the rule that catches designs sized on the easy section: a cable that runs in free air, then through a duct bank, then buried alongside others is rated by the worst of those sections, not the average. It is also why a sizing tool has to accept a route in sections rather than a single method.
Voltage drop (clause 525 and Annex G)
The voltage-drop formula and its limits are in Annex G, “Voltage drop in consumers' installations”, referenced by clause 525.
b = 1 for three-phase circuits and 2 for single-phase (a three-phase circuit with a completely unbalanced neutral counts as single-phase); ρ1 is the resistivity of the conductors in normal service, taken as 1,25 times the value at 20 °C — 0,0225 Ω·mm²/m for copper and 0,036 Ω·mm²/m for aluminium; λ is the reactance per unit length, taken as 0,08 mΩ/m in the absence of other data; cos φ is taken as 0,8 (sin φ = 0,6) when no precise value is known; IB is the design current; U0 is the line-to-neutral voltage.
| Type of installation | Lighting | Other uses |
|---|---|---|
| A — supplied directly from a public low-voltage distribution system | 3 % | 5 % |
| B — supplied from a private LV supply | 6 % | 8 % |
Annex G adds two practical allowances: where the main wiring is longer than 100 m the limits may be increased by 0,005 % per metre beyond 100 m, up to a maximum supplement of 0,5 %; and a larger drop is acceptable during motor starting or other inrush, provided the equipment's own standard limits are respected.
Calculation workflow
- Determine the design current IB of the circuit.
- Identify the reference installation method for each section of the route (Annex A).
- Read the tabulated capacity It for the insulation, conductor material and number of loaded conductors (Annex B).
- Apply the correction factors for ambient, grouping, soil resistivity and harmonics.
- Take the most onerous section of the route as governing (523.8).
- Check Iz ≥ IB, and that the protective device coordinates with Iz as IEC 60364-4-43 requires.
- Check the voltage drop against Table G.52.1 with the Annex G formula, at the resistivity in normal service.
- Check the cable against the fault duty as well — the thermal withstand from IEC 60909 currents and the adiabatic check of IEC 60364-5-54 or IEC 60949.
Worked example in the standard
Annex E.52.2 works through a harmonic-loaded circuit: a design current of 39 A on a four-core PVC copper cable clipped to a wall selects 6 mm², whose tabulated capacity is 41 A; applying the harmonic factor raises the required current to 45,35 A and moves the selection to 10 mm². That case is the reference the LV cable sizing calculator is checked against — see the validation page.
IECCalc calculators for IEC 60364-5-52
- #008 LV cable sizing, up to 1000 V Cross-section selection against the Annex B tables for methods A1 to G, with the ambient, grouping and soil-resistivity factors and the engineering drawing of each method.
- #001 Voltage drop, LV and MV to 33 kV The Annex G formula per segment with the resistivity in normal service, checked against the limits of Table G.52.1.
- #004 Cable ampacity by the thermal model Where the tables stop: IEC 60287 for the trench as built, with grouping and soil resistivity.