1. Conductor

a)Voltage drop,

b)Thermal Capacity,

c)Economy,

2. Insulator

3. Support structures.

Installation of a new transmission line is compulsory in a case where there is no feeder to remote loads. However, the installation in an existing power network can be avoided or at least delayed. Generally, it is known that, installation of new transmission lines is very expensive [1-3]. Considering the the later case, among the key factors is investigating the remaining capacity (not utilized) of the existing transmission lines. Utilization of the existing transmission networks may drop to 50% [4,5]. In this case, it is attractive to increase the utilization. This can take place by controlling the power flow in the existing lines mainly to force flow of more power.

To conclude, a significant factor that worth consideration before installing a new transmission line, is to investigate weather the existing network is fully utilized. 

Then, when you come to install a new transmission line, some key factors are the voltage level and the cross section of the conductors based on the distance (line length) and the amount of power that would flow.

References:

[1] Nazir, M. N., Omran, S., & Broadwater, R. (2016). Feasibility of DSR Applications in Transmission Grid Operation – Control of Power Flow and Imbalanced Voltage. Electric Power Systems Research, 131: 187-194.

[2] Abadie, L. M., & Chamorro, J. M. (2011). Valuing Expansions of the Electricity Transmission Network Under Uncertainty: the Binodal Case. Energies, 4 (10): 1696-1727.

[3] Irwin, P. (2002). Transmission at A Crossroads. Electrical world, 216(1): 13-18.

[4] Young II, M. A. (2015). Saturable Reactor for Power Flow Control in Electric Transmission Systems: Modeling and System Impact Study. PhD Thesis, University of Tennessee, USA.

[5] Johal, H., & Divan, D. (2006). Current Limiting Conductors: A Distributed Approach for Increasing T&D System Capacity and Enhancing Reliability. Proceedings from: Transmission and Distribution Conference and Exhibition 2005/2006 IEEE PES. pp. 1127-1133. IEEE.

well constructing a new transmission line is always a difficult task. for construction or extension of new transmission lines we need to consider both technical & economical aspects.

the technical aspects include the power to be transmitted through the proposed line, length of the line, nature of transmission (single circuit or double circuit), voltage at which tower to be transmitted, nature of route from which the lines will run, conductor type, conductor size etc.

the economic aspects include the loss per unit length, cost of auxiliaries etc. 

We may take into account that we have enough constructional support in establishing a whole transmission line network. The economical run might also be in consider. Choosing the conductor material we consider weather , temperature, area and other environmental aspects. The nature of conductor play an important role in establishing a transmission line.

Thanks everyone for your valuable responses.