The coupling currents loss for fusion conductors is frequently assessed applying a sinusoidal field sweep of fixed, small amplitude and variable frequency. From the initial slope of the loss curve, the coupling loss time constant is derived and applied in the loss calculation over the whole range of field transient. In this work, the traditional AC loss assessment is compared to an alternative experimental assessment made on a trapezoidal field change, which mimics the actual field change rate in the central solenoid of tokamaks during the plasma start-up. The comparative loss assessment is made using the test results of the Nb3Sn react&wind conductor developed at SPC and an ITER TF conductor. The proposed AC loss assessment allows an accurate estimate of the deposited energy during plasma start-up for the Central Solenoid.