120 Volt AC System



WARNING! 120 Volt systems can be dangerous, especially in marine environments. Don't use your system if you have any doubts about its safety. Don't work on your system if you are unsure of what you're doing.

Following are two key points to a safe 120 Volt system:

  • 1. The hot (black), neutral (white) and safety-ground (green or bare) wires must be intact and not mixed up (see AC System Warning Device, below)
  • 2. All current must flow in the hot and neutral wires only. Current flowing anywhere else is 'stray', a fault condition and presents a shock and corrosion hazard. GFCIs (Ground Fault Circuit Interrupters) ensure or an isolation transformer that current flows only in the hot and neutral wires. GFCIs trip if they detect a loss of current from the hot or neutral wire. Isolation transformers allow current to flow only in the hot and neutral wires.
    Note: No safety system protects against shock if you touch both hot and neutral wires at the same time. By touching both wires, you are no different from a light bulb or toaster since you are actually in the hot and neutral circuit. This can be fatal! Luckily, most fault conditions occur when current is able to flow outside of the hot and neutral circuit. This is the situation that GFCIs and isolation transformers protect against.
  • ship to shore plug connector has a locking cover and is insulated from the hull with a rubber gasket
  • ganged, double-pole main breaker is the first part of ship's system and it is easily accessible
    The main breaker must disconnect both hot (black) and neutral (white) wires simultaneously.
  • main breaker rating is appropriate for ship to shore plug connector and wiring used
  • all power indicating devices are wired to hot (black) and neutral (white) wires only unless switched by a 'momentary on' switch
    For example, reverse polarity (hot and neutral reversed) detectors are wired between neutral and safety ground or ship's ground. If the detector is permanently wired into the circuit, it can cause stray current corrosion by allowing current to flow in the safety-grounds or through the bonding system.

    Note: A momentary switch is not required if the device uses circuitry to keep it from drawing more than 1 mA in safety-grounds, ship's ground, or bonding system.
  • AC system warning device is testable and indicates reverse polarity, open safety-ground, hot on ground, etc. (all are dangerous conditions)
    Note: Unless your AC safety indicating system is quite sophisticated, it is a good idea to have a plug-in AC outlet tester. They cost $10 to $15 and test most dangerous conditions with their 3 lights. Do not leave this tester plugged in because it causes current to flow in the safety-ground, which is a cause of stray current corrosion.
  • no connection from either hot (black) or neutral (white) wires to any part of 12 Volt system, including bonding system (test at 120 VAC if qualified)
    A connection between 120 Volt AC hot or neutral wires and the 12 Volt system would be potentially dangerous and could cause stray current corrosion. This problem can occur with AC appliances, such as battery chargers or hot water heaters, and with poor insulation, wet connections, or broken wires.
  • wiring is stranded 3-conductor and is 14 gauge minimum
  • all connections (especially 'hot' ones) in switch-panel or other accessible areas are insulated
    Many commercially available panels contain both AC and DC systems, with all terminals exposed. The 120 Volt terminals must be insulated for safety.
  • all connections are accessible only with the use of tools
    We wouldn't want tiny exploring hands endangering themselves would we?
  • all switches, fuses and breakers disrupt the hot (black) wire
    Breakers may be the ganged, double-pole type, which disrupts both hot and neutral (white) wires simultaneously. The neutral wire must not be broken while the hot wire is intact.
  • no fuses, breakers, or switches in safety-ground (green or bare)
  • outlets in head and galley are protected by a GFCI (Ground Fault Circuit Interrupter)
    Most regulatory bodies require GFCI protection in heads and galleys since they tend to be especially wet spaces. However, since boats can be wet all over I highly recommended that GFCIs or an isolation transformer be installed to protect the entire 120 Volt system.
  • all 120 Volt outlets are 3 prong, grounding type and are incompatible with 12 Volt DC outlets
  • outlet faces clean and terminals coated with a moisture resistant sealant such as petroleum jelly
    120 Volt AC current leakage is likely with dirty or moist outlets. This leakage can cause GFCI devices to trip and, in extreme cases, can cause stray current corrosion or be a shock hazard.
  • if there's an on board 120 Volt AC power source (generator or inverter) wired into the system, there is a DPDT (double-pole, double-throw) switch in the hot (black) and neutral (white) wires that switches the system between shore power and ship's AC power source
    Shore power and the ship's AC power source must not connect to each other or sparks can fly!
  • all wiring enters its destination from below (or in a way that won't allow water drips to enter)

