The maximum voltage across a capacitor in an LRC series circuit is calculated differently for AC and DC inputs. For DC, the voltage across the capacitor equals the supply voltage (Vc = Vcc). In contrast, for AC, the maximum voltage depends on the frequency of the input voltage and the circuit's impedance, calculated using the formula Z = √(R² + (Xl - Xc)²), where Xl is inductive reactance and Xc is capacitive reactance. The voltage across the capacitor will vary over time due to the changing nature of the AC supply. Understanding these calculations is essential for analyzing LRC circuits effectively.