Ten Deadly Conditions

Large luxury yacht in a harbor

These deadly electrical problems can affect even luxury boats like this one.

By Tom Burden, Last updated: 4/4/2019

Blue Sea Systems’ engineering department has identified ten conditions that, when present in your boat’s electrical system, can cause serious problems.

1. Incorrectly Sized Wire

There are several problems that occur when sizing wire for a boat’s electrical system using the ABYC 105°C tables:

  • Using wire with less than 105°C insulation.
  • Bundling wires together or burying them in thermal insulation.

However, usually wire size isn't a problem because:

  • Most DC large loads are short term.
  • Most DC wire is chosen for voltage drop and is therefore larger than the minimum recommendations from the ABYC tables.
  • Wire is sold with different insulation temperature ratings. The highest rating in general use for shore side wiring is rated for 90°C and there are numerous common wire materials rated at 75°C, 80°C and 90°C. The ABYC 105°C table should only be used with wire that is rated at 105°C.
  • Heat is produced in wire by resistance to current flow. Wire temperature is a function of the heat produced by the losses in the wire and how effective the installation is at removing that heat. This heat escapes into the air or into heat sinks such as busbars made of solid copper. Efforts to neatly dress wire by tying it together or concealing them between hull and liner actually make the situation worse. The better job we do of providing physical protection for wire with conduits and installing between hull and liner, the harder it is for the heat to get out. The efforts to make neater installations can result in a potential hazard if the wire size is not increased to compensate.

The greatest concern here is with AC circuits that feed receptacles that can be easily overloaded.

Even when using 105°C rated wire at its maximum current rating, the wire may be too hot to touch without burning yourself. At these high temperatures, corrosion at terminals will be accelerated and the system may have shortened life.

2. Running Fuses Continuously at Full Ratings

When matching circuit protection to the wire it protects, two facts contribute to the complexity of this task:

  • The amperage at which fuses actually blow, and circuit breakers actually trip, is considerably higher than their nominal ratings, the rating usually marked on the unit.
  • Wire and circuit protection devices heat up dramatically when they carry 100 percent of their rated value for several minutes or more.
Melted fuses and wire insulation

Heat from high current can melt wire insulation and fuse blocks.

SEA, Maxi, ATO and AGC fuses, and most circuit breakers, blow or trip at about 130 percent of their rating. ANL fuses blow from 140 percent to as high as 266 percent of their rating. When fuses carry 100 percent of their rated current value, they generate excessive heat. When wires carry 100 percent of their rated current value, they also generate excessive heat. In combination, the heat produced by fuses and wires carrying high current can melt wire insulation and fuse blocks.

This heat generation may become critical when loads run for a considerable time. Large diameter wires take a long time to heat up, so short-term operations like bow thrusters, windlasses, and macerator pumps seldom run long enough for this kind of heating to occur. For example, a 2/0 wire may take 25 minutes to approach its maximum temperature. In contrast, small diameter wires reach near peak temperature in less than ten minutes.

For loads and appliances that run continuously for 10 to 30 minutes, choose circuit protection and wire so that current does not exceed 80 percent of their rating. For more information in this topic, refer to Blue Sea Systems’ Technical Brief: Choosing Circuit Protection.

burned cordset and shore power inlet

This boat nearly burned because a corroded shore power connection overheated (BoatUS Seaworthy photo).

3. Not Using the Shore Power Cord Locking Ring

The shore power cord locking ring maintains a solid connection between the power cord plug and hull receptacle. When this connection isn’t secure, motion can cause the plug to wiggle back and forth in the receptacle, compromise the electrical connection, and result in dangerous heating.

Shore power connectors can have both electrical and mechanical stresses applied. The locking ring keeps the plug from backing out and fixes the two elements together so that the connection is not moved by normal motions. The constant working of the connection between shore power cord plug and receptacle with the boat's motion can loosen the connection, increase corrosion and weaken spring contact tension.

The shore power cable and connection is the most easily overloaded point of wiring on the boat because it feeds all of the AC system including receptacle circuits. Every new appliance brought onboard can add a new load, and devices like electric grills, hair driers, air conditioners and space heaters are designed to use about 12 Amps each. It is easy to turn on more than the rated capacity of your system, and the circuit breaker system will not trip until your overload is at about 130 percent or more. In this condition, a weak connection between plug and receptacle can become a fire source.

4. No “Green Wire” or Poor-quality Connection Between DC Negative and AC Safety Ground

Without a good connection between DC negative and AC safety ground, stray AC current may enter the DC ground system. When this happens, AC current may enter the water around a boat and injure or kill swimmers near the boat. The green wire is the safety ground wire that connects the DC negative ground block to the AC safety ground bus. The purpose of this wire is to provide a lowest-resistance path to ground for any stray AC current that finds its way onto the DC ground system. There have been cases of AC current entering the water around a boat through the engine shaft and killing swimmers near the boat. For more on how to prevent this, see our West Advisor on ELCI/GFCI Electrical Shock Protection.

Diagram of proper DC and AC grounding

Lack of a good connection between DC negative and AC safety ground may put AC current in the water around your boat.

