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Glossary · Energy industry & energy transition

Cable Ampacity

Definition and scope

A network operator cannot commit to a connection merely because a line is drawn on the plan and the voltage level fits. Whether additional charging points, heat pumps, photovoltaic systems, battery storage or a data centre can be connected also depends on how much current an existing cable may carry under its actual operating conditions. This permissible current-carrying capacity is called cable ampacity in English, a portmanteau of ampere and capacity. It means the current at which the heat generated in the cable can be dissipated such that the permissible conductor temperature is not exceeded anywhere along the cable route. The term is closely related to the thermal loading capacity of equipment and to the more general thermal rating.

Ampacity is not a fixed type property like the conductor cross-section. It is a limit value that applies to a specific cable in a specific environment under a specific load profile. The same cable can therefore have different current-carrying capacities at two points along the same route. The decisive section is the one with the most unfavourable conditions, that is, the thermal bottleneck.

Physical core: a heat balance

A conductor carrying current generates heat losses that grow roughly with the square of the current. With alternating current, dielectric losses in the insulation as well as losses in screens, sheaths and armouring are added. This heat must overcome several thermal resistances in succession: insulation, protective covering, bedding material, soil, duct or air. The international IEC 60287 series therefore treats current rating as a heat balance. From the permissible temperature rise, the conductor resistance, the losses and the thermal resistances, the current is calculated at which the maximum permissible conductor temperature is just reached in continuous operation. For XLPE-insulated cables, this limit is usually 90 °C. Other insulation materials have other values.

If the heat is not dissipated sufficiently, the conductor temperature rises above the permissible value. Insulation then ages faster, joints and terminations are subjected to greater stress, and the technical service life is shortened. Such overloading often does not lead to immediate failure. However, it can increase the probability of later faults.

Influencing factors

  • Cable construction: conductor material (copper or aluminium), cross-section, insulation material, permissible conductor temperature and screen design.

  • Installation method: directly buried, in a duct, in a cable trench or free in air. As the installation depth increases, the current-carrying capacity in soil generally decreases.

  • Specific thermal resistivity of the soil (in K·m/W): moist soil conducts heat better than dry soil. Near the cable, the soil can dry out and form a dry zone with poorer conductivity. The calculation methods therefore distinguish between moist and dry soil.

  • Ambient and ground temperature as well as external heat sources such as district heating pipes or neighbouring cables.

  • Grouping: several loaded systems laid side by side heat each other. This is taken into account through reduction factors.

  • Load profile: continuous load, cyclic load with a daily pattern, temporary peak load or short-term emergency loading.

Standards and reference conditions

IEC 60287 contains the equations for current rating and losses in Part 1-1 and the thermal resistances in Part 2-1. Part 3-1 gives reference values for ambient temperatures and soil thermal resistivities in various countries. The calculation applies to a steady-state condition with constant continuous current, that is, with a load factor of 100 percent. For material properties, the standard provides uniform table values so that results remain comparable. Deviating values are permissible if they are substantiated and stated openly. The nominal soil thermal resistivities given in Part 3-1 range, depending on the country, from approximately 0.5 to 3.0 K·m/W. For calculations with a dry zone, some national specifications use 2.5 K·m/W.

In Germany, table values for power cables are based on the DIN VDE 0276 series of standards. DIN VDE 0276-1000 is authoritative for the conversion factors. The tables for underground installation are based on typical reference conditions: load factor 0.7 (the so-called utility load, EVU-Last), specific soil thermal resistivity 1.0 K·m/W, ground temperature 20 °C and installation depth 0.7 m. The load factor describes the ratio of average load to maximum load in a daily load cycle. It assumes that the cable can cool down during low-load periods. For loads without these recovery phases, the values must therefore be converted to a higher load factor. Technical literature gives as an example the conversion to 1.0 for photovoltaic systems that feed in evenly over long periods during the midday hours. For other conditions or more precise soil data, reference is made to the calculation according to IEC 60287.

