If the wiring between the cable and the junction box does not have to be carried out on site , then you could consider pre-wired, factory finished or pre-assembled might also fit the description.

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Note added at 1 hr (2009-01-23 19:30:35 GMT)
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For SuzieZ, to say that the junction box is supplied pre-equipped would give most British native speakers a bit of consternation, pre-wired sounds better. It depends on the recipient, pre-wired, " no need to wire up the junction box", "the junction box is delivered without further need for wiring", I would stay with the first suggestion.