Energy consumption of the optical core is expected to contribute to a significant portion of the energy consumed by the telecommunication networks. As an enhanced bandwidth utilization and management extent of the optical Wavelength Division Multiplexing (WDM) technology, multi-granular switching concept has been shown to guarantee energy savings when compared to the traditional traffic grooming. However, Multi-Granular Optical Transport (MG-OTN) design and planning still calls for legitimate and self-contained design specifications. In this paper, we develop a heuristic to design a green optical core network using the multi-granular switching concept. We present the design results for different topologies to give insights on the impact of optical reach on the power consumption of core networks.