For a taut cable, you need to know the straight line distance between the defining nodes of the cable member and the actual length of the unstressed cable. In case the later is shorter than the former length, the cable is definitely a taut cable. So, the force required to elongate the cable by that difference amount is the value of the initial tension. Though the self-weight of the cable has also a role to play in the determination of the initial tension, but for its small contribution you may neglect that.
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