For cyclic loads and emergency operation, there is the IEC 60853 series of standards. Part 2 contains, among other things, a method for calculating the emergency current rating for cables of all voltages. These methods use the thermal capacity of the cable and its surroundings. A temporarily higher load can be thermally permissible as long as it remains limited in time. To be considered separately is the short-circuit current rating. It concerns fault currents in the range of seconds with their own temperature limits. For cables and wires in building installations, separate sets of tables also apply, such as DIN VDE 0298-4.

Why a network plan is not enough

The network capacity of a cable route therefore cannot be read from a circuit diagram alone. A plan shows connections, cross-sections and equipment. It does not reliably show how dry a section of soil becomes in summer, whether further cables were later laid alongside, which joint is thermally pre-damaged, or whether the documented installation method corresponds to the actual execution. Old routes are often documented incompletely. Soil properties were frequently only estimated at the time of installation, and infill development in the street space or new heat sources change the surroundings. A standard value in the model can then feign a precision that does not exist in operation. Reliable statements emerge only when planning, documentation, measurement, operating experience and maintenance are brought together.

Decision situation for connection requests

When current-carrying capacity becomes tight, the answer is rarely a simple yes or no. Possible options include grid reinforcement, load management, limits in time or in power, a connection with conditions, a metering concept, time-limited operation or a rejection. Each variant distributes costs, risks and responsibility differently. An assessment that is too generous shifts the risk into the future: the connection succeeds at first, but cable damage, unplanned outages or costly renewal under time pressure follow later. An assessment that is too cautious delays connections, leaves existing capacity unused and drives away investment. Often it is then not a standard that is missing. What is missing is the ability to combine data, local knowledge, permissible assumptions and operational responsibility into a sound decision.

Behind every figure in a connection assessment there are prior decisions. These include the standard applied, the safety margins, the soil data and the assumed load factor. It must also be clarified who may deviate from standard assumptions, how unknown cable sections are handled and which emergency operation limits are documented.

Data situation, cybersecurity and incident response

Grid control, load control and connection assessment rely on digital data. These include the network model, geographic information systems, control systems, measured values from secondary substations, condition data and forecasts. If these data are incomplete, delayed or no longer trustworthy after an incident, operations cannot reliably recognise whether a cable is already critically loaded or whether a planned switching state creates new bottlenecks. Cybersecurity thus also becomes a question of grid control. In incident response, ampacity is a physical boundary condition. IT can isolate systems, block access and secure logs. However, which network areas continue to be operated under changed switching states, which consumers have priority and which emergency operation limits are justifiable must be decided by a joint situation command made up of grid operations, information security and executive management.

The significance of this limit also shows during restoration. When loads return and substitute switching arrangements are used, individual cables can be loaded significantly more heavily than in the normal state. A digital system can be available again while the infrastructure beneath it has only limited loading capacity. The methods for time-limited emergency loadings help only if the limit values have been determined and documented in advance.

Organisation, outsourcing and financing

Engineering firms, civil engineering contractors and software providers can deliver calculations, installation work and models. However, responsibility for the operating decision remains with the organisation that operates the network or releases the connection. If internally nobody can any longer explain the assumptions of a calculation, the organisation holds documents that it can no longer use sufficiently from a technical standpoint. Financing also affects ampacity. Free capacity in cables, routes and substations ties up capital and easily appears as oversizing in normal operation. In the event of load growth, a fault or a switchover, however, this reserve is the margin that keeps operations able to act. Regulatory evidence of the rules applied is necessary. But it does not answer whether a particular cable may be additionally loaded for a few hours on a hot day after a switchover.

Limits of informative value

Table values apply only to their reference conditions. Calculation results are only as good as the input data, especially for soil thermal resistivity and for the actual installation. Measurements of cable or soil temperature and methods of dynamic rating assessment can reduce uncertainties, but do not replace knowledge of the thermal bottleneck along the route. The status of the applicable editions of the standards should be checked before a specific design.

Sources

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Edited by eCMS on 11.11.2026 16:37

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