General Electrical Theory and Fundamentals
What this domain tests
General Electrical Theory and Fundamentals checks whether you can solve practical low-voltage circuit problems rather than just recall cable tables. Expect Ohm's law, power, series and parallel networks, AC impedance and power factor, transformer action, and voltage drop on long control or signaling runs.
Core rules
- Ohm's law is V = I × R. Power is P = V × I; for resistive loads, P = I²R and P = V²/R.
- Series circuits share current and add resistance. Parallel circuits share voltage and add conductance; use reciprocals for resistance.
- AC circuits use impedance Z. Inductive loads lag current; capacitive loads lead. Real power is V × I × power factor.
- A transformer changes voltage and current inversely and does not create power. A 24 V supply still has a VA or amp limit.
- Voltage drop is a loop value. Both supply and return conductors count, and conductor resistance rises with temperature.
- Batteries in series add voltage; batteries in parallel add capacity only when voltages and chemistries match.
Worked example
A 24 VDC access-control load draws 1.5 A at the end of a 150 ft one-way run of 16 AWG copper. Chapter 9 Table 8 gives about 4.89 Ω per 1,000 ft per conductor. The loop is 300 ft, so R = 0.300 × 4.89 = 1.47 Ω. Drop = 1.5 × 1.47 = 2.20 V, about 9.2 percent. The load sees 21.8 V. If the device needs 22 V minimum, use 14 AWG or shorten the run. The key is counting both conductors.
Traps
- Using one-way length in a voltage-drop calculation.
- Mixing RMS and peak values.
- Treating a 24 V supply as ideal when its current limit is only 1 A.
- Adding parallel resistances directly instead of using reciprocals.
- Ignoring AC resistance and reactance on long runs.
A quick sanity check: if a 12 V circuit drops 2 V, that is 16.7 percent, not 8.3 percent; low-voltage systems are less tolerant of the same absolute drop.