Abstract: Ferromagnetic components of overhead line fittings (suspension clamps, tension clamps, etc.) generate hysteresis and eddy-current losses under alternating current, which represent a long-underestimated source of power loss in transmission lines. Such magnetic loss is far smaller than bulk I²R loss of conductors, and its measurement is coupled with current fluctuations, conductor temperature rise, and varying heat-dissipation conditions; consequently, conventional power-difference methods (with versus without fittings) cannot effectively isolate true magnetic loss. To address these difficulties, a refined measurement method for magnetic loss of line fittings based on thermal-equilibrium correction was proposed in this paper. A current-stabilized platform was employed to ensure current consistency between two measurements; current-carrying layer temperature was determined through four-layer temperature measurements on steel core, aluminum strands, armour rods, and conductor surface; and measured power was corrected to a common reference temperature according to resistance temperature coefficients, whereby small magnetic loss was separated from large conductor loss. The apparent “negative-loss” phenomenon observed for preformed suspension clamps was mechanistically explained by combining thermo-electric coupled finite-element simulation with tests conducted in accordance with IEC 61284:1997: it arises from parallel current paths of armour rods, which reduce DC resistance, enlarge equivalent diameter, and enhance auxiliary heat dissipation, so that energy-saving gains outweigh the fitting’s own small magnetic loss. Results indicated that suspension clamps with aluminum-alloy armour rods did not increase total line loss but rather reduced overall power consumption. The proposed method could be used as a reference for magnetic-loss evaluation and selection of energy-saving fittings.