Why standard current-carrying capacity tables are not always sufficient
BS 7671 Appendix 4 provides current-carrying capacity tables that cover the vast majority of conventional electrical installation work. Those tables are built on a defined set of reference installation conditions: a soil thermal resistivity of 2.5 K·m/W, a burial depth of 0.7 m, an ambient ground temperature of 15°C, and cables installed individually without mutual heating from adjacent circuits. Where these conditions hold, the tables give reliable results.
High-power DC EV charging infrastructure frequently violates every one of those assumptions. Multiple charger feeder cables share a common trench or duct bank. Burial depths vary as the route crosses hard standings, roadways and grassed areas with different thermal characteristics. The soil itself may be urban made ground, sand, clay, or chalk — each with thermal resistivity values that can differ by a factor of three or more from the BS 7671 reference. And crucially, the cables in a busy charging hub may run at or near their continuous rated current for many hours each day.
The 70°C temperature limit and why it matters
BS 7671 specifies a maximum operating temperature of 70°C for PVC-insulated cables and 90°C for XLPE (cross-linked polyethylene) cables. These are not comfortable engineering margins — they are the temperatures at which insulation degradation begins to accelerate. A cable consistently operating above its design temperature will have a shortened service life. One operating significantly above it presents a risk of insulation breakdown, which in a charging hub context means a risk of fire in a space likely to contain vehicles with high-energy battery packs.
The risk is compounded by the installation geometry. A cable buried in a duct bank at the centre of a group dissipates heat more slowly than one at the periphery, because the surrounding cables raise the local temperature of the soil. This mutual heating effect means that the hottest cable in the group may be running significantly above the temperature predicted by a simple individual-cable calculation, and well above the BS 7671 derating factors if those factors do not accurately represent the actual installation arrangement.
What IEC 60287 thermal modelling addresses
IEC 60287 — Electric cables: calculation of the current rating — provides a first-principles method for determining the maximum permissible current in a cable for any specified installation condition. Rather than applying a lookup table with correction factors, it calculates the thermal resistance of every layer of the cable construction (conductor, insulation, bedding, sheath, armouring) and the thermal resistance of the surrounding ground to a reference ambient, accounting for the actual burial geometry and soil characteristics.
For grouped installations, the standard provides methods to account for mutual heating between cables and from cables in adjacent ducts. The result is the maximum continuous current that keeps the conductor at or below its rated temperature for the specified conditions — not a general-purpose value, but the value for that specific installation.
IEC 60287-2-1 extends the method to cyclic and emergency ratings, which is directly relevant to EV charging: chargers do not necessarily run at nameplate power continuously. A realistic load profile — accounting for dwell times, session lengths, and demand peaks — can legitimately increase the cyclic current rating above the continuous value, allowing more economical cable selection without compromising thermal safety.
Why few consultants offer this analysis
Performing a rigorous IEC 60287 analysis requires specific knowledge of the standard, familiarity with the calculation methodology, and either dedicated software or a structured calculation workbook. For typical domestic and light commercial electrical work, it is not necessary — the BS 7671 tables are appropriate. But as the power density of EV charging installations increases, and as cable routes become more complex, the gap between a table-based approach and a properly modelled one widens.
EV Design applies IEC 60287 analysis as standard on all DC and high-power AC cable systems — not as a premium add-on, but as the normal engineering basis for cable sizing in installations where the standard table assumptions do not hold. It is one of the clearest technical differentiators between a specialist EV charging designer and a general electrical consultancy that has moved into the sector.