Transmission lines which carry three phase power are usually configured as either single circuit or double circuit. A single circuit configuration has three conductors for the three phases. While a double circuit configuration has six conductors (three phases for each circuit).
Double Circuits are used where greater reliability is needed. This method of transmission enables the transfer of more power over a particular distance. The transmission is thus cheaper and requires less land and is considered ideal from an ecological and aesthetic point of view. However, running two circuits in close proximity to each other will involve inductive coupling between the conductors. This needs to be taken into account when calculating the fault level and while designing the protection schemes.
Double circuit transmission lines usually contain bundled conductors with the conductors placed as far as possible to minimize inductance.