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Posted by on Jul 1, 2012 in How To, Maintenance, Powerboating, Safety and Seamanship, Sailing, UK, US | 2 comments

Electrical Current and Connections: Why the Heat?

A heavy electrical current flowing through a poor connection creates extra heat, and the potential for fire.

Question: It has been really hot over the last several weeks and my boat, which is air-conditioned, has been plugged in and staying cool through it all. But, the other day I unplugged the boat to go for a spin and noticed that my shore power cord has some melting going on near the terminals, as you can see in the photo. The shore cord was new last year, so I really don’t understand why it should be melting.

The plug is almost new, but loose or corroded parts inside the shore-power fixture on the boat may have caused this melting.

The plug is almost new, but loose or corroded parts inside the shore-power fixture on the boat may have caused this melting.

Answer: Shore-power safety is very important. I have covered it before in a discussion of how and why a power cord can melt before tripping any circuit breakers, and your question is related.

It is important to remember one fundamental key regarding all things electrical: The primary by-product of excessive electrical resistance is heat. In this case, since you have been running your air-conditioner pretty heavily from the sounds of things, you are drawing a heavy electrical load as measured in amps. From the look of your plug, and since the cord and plug assembly are relatively new, my guess is that you may have some loose or corroded electrical connections (read electrical resistance) on your boat at the socket where you plug the shore cord into the boat. The loose or corroded connection, combined with the heavy electrical load from your air-conditioner system, is generating enough heat to melt the plastic on your new shore power cord.

The bottom line here is that you need to get these problems fixed ASAP or you could generate enough heat to actually start a fire. As for your shore cord, you should buy a replacement plug end for it and get this melted part out of the mix.

- Ed Sherman


  1. You should replace the entire shore power cord instead of just the plug end. Cutting off the old plug and replacing it with a new one does not meet the UL or National Electric code requirements. Trying to save a few bucks puts you at risk.

  2. Actually, well-designed and proper replacement components to make up watertight replacement plugs and sockets for shore power cables are widely available from companies such as Marinco, Hubbell, and Smart Plug. The cover of the latest Hubbell catalog shows samples of the 15, 20, 30, and 50 amp service replacement units they provide. The NEC (national electrical code) has no restrictions on these replacement parts as long as the parts meet recognized industry standards. Rest assured that all of the brands mentioned above meet accepted industry standards with their replacement plug kits.