General Electrical Knowledge and Theory
What this domain tests
Questions in this domain check whether you can predict circuit behavior before you touch a tool: Ohm's law, series and parallel networks, AC impedance and power factor, conductor resistance, voltage drop, and transformer relationships. Low-voltage work still depends on these basics because power-limited circuits have source-capacity and voltage-drop limits just as branch circuits do.
Core relationships
- Ohm's law: V = I × R. Power: P = V × I = I²R. In AC, use impedance Z and include power factor: P = V × I × PF for single-phase.
- Series: current is common; voltages add; resistances add. Parallel: voltage is common; currents add; 1/RT = 1/R1 + 1/R2.
- Balanced three-phase: P = √3 × V(line-to-line) × I × PF.
- Conductor resistance rises with temperature. Chapter 9 Table 8 gives DC resistance at 75°C; Table 9 gives AC resistance and reactance.
- Voltage drop for a single-phase two-wire load: VD = 2 × L × R × I, with L as the one-way length.
Worked example
A 120 V, 16 A load is 100 ft from the panel on 12 AWG copper. Table 9 gives about 1.93 Ω per 1,000 ft at 75°C. VD = 2 × 100 × (1.93/1000) × 16 = 6.2 V, or about 5.2 percent. NEC 210.19(A) informational notes recommend no more than 3 percent branch-circuit drop and 5 percent total. The circuit operates, but it is outside the recommendation; 10 AWG or a shorter run is the fix.
Traps
- Forgetting the factor of two for the return conductor.
- Mixing RMS and peak values, or using peak voltage in power calculations.
- Using 60°C resistance or ampacity when terminals are rated 75°C (110.14(C)).
- Confusing kVA with kW when power factor is not unity.