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Red Seal Construction Electrician study materials
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Red Seal Construction Electrician: Electrical Theory and Applied Calculations

Red Seal Construction Electrician · Work through your exam, one topic at a time.

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Electrical Theory and Applied Calculations

What this domain tests

Electrical Theory and Applied Calculations tests your ability to use Ohm's law, power relationships, three-phase formulas, voltage drop, and load calculations to choose conductors and explain measured results. Expect questions that mix line and phase values, power factor, efficiency, and series/parallel resistance.

Core relationships

  • Ohm's law: V = I × R; power: P = V × I for direct current or unity-power-factor AC.
  • Single-phase AC: P = V × I × PF. Three-phase: P = √3 × V_L × I × PF, where V_L is line-to-line voltage.
  • Include efficiency when the rating is mechanical output: input power = output power ÷ efficiency.
  • Voltage drop is proportional to current, conductor resistance, and circuit length. For a single-phase two-wire circuit, the return path doubles the length; for three-phase, use √3 with line current and one-way length.
  • Conductors in series add resistance; conductors in parallel divide current according to conductance.

Worked example

A 15 kW, 480 V, three-phase motor has a power factor of 0.85 and efficiency of 0.90. Find the line current. First convert output to input: 15,000 W ÷ 0.90 = 16,667 W. Then use P = √3 × V_L × I × PF: I = 16,667 ÷ (1.732 × 480 × 0.85). The denominator is 1.732 × 480 = 831.4; 831.4 × 0.85 = 706.7. Therefore I = 16,667 ÷ 706.7 = 23.6 A. The reasoning: the motor nameplate horsepower is shaft output; the circuit carries input current, so efficiency and power factor must both be applied before selecting conductors or overloads.

Traps

Using 480 V as a phase-to-neutral value; omitting power factor or efficiency; treating a three-phase load as single-phase; using one-way length in a single-phase voltage-drop calculation; mixing nameplate FLA with calculated current when code tables require a specific value.

Key points

  • Use √3 × V_L × I × PF for balanced three-phase loads.
  • Convert mechanical output to electrical input with efficiency.
  • Power factor affects current, not real power delivered.
  • Single-phase voltage drop uses the full loop length.
  • Keep line and phase values separate in wye and delta systems.