Method for measuring the power line thermal rating or maximum allowable current rating of an overhead power line with respect to a suspended/anchored cable span, comprising at least the following steps of: monitoring with an accelerometer a motion of at least one point P of said suspended/anchored cable span in at least two axes perpendicular to the cable over a time interval, the accelerometer signal being directly related to a sag value for the suspended/anchored cable span; monitoring actual line current I, in A, over said time interval; measuring or determining the effective wind speed of said suspended/anchored cable span over said time interval, said effective wind speed being a middle value of a wind class range; determining the relationship between said sag values and a measured cable temperature and adjusting sag values for temperature in order to get temperature-independent actual sag; determining said actual sag of said suspended/anchored cable, as a variable of actual line current; determining a sag reserve DF, in m, for thermal rating, which is the distance between the actual sag and a maximum allowable sag; determining the rate of change, tan ( α), in m/A 2, of said actual sag versus the square of the line current for said effective wind speed, said rate of change being the slope or angular coefficient of a linear fit to the points (actual sag, current-squared); and determining the power line thermal rating of the overhead power line, or ampacity, linked to a corresponding safety clearance, at measured or determined effective wind speed, by adding the square of actual current I to the ratio of the sag reserve DF by the sag rate of change, tan ( α), at the effective wind speed, and taking the square root of that addition, i.e. Ampacity = I 2 + DF tan α, wherein ampacity is in amperes. monitoring with an accelerometer a motion of at least one point P of said suspended/anchored cable span in at least two axes perpendicular to the cable over a time interval, the accelerometer signal being directly related to a sag value for the suspended/anchored cable span; monitoring actual line current I, in A, over said time interval; measuring or determining the effective wind speed of said suspended/anchored cable span over said time interval, said effective wind speed being a middle value of a wind class range; determining the relationship between said sag values and a measured cable temperature and adjusting sag values for temperature in order to get temperature-independent actual sag; determining said actual sag of said suspended/anchored cable, as a variable of actual line current; determining a sag reserve DF, in m, for thermal rating, which is the distance between the actual sag and a maximum allowable sag; determining the rate of change, tan ( α), in m/A 2, of said actual sag versus the square of the line current for said effective wind speed, said rate of change being the slope or angular coefficient of a linear fit to the points (actual sag, current-squared); and determining the power line thermal rating of the overhead power line, or ampacity, linked to a corresponding safety clearance, at measured or determined effective wind speed, by adding the square of actual current I to the ratio of the sag reserve DF by the sag rate of change, tan ( α), at the effective wind speed, and taking the square root of that addition, i.e. Ampacity = I 2 + DF tan α, wherein ampacity is in amperes.