The maximum efficiency of usage is being reached at maximum cable load which is characterised by the temperature in the cable close to a maximum allowed temperature. When exceeding this temperature, the ageing of cable insulation accelerates as cable service life shortens. The existing methods of determining allowable loads are based on information received from design firms and manufacturers. In the article, there is proposed the method to calculate allowable loads for high voltage single-conductor cables which is based mainly on the measurement data after performing cable installation. There are also the proposals in the article to be used for improvement of technical documentation.