AC system is one of the following three types:

    1.Fully GFCI protected:

  • o all circuits protected by GFCI (Ground Fault Circuit Interrupter) devices
    GFCIs are available as both circuit breakers and outlets. GFCI outlets must be in a circuit already protected by a circuit breaker (usually 20 Amp maximum)they do not function as an overload protector. GFCI outlets can be wired to protect the rest of the circuit, continuing from that outlet. GFCI devices must have a test feature and they should be tested monthly to ensure safety and corrosion protection.

    Note: The entire AC system could be protected by a single GFCI main breaker. However, this is not advisable if you have many AC circuits because the combined leakage of all AC devices could trip the GFCI unnecessarily. For large systems each circuit should have its own GFCI device.
  • shore safety-ground (green or bare) continues as far as first GFCI device, stops there and does not connect to anything at that point
  • ship's safety-ground (green or bare) starts at the first GFCI device, connects to GFCI and its box and continues from there
  • ship's safety-ground is connected to Common Ground Point
  • no connection from shore safety-ground to Common Ground Point (test at 120 VAC if qualified)
    The connection from shore safety-ground to ship's ground can allow stray current corrosion. This connection is safely avoided only with complete GFCI protection or an isolation transformer system.
  • 2. Isolation transformer protected:

  • all AC current supplied through an isolation transformer located between main breaker and switch-panel
    The transformer should be marine rated and large enough to supply all circuits used on board.
  • if neither secondary wires (ship side of transformer) are grounded to Common Ground Point, all circuit breakers are ganged, double-pole type
  • shore safety-ground (green or bare) connected to isolation transformer case only
  • ship's safety-ground (green or bare) connected to Common Ground Point
  • no connection from shore safety-ground to Common Ground Point (test at 120 VAC if qualified)
    The connection from shore safety-ground to ship's ground can allow stray current corrosion. This connection is safely avoided only with complete GFCI protection or an isolation transformer system.

    Note: GFCIs are not required with a correctly operating isolation transformer but may be added as protection against a malfunctioning transformer.
  • 3. Incomplete GFCIs, no isolation:

  • missing or incomplete GFCI (Ground Fault Interrupter) protection and no isolation transformer
  • AC safety-grounds (green or bare) from shore and ship are connected together and to Common Ground Point
    In this system, the connection between ship and shore safety grounds, and ship's ground is critical for protection against shock hazard and stray current corrosion. This connection does not protect against someone touching the hot (black) wire directly while in contact with bilge, sea, or bonding system. A GFCI or isolation transformer system would protect in this case.

    Note: Connecting the ship's safety-ground to ship's ground only, or to shore safety-ground only, can create a shock and corrosion hazard in this system.

    Note: Without complete GFCI or isolation transformer protection, the safety-ground system must be solid on boat and shore to get the protection it can provide. While this 120 Volt AC system is common, it provides limited safety protection and allows several kinds of stray current corrosion to occur since your boat's underwater metal parts are electrically connected to other boats and to the shore system ground. DC stray current can be blocked with a 'galvanic isolator' (diode or capacitor type) connected in series with the safety-ground wire. However, high voltage AC stray current cannot be blocked safely. For these reasons, the full GFCI or isolation transformer systems (#1 and #2 above), which safely eliminate the shore safety-ground to ship's ground connection, are highly recommended.