<p>Electrothermal convection in a horizontal capacitor of a low conducting liquid heated from above is theoretically studied using multi-mode Galerkin expansions. Under the alternating electric field the model exhibits oscillatory convection patterns. The synchronous oscillations appear at the electroconvection threshold. They transform into quasiperiodic oscillations at higher electric driving force. Further growth of the electric Rayleigh number leads to chaotic oscillations. The region of coexistence of chaotic and synchronous oscillations is found.</p>