What is the relation between peak voltage and rms voltage ?

The Peak voltage is the maximum voltage that the voltage reaches in an ac power supply.  It is the topmost and the lower most point of the sine curve.

The normal value with which we refer to the voltage is called the rms voltage.  It is the root mean square of the instantaneous voltage supply

V _rms = Sq. root (v₁+v₂+v₃+v₄.....v_n)/n

Where v₁,  v₂ , v₃ and v_n are the instantaneous voltages.

The relation between the rms voltage and the peak voltage is as follows.

V_rms = 0.707 x V _peak