•Cables A and B with the same chemical composition showed different fatigue strength.•Wires removed from cable A had fatigue life shorter than wires removed from cable B.•Cable A had lower amount of Mg2Si precipitates and higher number of iron impurities.•Inadequate aging step of heat treatment resulted in lower fatigue life of cable A. Aeolian vibration can cause failure by fatigue in electric conductors and, in some cases, possible blackout. In this study, wires extracted from two cables (A and B) made of 6201 aluminum alloy were tested under fatigue. These tests were carried out aiming to evaluate causes of differences observed on fatigue resistance of the two cables, manufactured with the same chemical composition and, apparently, subjected to the same heat treatment (T81). Wires extracted from cable “A” exhibited lower fatigue life than wires of cable “B”. Scanning electron microscopy analyses of fractured wire surfaces demonstrated that failures were caused by fretting fatigue. Images and analyses using transmission electron microscopy and energy-dispersive spectroscopy demonstrated a lower concentration of Mg2Si hardening precipitates and appreciable concentration of plate-shaped β-Al5FeSi phases in cable “A” wires. The lower fatigue resistance of cable “A” wires compared to those of cable “B” are related to inefficient heat treatment applied to cable “A” and existing iron impurities possible originated from raw materials used during the manufacturing process of this conductor. The inadequate aging step of heat treatment gives rise to fewer hardening Mg2Si precipitates. This effect added to existing iron impurities shortened the mechanical properties of cable “A” and, thereby, its fatigue life, when compared to cable “B”.