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Voltage‐gated Na+ channels are crucial to action potential propagation in excitable tissues. Because of the high amplitude and rapid activation of the Na+ current, voltage‐clamp measurements are very challenging and are usually performed at room temperature. In this study, we measured Na+ current voltage‐dependence in stem cell‐derived cardiomyocytes at physiological temperature. While the apparent activation and inactivation curves, measured as the dependence of current amplitude on voltage, fall within the range reported in previous studies, we identified a systematic error in our measurements. This error is caused by the deviation of the membrane potential from the command potential of the amplifier. We demonstrate that it is possible to account for this artifact using computer simulation of the patch‐clamp experiment. We obtained surprising results through patch‐clamp model optimization: a half‐activation of −11.5 mV and a half‐inactivation of −87 mV. Although the half‐activation deviates from previous research, we demonstrate that this estimate reproduces the conduction velocity dependence on extracellular potassium concentration. Key points: Voltage‐gated Na+ currents play a crucial role in excitable tissues including neurons, cardiac and skeletal muscle.Measurement of Na+ current is challenging because of its high amplitude and rapid kinetics, especially at physiological temperature.We have used the patch‐clamp technique to measure human Na+ current voltage‐dependence in human induced pluripotent stem cell‐derived cardiomyocytes.The patch‐clamp data were processed by optimization of the model accounting for voltage‐clamp experiment artifacts, revealing a large difference between apparent parameters of Na+ current and the results of the optimization.We conclude that actual Na+ current activation is extremely depolarized in comparison to previous studies.The new Na+ current model provides a better understanding of action potential propagation; we demonstrate that it explains propagation in hyperkalaemic conditions. [ABSTRACT FROM AUTHOR]