Build a DIY shore power test adapter

Dave Osmolski


When you’re out cruising this summer, you may find yourself at a marina slip wanting to use shore power to charge your batteries and run those devices you wouldn’t normally run through your inverter.

Testing for problems

In the Summer 2012 issue, I discussed testing for potential shore power connection dangers using a small inexpensive device with four lights that will detect wiring faults when plugged into a 110-120 volt single-phase circuit. The pattern of lights tells the type of fault. These testers are for standard three-prong polarized plugs with a ground—a type more commonly found in homes not in shore power.

Shore power connectors come in many configurations, the most common of which is a turn locking plug. To use your three-prong circuit tester on this type of plug, you need to make an adaptor. Although you can buy lighted turn lock test units, they are pricey and can only test turn locking plugs. You can save a few dollars by building an adapter that will test both.

Building an adapter

Turn lock connectors come in three-, four- or five-blade configurations. The National Electrical Manufacturers Association has categorized them to prevent intermixing. Three-blade grounded connectors have the designation L5; within that designation are five categories indicated by amperage. Most marinas have 30 amp outlets, so for this project you need an L5-30. The L5-30 category has three designations: P, C or R, for plug, connector or receptacle. For this project, you need an L5-30P.

You may not be able to find these plugs in your local big-box or mom-and-pop hardware store, but you can purchase them in a marine or electrical supply store for about $25.

Next, you need a circuit tester, which you can find in most any hardware store for about $10. You need a standard three-prong grounded connector, the same as you’d find on the extension cord for your string-trimmer. Finally, you need a short, 10- to 12-inch-long piece of 12-gauge wire with two conductors and a ground, which is readily available in any hardware store.

Each conductor has a color-coded coating. The hot wire is black, the neutral wire is white, and the ground is green. Attach the black wire to the copper- or brass-colored screw in both the plug and the connector. Likewise, attach the white wire to the silver-colored screw, and the green wire to the green-colored screw. Assemble the cases on both the plug and the receptacle. Plug the tester into the receptacle, and you are ready to test.

If the outlet at your slip doesn’t test properly, ask for a new slip. A faulty electrical outlet is a disaster waiting to happen.

A word of caution

Even if you find yourself in a slip with a properly wired power head, never go in the water at your slip or anywhere near the docks of a marina. Don’t even reach in to rinse your hand or cool your feet. The marina may be perfectly wired, your boat may be perfectly wired, but you never know about the wiring on your neighbor’s boat. It could kill you.

David H. Osmolski of Charlotte Power Squadron/27 has been repairing boats since high school when his first boat, a canvas-covered canoe with cedar ribs, leaked in gallons per minute and required constant repair.

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