There is a downside to this green wire connection. This safety ground can also provide a path for galvanic current if the boat is not adequately protected with galvanic isolators. However, most marine industry organizations and professionals now consider it standard practice to install this wire. Safety requires providing the grounding wire, either directly or through a galvanic isolator, or using a properly installed marine isolation transformer. Some people have left off the ground wire in a mistaken notion that they are providing galvanic protection, but forget that they are compromising safety for those on the boat, on the dock, and in the water. Electrically induced drowning is now recognized as a previously undocumented cause of death. The Coast Guard is funding a study to isolate and investigate this hazard.

The green wire can be tested and indicate continuity but be unable to safely carry enough current to trip a circuit breaker during a fault. There are ways to check the quality of the connection.

An Ohmmeter test may show very little resistance in a green wire installation, yet the wire may be incapable of carrying 30 amperes or the higher currents needed to trip a circuit breaker during a fault. The minimum resistance reading of an Ohmmeter will not necessarily indicate if a connection is compromised, such as a connection making to only a single strand of wire. There are specialized ground resistance testers that apply significant current, but they are uncommon. Careful visual inspection of the grounding connections helps, but even a careful surveyor may have a hard time finding all connections and tracing the wiring path.

One way to test the green wire connection quality is to connect a spotlight or other heavy 12V load, positive to the boat’s battery, and the negative to the safety ground pin of the shore cord. In a properly wired boat, the safety ground pin should return to the battery negative after first connecting at the AC panel. If the light burns bright and steady, there probably is a good grounding system. This is a good check to perform if a boat has an unknown maintenance history, has been rewired, or is being repaired after damage.

5. Using Ordinary Plug-in AC Receptacle Testers to Check Ground Integrity

Ordinary plug-in AC receptacle testers are so sensitive that they will indicate a good ground even if the only connection is through a prop shaft or thru-hull fitting to water. A better way to test for ground integrity is to connect the shore cord to the boat and bring the shore plug back to a position near the electrical panel. With all on-board AC sources turned off, use an Ohmmeter to check that the ground prong is solidly connected to the boat’s safety ground system. Check to the “U” ground at each receptacle by dragging around the shore cord end and meter to test at each receptacle.

6. Using Non-ignition Protected Devices in Explosive Areas or Making Areas Explosive with Dinghy Fuel and Propane Bottles

It is dangerous to store dinghy fuel or portable propane bottles on board boats that are not designed for ignition protection. This includes diesel propulsion fuel or gasoline-fueled systems for externally mounted outboard motors. Bringing alternative fuels on board, even in small quantities, can lead to explosive situations.

Ignition protected breaker

A half gallon of gasoline carried on board to fuel the dinghy motor turns a diesel boat into a gasoline boat. The starter, alternator, switches, and circuit breakers have all been selected according to the rules for a diesel engine system. Dinghy fuel, if carried on board, should be stored in lockers that vent overboard just like propane lockers. Gasoline vapors are heavier than air and will pour down the companionway or from internally vented lockers right into the bilge and engine spaces.

GFCI receptacles have contacts to open when a fault is detected and as of this writing there are no such devices with ignition protection, and it may not be possible to do so. Any receptacle in a gasoline engine room is suspect because plugging in or unplugging an appliance can generate sparks. The use of portable power tools in such spaces is also very dangerous because of brush arcing in the universal motors.

These are the ABYC guidelines for ignition protection:

  • Potential electrical sources of ignition located in spaces containing gasoline powered machinery, or gasoline fuel tank(s), or joint fitting(s), or other connection(s) between components of a gasoline system, shall be ignition protected.
  • If LPG or CNG is provided on the boat, all electrical potential sources of ignition located in compartments containing LPG/CNG appliances, cylinders, fittings, valves or regulators shall be ignition protected. An exception is made for open compartments and accommodation spaces if everything is installed according to ABYC- A-1, ABYC A-33 and ABYC A-3 for gas systems and appliances.

7. Hot/Neutral Reversal on AC Connectors at the Dock or Onboard

If a shore cord is incorrectly wired so that the hot and neutral wires are reversed, there is inadequate AC circuit protection. Your boat should have a two pole main breaker if wired for 120V. It should also have a reverse polarity indicator or warning device.

Diagram of reversed hot and neutral wires.

Hot and neutral wires are reversed.

The power companies have developed a standard for 120 volt single-phase power distribution in which one of the two conductors is designated the “hot” wire (the black wire in North America) and the other, by default, the neutral (the white wire). However, neither power conductor in an AC system is permanently at either a positive or a negative potential. Their respective potentials alternate between zero, the maximum positive, zero, and the maximum negative potential. This is what makes it AC.

If a shore cord is incorrectly wired so that the hot and neutral wires are reversed, the “new” hot line now has no overcurrent protection because the branch circuit wiring is only protected by a 30 or 50-ampere main circuit breaker. Worse, turning off the branch circuit breaker leaves the circuit hot. The circuit breaker does turn the load on and off, so all seems normal.

