This is an extract from a white paper circulated by APC!
A dedicated line is a power line, which runs from the circuit breaker panel to the critical load and has no other loads connected to it. Normally a circuit breaker feeds multiple receptacles; with a dedicated line, a single circuit breaker feeds only the one receptacle that the protected load is connected to. The advantages of a dedicated line are threefold:
First, the protected load is not subjected to input voltage variations that could be caused by other loads sharing the same circuit. Such variations could result from the voltage drops in the building wiring, which are caused by the currents drawn by the adjacent loads. These variations are prevented by the dedicated line since the adjacent loads no longer share the same building wiring. Second, the protected load is not subjected to variations in the voltage of the grounding wire, which might result from ground noise injection from adjacent loads. This reduces intersystem ground noise.
Third, the protected load is not subject to the potential hazard of having its source circuit breaker tripped by the malfunction of another load, since with a dedicated line no other load shares the same circuit breaker.
A dedicated line may be installed at any time. An electrician simply installs a new circuit breaker in the circuit breaker panel and runs a new wire to either a new or existing AC receptacle.