DC Electrical Theory, Circuits, and Ohm's Law
Core concepts
Every DC question reduces to Ohm's law (E = I × R), power (P = E × I), and Kirchhoff's laws. In series, current is common and voltages add; in parallel, voltage is common and currents add. Equivalent resistance in parallel is always less than the smallest branch. Marine conductors are stranded copper sized by ampacity and voltage drop, not by physical fit. ABYC practice caps voltage drop at 3% for navigation lights, electronics, and other critical loads, and allows about 10% for motor-starting circuits. Ampacity tables assume a given ambient temperature and bundle; engine-room heat and conduit fill force derating. An amp-hour is a capacity unit, not a current; a 100 Ah bank delivers 5 A for 20 h only at that rate, and the Peukert effect reduces usable capacity at high discharge rates.
Traps
Watch for series/parallel mix-ups, using open-circuit battery voltage as a state-of-charge reading under load, forgetting the return conductor in voltage-drop calculations, and assuming a fuse protects equipment rather than the conductor. Another common error is treating AC and DC resistance or color codes as interchangeable.
Worked example
A 10 A pump is 25 ft from the battery, so the round trip is 50 ft. At 12.6 V, 3% is 0.378 V; maximum loop resistance is 0.378 ÷ 10 = 0.0378 Ω, or about 0.76 mΩ/ft. 10 AWG copper is roughly 1.0 mΩ/ft, giving about 0.50 V drop (4%). 8 AWG at about 0.63 mΩ/ft gives about 0.32 V (2.5%), so 8 AWG is the better choice. Rule of thumb: size for voltage drop first on long runs, then confirm ampacity and overcurrent protection.