What is the peak value of current if its rms value is 2A?

Answer 1

If an AC current is sinusoidal, then the peak value of a current with an RMS value of 2A is 2.8A, or 2A times the square root of 2.

However, very few current waveforms are sinusoidal. In fact, they are often far from sinusoidal - they are more often pulsating - because most power supplies, even switching power supplies, only draw current when the rectifier diodes conduct to recharge the primary filter capacitor. That only occurs once (half wave rectifier) or twice (full wave rectifier) each line cycle.

The square root of 2 rule does apply if the load is purely resistive and the voltage is also sinusoidal. Most voltages are also not sinudoidal, because of the pulsating current issue, and because conductors are not perfect zero impedance conductors. Voltage waveforms, however, are more closely sinudoidal than current waveforms.

The most accurate way to measure the peak current value is to observe it with an oscilloscope. If you want the analytic approach, you need to back calculate from peak value to RMS value, knowing that RMS value is the square root of the sum of the squares of the observations divided by N, as delta T approaches zero, and N approaches infinity. (Best to just use the oscilloscope.)