How to use this calculator
- Pick the insulation (PVC 70 °C or XLPE/EPR 90 °C), the number of loaded conductors (2 for single-phase, 3 for balanced three-phase) and the installation method A1, B1 or C from Table B.52.1.
- Enter the design current Ib. If you already know the protective device rating In, enter it too, so the cable is sized for Iz ≥ In.
- Enter the ambient air temperature. The factor Ca is filled in from Table B.52.14; you can overwrite it with a value from your own table.
- Choose the grouping arrangement and the number of circuits. The factor Cg is filled in from Table B.52.17 and can be overwritten too.
- Fill in the voltage-drop data (system, voltage, one-way length, power factor and limit) and read the recommended size, the governing check and the table used.
How it works: the formula
The tables give the reference capacity It of a cable in a defined installation method at 30 °C ambient air, with no other circuits nearby. A hotter room or neighbouring loaded cables reduce the heat the cable can shed, so the capacity is multiplied by Ca and Cg. Turned round, the cable must have a tabulated It of at least Ib / (Ca × Cg). The smallest size that meets this is the ampacity size. The voltage-drop size is the smallest IEC size whose drop stays within your limit at the conductor's operating temperature. The cable has to satisfy both, so the larger one is chosen.
Variables and units
| Symbol | Meaning | Unit |
|---|---|---|
| Ib | Design current of the circuit | A |
| In | Rated current of the protective device (optional) | A |
| It | Tabulated reference current-carrying capacity, IEC 60364-5-52 Tables B.52.2–B.52.5 | A |
| Ca | Ambient temperature factor, Table B.52.14 | — |
| Cg | Grouping factor, Table B.52.17 | — |
| Iz | Corrected current-carrying capacity | A |
| θmax, θa | Maximum conductor temperature and ambient air temperature | °C |
Worked example
PVC-insulated copper, three loaded conductors, method C, Ib = 40 A, ambient 40 °C, three circuits bunched; three-phase 400 V, 60 m, PF 0.85, limit 3 %:
With the same load at 200 m, 16 mm² gives a 3.94 % drop and fails. 25 mm² gives 2.58 %, so voltage drop governs and the recommendation becomes 25 mm².
Assumptions
- Copper conductors and the reference capacities of IEC 60364-5-52:2009 Annex B for methods A1, B1 and C at 30 °C ambient air.
- Method B1 values are included up to 120 mm² only.
- Ca and Cg apply to the whole route; where conditions change along the route, the worst section governs.
- The voltage-drop check uses one conductor per phase at 70 °C (PVC) or 90 °C (XLPE), with resistance from resistivity.
Limitations
- Aluminium conductors, buried cables, ducts, trays, ladders and other installation methods are not built in. Use the custom mode with values from the standard or the cable maker.
- Harmonic currents in the neutral (IEC 60364-5-52 Annex E) and thermal insulation around the cable beyond method A1 need separate treatment.
- The cable must also be protected against short-circuit (k²S² ≥ I²t, IEC 60364-4-43) and must allow automatic disconnection in time (IEC 60364-4-41).
- Grouping factors are given here for 1 to 9 circuits; larger groups and spaced arrangements are in the full table.
- National tables (for example BS 7671 Appendix 4) contain the same kind of data and may differ slightly; follow the code that applies to your installation.
References
Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.
- IEC 60364-5-52:2009, Selection and erection of electrical equipment — Wiring systems: Table B.52.1 (installation methods), Tables B.52.2 to B.52.5 (current-carrying capacities), Table B.52.14 (ambient temperature) and Table B.52.17 (grouping).
- IEC 60364-4-43, Protection for safety — Protection against overcurrent (Ib ≤ In ≤ Iz, short-circuit withstand).
- IEC 60364-4-41, Protection for safety — Protection against electric shock (automatic disconnection of supply).
- BS 7671, Requirements for Electrical Installations, Appendix 4 — current-carrying capacity and voltage drop tables for UK practice.
Frequently asked questions
Why do I have to choose an installation method?
A cable can only carry the current at which it reaches its maximum conductor temperature. How fast it sheds heat depends on where it is: in a conduit inside an insulated wall (A1) it runs much hotter than clipped to a masonry wall (C). The same 16 mm² PVC cable is rated 61 A in method A1 and 85 A in method C for two loaded conductors.
When do I count 2 and when 3 loaded conductors?
A single-phase circuit has 2 loaded conductors (line and neutral). A balanced three-phase circuit has 3; the neutral carries little current and is not counted. If the neutral carries significant harmonic current, IEC 60364-5-52 Annex E applies a further reduction.
Can I enter my own Ca and Cg?
Yes. The fields are filled in from Tables B.52.14 and B.52.17 but stay editable, so you can use values from your national code, the cable maker or a combined factor for ground temperature or soil resistivity.
Why is aluminium not in the built-in table?
The aluminium columns of the standard could not be checked to the same level for this page, so they are left out rather than risk wrong values. Use the custom mode and type in the tabulated capacity for your size from the standard or the maker's data.
Should the cable be sized for Ib or for the breaker rating?
IEC 60364-4-43 requires Ib ≤ In ≤ Iz, so the corrected capacity Iz must be at least the device rating In, not just the load current. Entering In makes the calculator size the cable for In.