Types of shore power connection found in the US
120 Volt 30 Amp Shore Power Connections
Single Phase 120-Volt System with Shore-Grounded (White) Neutral Conductor, Hot (Black) Conductor and Grounding (Green) Conductor. A 30 Amp service can supply two 15 Amp receptacle circuits allowing simultaneous use of two appliances such as a heater, toaster, hair dryer or vacuum cleaner. Boats with Air Conditioning will find that a 30 Amp service is barely adequate to run a single air conditioner and have power left for anything else. Boats with a 120 Volt System get a double pole main breaker with reverse polarity indicator and branch circuits have single pole breakers since only one leg is Hot.
120 Volt 50 Amp Shore Power Connections
Single Phase 120-Volt System with Shore-Grounded (White) Neutral Conductor, Hot (Black) Conductor and Grounding (Green) Conductor. A 50 Amp service can typically supply an air conditioner and two simultaneous appliances such as a heater, toaster, hair dryer or vacuum cleaner. Boats with a 120 Volt System get a double pole main breaker with reverse polarity indicator and branch circuits have single pole breakers since only one leg is Hot.
120/240 Volt 50 Amp Shore Power Connections
Single Phase 120/240 Volt System With Shore Grounded (White) Neutral Conductor, two Hot (Red and Black) Conductors and a Grounding (Green) Conductor. A 120/240 system provides 120 Volts between either Hot conductor and Neutral while proving 240 Volts between the two Hot Conductors. It allows high current devices such as clothes dryers, electric stoves and larger air conditioners to run at 240 volts while providing adequate power for receptacles and smaller devices running at 120 Volts. 240 Volt devices get double pole breakers since both legs are Hot. 120 Volt devices get a single pole breaker since only one leg is Hot.
Looking out for problems with Shore Power Cords
Shore power cords should be regularly inspected for corrosion, melted plastic, or blackened connections. If shore power connections get salt or moisture on the conductors corrosion can cause a voltage drop across the contacts. The voltage drop produces heat which further accelerates the deterioration. Although the male prongs of the plug can be reasonably easily cleaned with a piece of sandpaper or a Dremel tool it is virtually impossible to clean the female contacts in the socket. The only way to prevent corroded contacts in the socket is to keep it sealed at all times, either with a tight fitting plug in place or with a tight fitting lid securely fastened. A small amount of WD40 sprayed into the socket might also help.
Plugs that are worn, loose, cracked, heat damaged or melted should be replaced. It is normally advisable to replace the plug and socket at the same time as heat from one will cause damage to the other. Shore power cords should be stored in a dry place and when shore power cords are not in use the male end should be plugged in to the female end to help avoid getting moisture on the connectors.
Overheated shore power plugs are a major cause of boat fires. Its not worth trying to economize on this vital area of your boats electrical system. If your plug or socket don't look right replace them before it is too late.