"Provides a safer hook-up to 30A dockside power for up to three attached tools, and provides protection against potentially lethal electric shock. GFCI (Ground Fault Circuit Interruptor) trips at 4–6mA. Total load of attached tools must not exceed 15A."
I interpret this as meaning that an extension cord rated at 15 amps will work just fine. Loads above this will trip the breaker. The device simply allows hook up to a 30A plug commonly found at a dockside power box with a 15A breaker and GFCI circuit with 3 female outlets. I have a 75 ft cord used for travel that works great. Basically only charging the batteries, running the fridge, and charging misc electronics