Every once in awhile we are asked to service jobs that were originally installed by homeowners or landscapers that didn’t really know what they were doing.
In this specific case the previous installer ran out of wire so he thought the right thing to do was to create a j-box to house the wire nuts and the junction between two runs of wire. If this was on a wall this may have worked. But when you bury a metal box in the ground it will eventually fail. The gasket is not made to be wet all the time and the metal will eventually rust.
We make it a point never to create an underground junction in a metal box that will fail. The best choice is to use a new wire that can make it the distance you need. Should there be no other choice than to create an underground junction then the wire connections should be potted to prevent air and moisture from creating corrosion.
Should you need help planning and installing your lighting system for long run success call the Landscape Lighting Guru. 210-710-7737