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Glossary · Energy industry & energy transition

Available Transfer Capacity, ATC

Available transfer capacity ATC transmission capacity electricity market Interconnector congestion management

Available Transfer Capacity, ATC for short, is the transmission capacity between two bidding zones or grid areas that can still be allocated for trading in a given period. The value always applies to a specific direction and a specific market time unit. ATC is stated in megawatts (MW). It is therefore a power value and not an energy quantity in megawatt hours. An example: if the ATC from area A to area B is 1,000 MW for one hour, additional cross-border schedules up to this power can be allocated for that hour. This corresponds to at most 1,000 MWh of energy in that hour. A completely different value may apply in the opposite direction.

ATC is not a measured value that can be read off a line. The value is the result of a grid calculation. The transmission system operators examine which cross-border exchange the grid can handle safely under the expected conditions. They take into account thermal limits of lines and transformers, voltage and stability limits, and the N-1 criterion. Under this criterion, the grid must continue to operate safely even if a single important element of equipment fails.

Calculation chain: from TTC via NTC to ATC

The classic definitions date from the years 1999 to 2001 and come from ETSO, a predecessor of today's association ENTSO-E. The calculation takes place in three steps:

  • Total Transfer Capacity (TTC): the largest exchange between two areas that is still compatible with the grid's security rules. It is based on certain assumptions about generation, load and grid topology.

  • Transmission Reliability Margin (TRM): a safety margin. It absorbs uncertainties, such as forecast errors, unplanned load flows or inaccuracies in the grid model. It follows that NTC = TTC − TRM.

  • Already Allocated Capacity (AAC): capacity that has already been allocated, for example in annual or monthly auctions. It follows that ATC = NTC − AAC. The ATC is thus the part of the Net Transfer Capacity that remains available for further trading after each allocation round.

The ATC therefore changes from one allocation period to the next, that is, from the annual through the monthly and day-ahead to the intraday timeframe. All quantities in this chain apply to one direction. In the outward and return directions they generally differ.

Distinction from NTC and interconnector capacity

ATC is neither the same as NTC nor the same as the technical capacity of an interconnector. The NTC is the trading capacity after deduction of the safety margin. The ATC is only the part of it that has not yet been allocated. The interconnector capacity, by contrast, describes the technical design of an individual connection. For AC tie lines, the tradable capacity is often significantly below this design value. Reasons include bottlenecks in the adjacent domestic grid, maintenance, unfavourable load flow distributions or safety reserves. For controllable high-voltage direct current connections, the gap is usually smaller. In the Nordic calculation principles, for example, the thermal capacity is normally used as the trading value for such connections, and the safety margin is zero.

ATC also does not describe a fixed physical path of the current. In a meshed AC grid, the current follows the impedances, that is, the electrical resistances of the lines, and not the trade contracts. A transaction between two neighbouring areas therefore also loads lines in third countries. If borders are assessed individually and bilaterally, this interaction is captured only roughly. If the values of two neighbouring grid operators differ, in practice the lower value generally applies.

ATC and flow-based methods

In Flow-Based Market Coupling, each border is not assigned a fixed value on its own. Instead, the method models how the net positions of all bidding zones jointly affect critical network elements. In Central Europe, the day-ahead market has been coupled in this way since 2015, initially in the CWE region. Since 2022 the method has applied to the entire Core region, and since 29 October 2024 also to Denmark, Finland, Norway and Sweden.

However, the ATC principle has not disappeared as a result. In the European intraday market SIDC, capacity in the Core region has indeed been calculated on a flow-based basis since 2024. As long as SIDC cannot process flow-based parameters directly, however, these results are converted into ATC values per border and direction. This step is called ATC extraction. The calculation rounds have been extended step by step. According to ENTSO-E, the fifth round, IDCC(e), was added in autumn 2026. For market participants, ATC therefore remains the visible trading quantity in many timeframes and at many borders.

Legal framework in the EU

The decisive text is the Electricity Market Regulation (EU) 2019/943. The rules cited here correspond to the consolidated version of 16.07.2024. The Regulation describes coordinated net transmission capacity as a method in which the maximum energy exchange between adjacent bidding zones is assessed and determined in advance. Article 16 prescribes a minimum capacity for cross-zonal trade. At borders using the NTC approach, this is 70 percent of the transmission capacity that respects the operational security limits, taking contingencies into account. For flow-based methods, the 70 percent threshold applies to the capacity of the critical network elements. In addition, exceptions and transitional paths are provided for, such as linear trajectories within the framework of national action plans.

Article 19 governs the use of congestion revenues. They arise when scarce capacity leads to price differences between bidding zones. Grid operators must determine in advance how they use these revenues and report to the regulatory authorities on the actual use. The revenues primarily serve to guarantee the actual availability of allocated capacity and to maintain or increase cross-zonal capacity. Only under certain conditions may they be included in the calculation of network tariffs.

Significance for market and grid

If the ATC is sufficient, the prices of neighbouring bidding zones converge. If it is exhausted, a price difference remains, and this indicates the congestion. In coupled markets, capacity is allocated implicitly: the coupling algorithm uses it automatically together with the energy bids. In explicit auctions, for example for long-term transmission rights, traders by contrast bid for the capacity separately from energy trading. In both cases, the grid operator must determine beforehand how much capacity it makes available to the market.

Higher ATC values make it easier to distribute regional wind and solar surpluses over larger areas. However, if a grid operator releases a lot of capacity even though there are bottlenecks within its bidding zone, the costs often shift to redispatch. If it releases little, the price differences between zones remain larger. It is precisely between these two effects that the 70 percent rule sets a binding minimum standard.

Typical misunderstandings

  • A low ATC does not automatically mean that capacity is being withheld arbitrarily. Real grid restrictions are often behind it. However, EU law prescribes minimum values and obligations to provide justification precisely for such cases.

  • A high installed interconnection capacity does not mean that a correspondingly large amount of electricity can be imported in every hour. The ATC depends, among other things, on simultaneous flows, power plant dispatch, wind and solar feed-in, and maintenance. Annual averages conceal precisely the critical hours.

  • Trading capacity is not the same as security of supply. If several neighbouring countries are short of generation at the same time, a nominally free interconnection capacity helps little.

  • The term is not used identically everywhere. In North America, ATC stands for Available Transfer Capability and is calculated differently, including with additional margins. Figures from different systems therefore cannot be compared directly.

In summary, ATC shows where trading meets the physical limits of the grid. The value says nothing about the quantity, origin or climate footprint of the electricity generated. It only describes how much additional cross-border trading is still possible under certain grid assumptions, in one direction and in one period.

Sources

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Edited by eCMS on 11.11.2026 14:55

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