Limitations imposed by Four-Wave Mixing (FWM), Amplified Spontaneous Emission (ASE), dispersion and Stimulated Raman Scattering (SRS) on a multi-spam DWDM system are theoretically studied. In this work, expression for the linear dispersion parameter D has been defined as a function of number of channels in order to separate FWM and SRS effects and calculates both maximum fibre length and optimum power. Additionally, our simulation results consider the effect of ASE noise from Erbium Doped Fibre Amplifiers (EDFAs). This theoretical analysis yields a set of design criteria from optimized multi-span DWDM systems.