The measurement of the actual thermal strain, epsilon th, in a cable in conduit conductor (CICC) of Nb3Sn is of paramount importance to predict the CICC performance in operation starting from the strand scaling laws for the critical current density J(c)(B,T,epsilon). The behavior of the critical temperature as a function of the axial strain at constant field, T-c(B, epsilon), is known from the literature and is accurately described by interpolative scaling laws. In the scope of this work, the critical temperature as a function of the applied field, T-c(B), is measured for the CICC in-situ and for the free standing filaments used for the cable manufacture. These results are compared with the T-c(B, epsilon) curve to draw the thermal strain in the CICC. A small AC field is generated at the sample and its inductive response is recorded. At the superconducting transition the change from a diamagnetic to a paramagnetic response is observed. The experimental assessment of the epsilon th in a CICC through T-c measurements is the first step toward the quantitative evaluation of the irreversible degradation in Nb3Sn CICC, which is estimated from the discrepancy of the predicted vs. actual CICC performance. This paper describes a novel method for the in-situ measurement of the T-c in a large CICC, including the monitoring of T-c during the electromagnetic cyclic loading test campaign in SULTAN of the ITER conductors.