The working voltage of a capacitor in an ac circuit should be

• A. Equal to the highest applied voltage.
• B. At least 20 percent greater than the highest applied voltage.
• C. At least 50 percent greater than the highest applied voltage.
• D. Less than the highest applied voltage.

Answer: (C)

In an AC circuit the voltage across a capacitor swings with the waveform, so the capacitor must be rated to withstand the maximum instantaneous voltage it will see. That peak value is higher than the RMS value of the supply, and capacitors also have tolerances and can experience transient spikes. To avoid dielectric failure, you select a working voltage that provides a healthy margin above the highest voltage the circuit reaches. A margin of about 50% greater than the peak voltage is a common design guideline, offering protection against tolerances, aging, and brief transients. So the working voltage should be at least 50% higher than the highest applied voltage. For example, with a 120 V RMS supply (peak about 170 V), using a capacitor rated around 250 V gives that margin. The other options either offer too little margin, or no margin at all.