ABYC Marine Electrical cheat sheet

Ohm's law: E = I × R; rearranged, I = E/R and R = E/I, with E in volts, I in amperes, and R in ohms.

DC power: P = E × I; equivalent forms are P = I²R and P = E²/R, with P in watts.

Series circuit: one common current; R_total = R1 + R2 + ...; source voltage equals the sum of individual voltage drops.

Parallel circuit: one common voltage; branch currents add; 1/R_total = 1/R1 + 1/R2 + ... .

Kirchhoff's laws: current entering a node equals current leaving; voltage drops around a closed loop sum to zero.

Two-wire DC voltage drop: V_drop = 2 × L × I × R_conductor, where L is one-way length; total drop should not exceed 10% of nominal, or 3% for critical loads.

Ampacity varies with conductor gauge, insulation temperature rating, and bundling; marine cable commonly has 105°C insulation.

A 12 V battery can deliver hundreds of amperes into a low-resistance short; conductor and overcurrent protection must withstand available fault current.

The ungrounded (positive) DC conductor is switched and overcurrent-protected; the negative return is the DC ground reference.

Rested flooded lead-acid state of charge at 77°F: 12.65 V = 100%, 12.45 V = 75%, 12.24 V = 50%, 12.06 V = 25%, about 11.9 V = 0%.

Specific gravity: about 1.265 fully charged and 1.120 discharged; correct hydrometer reading by adding 0.004 per 10°F above 80°F and subtracting below.

Three-stage charging: bulk constant current, absorption constant voltage (about 14.4 V flooded at 77°F), then float (about 13.2–13.5 V).

Temperature compensation for a 12 V lead-acid battery is roughly −0.03 V per °C; charging voltage must fall as battery temperature rises.

Capacity ratings: amp-hours at the 20-hour rate, reserve capacity in minutes at 25 A, and cold cranking amps at 0°F; Peukert effect reduces usable capacity at high discharge rates.

Battery installation: secure against movement, cover terminals, exclude ignition sources, ventilate flooded batteries for hydrogen, and contain spilled electrolyte.

Battery switch positions ON/OFF/1/BOTH/2; do not turn the switch to OFF while the engine is running because the alternator can suffer a load-dump voltage spike.

LiFePO4 batteries require a battery management system, no equalization, and no charging below 0°C; ABYC E-13 addresses lithium-ion installation.

ABYC E-11 overcurrent protection: within 7 in (178 mm) of the power source, or within 40 in (1.0 m) when the conductor is enclosed in a sheath or raceway.

Overcurrent device rating must not exceed the conductor ampacity after derating for temperature and bundling; the device protects the wire, not the appliance.

DC circuit breakers and fuses must be DC-rated; AC-rated breakers may not safely interrupt DC arc faults.

ABYC color code: DC positive red, DC negative yellow, AC ungrounded hot black, AC neutral white, AC safety ground green.

Marine conductors are stranded tinned copper with 105°C insulation, crimped solderless terminals, and adhesive heat-shrink; wire nuts and solid wire are not acceptable.

Support conductors at intervals not exceeding 18 in (450 mm); protect them from chafe, sharp edges, heat, and fuel-system contact.

Use 3% maximum voltage drop for navigation lights, electronics, and critical circuits; 10% is the general maximum for other DC circuits.

DC grounding: the negative system bonds to the engine block or a common grounding bus, which also connects to the AC safety ground at one point.

Panel distribution: main breaker feeds branch breakers through bus bars; label all circuits and keep AC and DC panels physically separate.

Common shore power: 120 V 30 A with a NEMA TT-30 plug, or 125/250 V 50 A with a NEMA SS2-50 plug; inlet, cord, and main breaker must match.

Shore power neutral is bonded to ground at the shore source only; bonding neutral to ground on board creates a parallel ground path and can energize metal parts.

Reversed polarity means hot and neutral are swapped; a polarity tester or ELCI must detect it before the boat is connected.

ELCI: equipment leakage circuit interrupter, about 30 mA trip, installed at or within 10 ft of the shore power inlet to protect the whole boat.

GFCI: ground fault circuit interrupter, about 5 mA trip, required for galley, head, weather-deck, and other wet-location receptacles.

Galvanic isolator: in series with the shore safety ground, blocks low-voltage galvanic DC current while passing AC fault current.

Isolation transformer: breaks the shore ground galvanic path and provides a separately derived AC source; its secondary neutral is bonded to ground at the transformer.

Generator or inverter as a separately derived source: neutral bonded to ground at the source, and the transfer switch must switch the neutral.

Never parallel shore power and generator/inverter outputs; use a transfer switch or interlocked breakers.

Bonding connects all underwater metal fittings to a common bus and to anodes so they sit at the same electrical potential.

AC safety ground, DC negative, and bonding system meet at one common ground point, usually the engine block or a dedicated grounding bus.

Galvanic corrosion: dissimilar metals in an electrolyte form a cell; the less noble metal becomes the anode and corrodes.

Anode selection: zinc or aluminum for salt water, magnesium for fresh water; replace when about 50% consumed.

Cathodic protection requires a continuous electrical path from anode to protected metal; a reference electrode verifies protection potential.

Stray current corrosion: external DC current leaves submerged metal into water; clamp-meter testing and isolation of shore ground help locate it.

Lightning protection: air terminal at the highest point, down conductor of at least 8 AWG copper, and a ground plate bonded to major metal fittings.

The lightning ground system is separate from the AC safety ground; bonding large metal objects reduces side-flash hazards.

Motor branch circuit: short-circuit protection sized to the conductor and separate overload protection sized to the motor nameplate current.

Generator ratings: continuous kW is for indefinite operation; standby or intermittent ratings apply only for short periods.

Inverter output: pure sine wave runs sensitive electronics; modified sine wave is cheaper but can cause motor heating and interference.

Inverter/charger installation: DC input fused at the battery, AC input protected, case grounded, and neutral-ground bonding correct for the source mode.

Panel layout: main breaker, branch breakers, bus bars, and clear labels; DC breakers must be DC-rated and AC panels separated from DC panels.

Voltage drop test: measure across a connection or conductor under load; excessive drop means corrosion, loose terminals, or undersized wire.

Insulation resistance test: a megohmmeter checks an isolated conductor to ground; low readings indicate moisture, damage, or insulation breakdown.

Troubleshooting sequence: verify battery voltage, check fuses/breakers, test continuity with power off, then measure voltage and current under load.