This leads to several possible hazards:

  • The branch circuit wires are only protected by the larger main circuit breaker in the event of a failure of the white wire to ground.
  • Anyone working on the wiring and turning off the branch circuit breaker will still be in contact with a live wire if the main breaker isn’t off. This can be a serious shock hazard.
  • The outer shell of conventional screw in light bulbs is intended to be the neutral conductor with a voltage near ground. Any broken bulb is dangerous to remove when energized, and reverse polarity makes it worse.
  • Boats not wired to present-day ABYC standards may have only a single pole main breaker and no reverse polarity lights to indicate a reverse polarity fault. This can occur in internal wiring or more probably in marina wiring that is not performed correctly. Private docks, and some commercial docks, may not always be wired by skilled electricians and properly inspected.

Your boat should have a two pole main breaker if wired for 120V. It should also have a reverse polarity indicator or warning device. A three pole breaker is appropriate for the main breaker in a 120/240V system, because neutral faults can also exist in these systems, with either an open neutral or reverse polarity generating hazards or damage to equipment.

8. Undersized or Absent Inverter and Charger DC Grounding

Inverters and battery chargers are bridges between a boat's AC and DC power systems. The high voltage of the AC system presents a shock hazard and can be lethal if it exists on an ungrounded case. The DC system is not normally a shock hazard but can provide a lot of current, and so is potentially a source of fire. A suitable ground must be installed between the AC and DC system.

Inverters and changers are provided with grounding connections on both the AC side and DC side. This grounding can prevent shocks from AC, and fire hazard from DC. Frequently only the AC grounding connection is made. A fault in the DC side of the system could provide enough current to overheat the AC grounding conductor without blowing the large DC fuse. Therefore, a high amperage capacity DC grounding path back to the DC system is required.

There is no mention of adequate measures to wire inverters and chargers in ABYC E-11 where most guidelines for electrical systems are found, so knowledge of proper installation is readily overlooked. This information is actually contained in ABYC A-25 for inverters, and A-24 for chargers.

Because this hazard has only recently been recognized, the installation instructions for older generation inverters typically do not include this precaution, but most newly designed units do. You may buy a new inverter from off the dealer shelf today and not get the proper information because the instructions may have been written a few years ago. For example, the instructions for a newly introduced inverter recommend a 2/0 cable for a 3 KVA inverter, but the instructions from the same manufacturer on their older models give no recommendation at all.

Follow these general guidelines for wiring inverters and chargers:

  • The ground wire to the DC grounding system should not be smaller than one size below the wire size required for the DC current carrying conductors.
  • The DC overcurrent protection device should not be sized at more than 150 percent of the capacity of the grounding conductor.

9. Missing or Faulty GFCI

GFCI wiring diagram

According to ABYC, when an AC receptacle is installed in a head, galley, machinery space, or on a weather deck, it shall be protected by a Type A (nominal 5 milliamperes) Ground Fault Circuit Interrupter (GFCI). (There is a little natural imbalance from the tiny radiation from an AC system, so the limit at 5mA is set below a safe threshold for humans; if it were zero it would be tripping unnecessarily.) GFCI devices compare the current flowing in the hot (black) wire and the neutral (white) wire. If all of the current going out does not come back on the paired conductor, it is going somewhere not intended, and that might be through a person! In this condition, the GFCI will trip.

The ABYC Standards call for GFCIs to protect receptacles and therefore the appliances that are plugged into the receptacle. However, the Standards do not require GFCIs for permanently-wired appliances such as hot water heaters, space heaters, battery chargers and air conditioners. These devices are assumed to be protected by a solid safety ground connection to the case. However, it is good practice to also protect these devices with GFCIs. It is possible to use the output of one GFCI receptacle to feed other receptacles and permanently wired devices. By doing so, you can choose to provide more protection than the minimum in the ABYC Standard thereby protecting these downstream receptacles and devices.

GFCIs should be tested frequently, especially in areas subject to lightning or if there has been any electrical malfunction onboard. Because of the corrosive atmosphere of the marine environment, these devices may have a shorter life span on a boat than they have ashore.

10. Operating Power Tools in Potentially Explosive Atmospheres

12V DC Vacuum Cleaner

Be careful when using a wet-dry vacuum or other power tools in areas where fuel or other explosive vapors are present.

Portable power tools operated by batteries or by AC have brush-type universal or DC motors. The spinning commutator generates sparks and the space is usually well ventilated and even has a fan forcing outside air through the space. Electric drills, routers and saws are bad. Shop vacuums are worse. They are designed to collect air and dirt through the hose and bring it inside. The air passes through a filter and then some of it through the motor. More than one explosion has taken place when a wet/dry vacuum was used to vacuum up a fuel spill in the bilge. A shop vacuum is a wonderful way to pick up water spills, but it also excellent at mixing and igniting explosive gas. Nobody thinks about getting killed by a vacuum cleaner but it can take out the operator, the boat and possibly the neighbors!


Avoiding these deadly conditions will make your boating experience far more safe and pleasurable. Take time to check your boat’s electrical system and look for these conditions. Contact your local ABYC electrician.