Many countries, authorities and standards organisations publish cable sizing standards. These standards provide engineers with a consistent method for selecting cable sizes and a recognised basis for validating a design.
myCableEngineering supports several sustained-current cable sizing standards. The notes below summarise the standards originally covered in the knowledge base and their relationship to current-carrying capacity, voltage drop, and derating factors.
Standards supported by myCableEngineering
| Standard | Scope in the original knowledge base |
|---|---|
| BS 7671:2018 | Requirements for Electrical Installations, Appendix 4 current-carrying capacity for a.c. and d.c. low-voltage cables. |
| ERA 69-30 Part III:2009 | Sustained current ratings for 600/1000 V and 1900/3300 V cables with 70 °C thermoplastic insulation, a.c. 50 Hz and d.c. |
| ERA 69-30 Part IV:2009 | Sustained current ratings for 600/1000 V and 1900/3300 V cables with 90 °C thermosetting insulation, a.c. 50 Hz and d.c. |
| IEC 60502-2:2005 | Power cables for rated voltages from 6 kV (Um = 7.2 kV) up to 30 kV (Um = 36 kV), covering medium-voltage cables. |
| BS EN 50618:2014 | Electric cables for photovoltaic systems, single-core d.c. cables. Listed in the original source as coming soon. |
BS 7671
BS 7671 is the UK requirements for electrical installations standard, commonly known as the Wiring Regulations. Appendix 4 gives guidance for determining current-carrying capacity and voltage drop for cables.
Current capacity
BS 7671 current-carrying capacity calculations are based on the derating factor method. The derating tables are derived from IEC 60287 and aligned with IEC 60364, although there are some fundamental differences.
The parameters considered include ambient temperature, soil thermal resistivity, installation method and cable grouping.
Voltage drop
Voltage drop calculations in BS 7671 use tables of resistive and reactive voltage drop. The tables give the voltage drop in mV/A/m for different installation conditions. Values are related to line voltage in three-phase circuits and to phase voltage in single-phase circuits.
As noted in the standard, use of the tabulated voltage drops may lead to pessimistically high calculated values. myCableEngineering does not use the BS 7671 voltage drop tables directly; instead, it calculates circuit impedances in accordance with IEC 60909 and then calculates voltage drop from those impedances.
ERA 69-30
ERA 69-30 is a nine-volume set of sustained current rating recommendations published by ERA. Cable rating calculations follow the derating factor method.
ERA is a UK-based organisation, and the recommendations are particularly popular with distribution network operators and supply authorities. Each volume covers a particular cable type. For example, one part covers 600/1000 V and 1900/3300 V cables with thermosetting insulation.
The recommendations typically cover three-phase a.c. 50 Hz and d.c. cable ratings for installations in the air and buried in the ground. In addition to current capacity, the recommendations provide estimates of power dissipation and guidance on average soil temperatures within the UK.
IEC 60502
IEC 60502 is an International Electrotechnical Commission standard for power cables. The original knowledge base note refers to current ratings for medium-voltage cables and the use of the derating factor method.
The standard is divided into parts. Part 1 covers voltages up to 1 kV, with current capacity generally addressed by IEC 60364. Part 2 covers voltages from 1 kV to 30 kV, and Annex B includes a method for determining cable current capacities within that voltage range.
The current rating tables include three-phase cables installed both in the air and buried in the ground. Derating factors include ambient temperature, depth of laying, soil thermal resistivity for buried cables and cable grouping.
For life-cycle cost- and energy-loss-based conductor selection, see Economic Optimisation of Cable Size.
For a comparison of IEC cable construction standards by voltage range, see IEC 60502 vs IEC 60840.
For a practical, low-voltage workflow that uses these design checks, see “8 Steps to Low Voltage Power Cable Selection and Sizing.”