Articles: Power

Double Pole AC Circuit Breaker

Most boats have a shore power inlet to plug into dockside power.  The power supplied is AC (alternating current), and it is usually available in 240V/120V, with a 30A, 50A or 100A service offered.  Most mid-sized boats (32 to 45 foot) will have a 30A inlet installed, as original equipment, by the the boat manufacturer.  The shore power supply will contain three or four wires; one or two “hot” lines (black and red), a “safety ground” (green or green with a yellow stripe), and a “neutral” (white) which is used as a return path to balance the loads.  A common 120VAC arrangement will have the familiar black, white, and green configuration.

On a common 120 VAC systems, a double pole breaker should be installed within 10 feet or 3 meters of the shore power inlet.  The current rating for the breaker will be the same and matched to the AC distribution system aboard the boat, it is physically linked externally and functionally linked internally.  The purpose of this breaker is to provide over-current protection on both lines.  It is intuitive, in that the “hot” line will have over-current protection, but in case where there is dockside reverse polarity, the “neutral” line will also have over-current protection. 

The breaker should have two indicators, one to indicate if power is available (usually a green light) and a second indicator that shows reverse polarity (usually a red light).

- Lyle, PYS Electrical Technician 


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