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In this paper losses in superconducting composite tapes carrying ac transport current in external ac magnetic field are studied by using a numerical model. The external field and the transport current are in phase, as usual in superconducting coils. The computational model is based on Maxwell's equations and a non-linear constitutive law between the current density and the electric field. By using the model ac loss behaviour between high temperature (HTS) and low temperature superconductors (LTS) is compared. Finally, the temperature dependence of losses in HTS tapes are studied at temperatures 20-40 K at constant amplitudes of the transport current and the external magnetic field. According to the results the ac losses decrease with temperature if the peak value of the current ramp is lower than the current which fully penetrates the conductor cross-section, i*. It is proposed, that frequency dependent i* is a more appropriate design parameter for HTS than a critical current defined by an electric field criterion.