National Home Inspector Exam Prep Podcast

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

• Season 1 - Electrical Systems ⚡ • Season 1 • Episode 9

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Current Types

Direct Current (DC):

  • Flows one direction (positive to negative pole)
  • Sources: Batteries, some power supplies for electronics
  • Not used in utility distribution

Alternating Current (AC):

  • Flows back and forth at 60 cycles/second (North America)
  • Used in modern utility electrical distribution
  • Don't try to visualize how it flows—just trust that it does

Three Phase vs. Single Phase

Three Phase Electricity:

  • Generated by utilities
  • Three energized conductors (think: three TV channels)
  • More potential power than single phase
  • Rarely seen in residential inspections
  • Used for commercial/industrial applications

Single Phase Electricity:

  • Converted from three phase for residential use
  • Two energized conductors (one TV channel)
  • Almost all residential electrical systems
  • 240/120 volts standard

Residential Voltage:

  • Two energized conductors: 120 volts each
  • Voltage between two energized conductors: 240 volts
  • Voltage between each energized and grounded conductor: 120 volts
  • Electricity leaves through uninsulated grounded conductor

Fundamental Electrical Laws

Ohm's Law:

  • Current (I) = Voltage (E) / Resistance (R)
  • Expresses relationship between current, voltage, and resistance
  • I = E / R
  • E = I × R
  • R = E / I

Watt's Law:

  • Power (P) = Voltage (E) × Current (I)
  • Expresses relationship between power, voltage, and current
  • P = E × I
  • E = P / I
  • I = P / E

Joule's Law:

  • Heat = Current² × Resistance × Time
  • Expresses heat produced in electrical circuit
  • Heat = I² × R × Time
  • Critical for understanding why resistance creates fire hazards

Practical Applications

Example 1: Common 15-Amp Circuit

  • 15 amp, 120 volt general lighting circuit
  • Available power: P = I × E = 15 × 120 = 1,800 watts

Example 2: Light Bulb Current Draw

  • 100-watt light bulb at 120 volts
  • Current draw: I = P / E = 100 / 120 = 0.833 amps

Example 3: Voltage Drop from Resistance

  • Voltage decreases as resistance increases (given constant current)
  • Important for long wire lengths and small conductor sizes
  • Longer wires = more resistance
  • Smaller wires = more resistance
  • Results in voltage drop at end of circuit

Example 4: Wire Size Selection

  • #12 AWG wire for 20-amp circuit (not #14)
  • #10 AWG wire for 30-amp circuit
  • Undersized wire = excess resistance = excess heat = fire hazard

Why This Matters for Home Inspectors

Understanding Overcurrent:

  • Circuit breaker/fuse rating must match wire size
  • Oversized OPD allows too much current for wire capacity
  • Excess current = excess heat (Joule's Law)
  • Heat melts insulation and causes fires

Key Takeaways

  • Ohm'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 current
  • AC flows back/forth at 60 cycles per second
  • Single-phase 240/120V is standard residential service
  • Resistance in connections creates heat and fire hazards
  • Wire size

For complete training with visual materials, practice exams, and certification support, visit nhiexamprep.com

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