ABYC Marine Electrical key terms

Ohm's Law expresses the fixed relationship among voltage, current, and resistance in a circuit: current equals voltage divided by resistance. Knowing any two values lets a technician calculate the third.

Voltage lost as current flows through the resistance of conductors and connections, leaving less voltage at the load. ABYC recommends no more than 3 percent drop on critical circuits and 10 percent total.

A circuit with only one path for current, so the same current flows through every component and the total resistance is the sum of the individual resistances. A break anywhere stops current everywhere.

A circuit with multiple current paths, where each branch sees the same voltage and total current is the sum of the branch currents. Total resistance is always lower than the smallest branch resistance.

An unintended low-resistance path that bypasses the load, allowing excessive current to flow. It can overheat conductors, damage equipment, and start a fire unless an overcurrent device opens the circuit quickly.

The amount of energy remaining in a battery, usually expressed as a percentage of its full capacity. It is estimated from resting voltage, specific gravity, or coulomb counting by a battery monitor.

The density of battery electrolyte compared with pure water, used to estimate lead-acid state of charge. A fully charged flooded cell typically reads about 1.265 at 77 degrees Fahrenheit.

The first charging stage, in which the charger delivers its maximum safe current into the battery while voltage rises toward the absorption set point. It restores most of the missing capacity quickly.

A charging stage that holds battery voltage at an elevated set point while current tapers, allowing the last portion of capacity to recharge without overheating or excessive gassing.

A low-voltage maintenance stage that keeps a fully charged battery topped up while supplying small loads. It counteracts self-discharge without causing significant gassing or water loss.

The buildup of hard lead sulfate crystals on battery plates, which reduces capacity and cannot always be reversed. It develops when batteries sit partially discharged or are left uncharged for long periods.

Provision of airflow that disperses hydrogen gas released during charging and prevents explosive concentrations. Enclosures must be vented to the atmosphere, and ignition sources should be kept clear.

A lithium battery chemistry popular on boats for its high energy density, long cycle life, and relatively stable thermal behavior. It requires a battery management system and charging voltages different from lead-acid.

The maximum continuous current a conductor can carry without exceeding its insulation temperature rating under specified conditions. ABYC tables give allowable current for each wire size and insulation type.

A fuse or circuit breaker that opens a circuit when current exceeds its rating for a set time, protecting conductors from overheating. It must be sized to protect the conductor it serves.

A manually operated switch that connects or isolates the battery bank from the boat's DC distribution system. It must be rated for the continuous current and be readily accessible.

An ABYC requirement that overcurrent protection be installed within seven inches of the point where a conductor leaves the battery source, unless the conductor is sheathed or in conduit for up to 40 inches.

A metal bar used to collect and distribute current among multiple circuits or batteries. It provides a tidy, low-resistance termination point for large cables and small branch conductors.

Sleeving, grommets, clamps, or conduit that prevents wire insulation from rubbing against sharp edges or vibrating structures. Damaged insulation exposes conductors and creates fire and shock hazards.

ABYC conventions that identify DC conductors by color: red for positive, yellow or black for negative, and yellow with a red stripe for the DC grounding conductor.

The flexible cord and molded connectors that bring dock AC power to the boat's inlet. It must be rated for the circuit, kept free of damage, and supported so strain is not carried by the terminals.

A dangerous condition in which the shore power hot and neutral conductors are swapped, energizing what should be the neutral. It is detected by a polarity tester or indicator and must be corrected before use.

A device that trips when it detects a small difference between hot and neutral current, indicating leakage to ground. GFCIs protect people from electric shock in wet or damp locations.

A shore-power device that trips when leakage current from the boat's AC system to ground exceeds a low threshold. It protects the whole boat rather than a single receptacle.

A device installed in the shore power grounding conductor that blocks low-voltage galvanic currents while still passing dangerous AC fault current. It reduces corrosion of underwater metals connected to shore ground.

A transformer that magnetically couples shore power to the boat without a direct conductor connection, breaking galvanic paths and reducing corrosion and shock risk. It can also correct voltage and isolate neutral.

The intentional connection between the AC neutral and the safety ground at the source. On a boat it must exist at only one point at a time, typically at the generator or inverter when that unit is the source.

The intentional connection of all major metallic components exposed to seawater to a common conductor, equalizing electrical potential and reducing galvanic corrosion and shock hazards.

Corrosion caused when two dissimilar metals in an electrolyte are electrically connected, forming a battery cell that eats away the less noble metal. Seawater is a strong electrolyte.

A zinc, aluminum, or magnesium fitting connected to underwater metals so it corrodes preferentially, protecting the more noble metals. Anodes must be inspected and replaced as they waste away.

A system that prevents corrosion by making the protected metal the cathode of an electrochemical cell, usually through sacrificial anodes or an impressed current system.

Metal loss caused by DC current leaking through seawater from a boat's wiring or a neighboring boat, rather than by natural galvanic action. It can destroy propellers and thru-hulls quickly.

A network of air terminals, a main conductor, and a grounding path that gives lightning a controlled route to the water. It also bonds major metal fittings to reduce side flashes.

An electronic device that converts DC battery power into AC power for receptacles and appliances. Inverters are rated by continuous output and surge capacity.

An engine-driven generator that produces DC charging current for the battery bank. Its output depends on rotor speed, temperature, and the voltage regulator's set point.

A switch or relay that selects between shore power and an onboard generator or inverter, preventing two AC sources from being connected together at the same time.

A handheld test instrument that measures voltage, resistance, and often current and continuity. It is the basic troubleshooting tool for verifying circuits and finding opens or shorts.

A meter that measures current by sensing the magnetic field around a single conductor without breaking the circuit. It is useful for checking alternator, inverter, and branch circuit loading.

An instrument that applies a high test voltage to measure insulation resistance, revealing degraded wire insulation or moisture intrusion. Tests are performed with the circuit de-energized and disconnected.