A sine wave varies its voltage up and down. The voltage of the supply is quoted as the same as would produce the same average power as DC in a resistor. This calculated as the root mean square (RMS). It follows that at some times the voltage will be above the RMS and at other times it will be below. A sine wave will have a peak voltage of the RMS * root 2 and an average voltage of 0.9 of the RMS.why we multiply with root 2??

You can prove these relationships with calculus. Note that LTspice uses the peak voltage in their simulations.