09 - Basic Electricity and Calculations: Ohm's Law, Watt's Law, Joule's Law

Send us Fan MailCurrent TypesDirect Current (DC):Flows one direction (positive to negative pole)Sources: Batteries, some power supplies for electronicsNot used in utility distributionAlternating Current (AC):Flows back and forth at 60 cycles/second (North America)Used in modern utility electrical distributionDon't try to visualize how it flows—just trust that it doesThree Phase vs. Single PhaseThree Phase Electricity:Generated by utilitiesThree energized conductors (think: three TV channels)More potential power than single phaseRarely seen in residential inspectionsUsed for commercial/industrial applicationsSingle Phase Electricity:Converted from three phase for residential useTwo energized conductors (one TV channel)Almost all residential electrical systems240/120 volts standardResidential Voltage:Two energized conductors: 120 volts eachVoltage between two energized conductors: 240 voltsVoltage between each energized and grounded conductor: 120 voltsElectricity leaves through uninsulated grounded conductorFundamental Electrical LawsOhm's Law:Current (I) = Voltage (E) / Resistance (R)Expresses relationship between current, voltage, and resistanceI = E / RE = I × RR = E / IWatt's Law:Power (P) = Voltage (E) × Current (I)Expresses relationship between power, voltage, and currentP = E × IE = P / II = P / EJoule's Law:Heat = Current² × Resistance × TimeExpresses heat produced in electrical circuitHeat = I² × R × TimeCritical for understanding why resistance creates fire hazardsPractical ApplicationsExample 1: Common 15-Amp Circuit15 amp, 120 volt general lighting circuitAvailable power: P = I × E = 15 × 120 = 1,800 wattsExample 2: Light Bulb Current Draw100-watt light bulb at 120 voltsCurrent draw: I = P / E = 100 / 120 = 0.833 ampsExample 3: Voltage Drop from ResistanceVoltage decreases as resistance increases (given constant current)Important for long wire lengths and small conductor sizesLonger wires = more resistanceSmaller wires = more resistanceResults in voltage drop at end of circuitExample 4: Wire Size Selection#12 AWG wire for 20-amp circuit (not #14)#10 AWG wire for 30-amp circuitUndersized wire = excess resistance = excess heat = fire hazardWhy This Matters for Home InspectorsUnderstanding Overcurrent:Circuit breaker/fuse rating must match wire sizeOversized OPD allows too much current for wire capacityExcess current = excess heat (Joule's Law)Heat melts insulation and causes firesKey TakeawaysOhm's Law: I = E / R (current, voltage, resistance relationship)Watt's Law: P = E × I (power, voltage, current relationship)Joule's Law: Heat increases with resistance and currentAC flows back/forth at 60 cycles per secondSingle-phase 240/120V is standard residential serviceResistance in connections creates heat and fire hazardsWire size must match circuit breaker/fuse ratingIntro This episode is hosted by Charlie Bellefontaine of Chicagoland Home InspectorsFor complete training with visual materials, practice exams, and certification support, visit nhiexamprep.com© 2025 National Home Inspector Exam Prep Podcast. All rights reserved.