General Electrical Knowledge and Ohm's Law
Core concepts
Low voltage work still rests on the same physics as line-voltage work. Candidates must be fluent with Ohm's Law (E = I × R, with the three algebraic rearrangements), the power relationships (P = E × I, P = I²R, P = E²/R), and the difference between AC and DC behavior. Know that series circuits share current, parallel circuits share voltage, and that resistance adds in series while conductance adds in parallel. Understand voltage drop as a separate problem from conductor ampacity: a conductor can be thermally adequate yet still produce unacceptable drop on a long run.
Traps to expect
Questions often give you two of the three Ohm's Law quantities in mixed units - milliamps with kilohms, or watts with milliamps - and expect a clean conversion first. Watch for "nominal" versus "actual" values (a 24 V system that measures 27.5 V; a 12 V battery at 12.6 V). Distinguish RMS from peak for AC sine waves (peak = RMS × 1.414). Another common trap is confusing ampacity tables with voltage-drop calculations, or assuming a device's nameplate current equals its actual draw.
Worked example
A 24 V DC camera draws 350 mA over a 180 ft one-way run of copper with 10.2 Ω per 1000 ft. Round-trip resistance = 360 ft × 10.2/1000 = 3.672 Ω. Voltage drop = 0.350 A × 3.672 Ω ≈ 1.29 V, about 5.4 percent - acceptable for most 24 V equipment but marginal for devices rated ±10 percent at low supply. Rule of thumb: keep low voltage drops under 5 percent by upsizing conductors or shortening runs, not by raising supply voltage beyond equipment ratings.