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The Official Magazine of the Bluewater Cruising Association

Plugging In

Jeff Cote

Pacific Yacht Systems
April 11th, 2016

It is easy to become complacent when it comes to plugging an electrical cord into a socket.  This convenient arrangement for making an electrical connection has become such a casual part of our lives.  It is commonplace to just plug-in if it fits and sometimes, with a little encouragement, it can be made to fit.

At home when we need to plug in a lamp that does not quite reach the closest receptacle, an extension cord can be employed to manage the connection. The two blade lamp plugs easily and safely into the two slot receiver of the extension cord.  Sometimes the device that does not quite reach has two blades and a pin, but the extension has only two slots. With a little encouragement the third pin can be made to slide unconnected beneath the extension cord’s receiver body.  No harm done, except if there was a short in the device when that third connection (safety ground pin) should come into service to provide a safe path to ground, and prevent the device from becoming energized.

NEMA (The National Electrical Manufactures Association) has been around for almost a Century. They have standardized the configurations for plugs and receptacles to try and prevent dangerous and harmful connections being made between dissimilar voltages and/or amperage; sometimes despite their best efforts, the configuration is “close enough” that it can be “made to fit”, with sometimes dangerous or disastrous results.

We have seen 30 Amp cord sets plugged into 50 Amp receptacles and recently, a 125V system plugged into a 208V receptacle.  In both cases, thankfully, no persons were harmed (although there was a very real possibility of this happening), and in both cases there was extensive equipment damage.

If you have any doubts about the connection you are about to make, many NEMA plugs and receptacles have molded into their ends, by convention, a description of the voltage and amperage  for which the plug or receptacle is designed.  If they do not match, do not make the connection. The possibility of a damaging, or potentially lethal, situation developing is almost guaranteed.


  1. Allen says:

    Interesting. Adapters are widely available in marine chandleries to connect 30 amp shore power cords to 50 amp outlets on docks. The problem here, I suppose, is that the fusing on the circuit would be 50 amps and the cords are only designed for 30. The boat should be protected by its main breaker, but the cord and connection to the boat are not. I’m surprised that these adapters exist, or am I missing something?

    1. Jeff Cote says:

      Hi Allen,

      You bring a good point about safety, especially when going from a 50 amp shoreside connection to a 30 amp connection. The shoreside pedestal is sized for 50 amp but yet the shorepower cord via the 50 to 30 amp pigtail is only sized for 30 amp. In a situation like this, you are hoping that your on boat 30 amp breaker will do it’s job flawlessly. Furthermore, you also expect that there will be NO fault in between the 50 amp shoreside breaker and the 30 amp boat side breaker. Unfortunately, it’s common for the AC wiring from shore to be wired for a considerable distance aboard a boat until it reaches the double pole main AC breaker. The ABYC code stipulates 10 feet but that is a rule that is often overlooked.

      1. Allen says:

        Additionally, I have heard of several recent fires at the twist-lock connection between the shore power cord and the boat. (This would be fused at 50 Amps if using the adapter.) Apparently this connection is a weak point and if there is corrosion or a bad connection due to carelessness, heat can be sufficient to cause local damage or even burn up the entire boat. I have been guilty myself of not tightening the black collar that presses the plug into the receptacle due to awkward working position and at other times because that black ring was cracked, but I have smartened up a little since then.

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