Having a common ground is nothing uncommon. It's kind of like a plumbing system. Seperate pipes (wires) for the supply water, and feeding into a common drain pipe as soon as efficiently done (usually in the basement). It saves a lot on switches as they need only switch one wire, and on wires. If you have three lights on the mast (spreaders, steaming, and mast head) you need three feeder wires, and one common wire to get the electrity back to the battery. That's four wires. Granted the common return wire has should be larger to provide the capacity for all cirucuits to return the electricity back to the battery. Or if you have return wires for all three circuits, that would be six wires, total. And, bundling four wires together and fixing them down is a lot easier than doing so for six wires. In my mast, the wires go up a piece of plastic pipe, to protect them from sharp metal things in the mast (screw tips, etc.) and to confine them a bit so that don't slap around in the mast and have the insulation on them wear off. It's easier to thread four wires through the plastic pipe than six smaller wires.
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