Master Electrician Exam (NEC)General Electrical KnowledgeEasy
A maintenance electrician is replacing a faulty contactor coil in a motor control circuit. The circuit operates at 24V AC. The new coil must have the correct impedance to ensure proper operation and prevent damage. If the coil draws 0.5A at 24V AC, what is its approximate impedance?
- A48 Ohms
- B12 Ohms
- C0.02 Ohms
- D24 Ohms
Show answer & explanationAnswer & explanation
Correct answer: A. 48 Ohms
Impedance (Z) in an AC circuit is analogous to resistance in a DC circuit and can be calculated using Ohm's Law: Z = V / I. Z = 24V / 0.5A = 48 Ohms.
Why the other options are wrong
- B. Incorrect. This would be the result of 24/2 or 24 * 0.5.
- C. Incorrect. This value is significantly too low and a result of incorrect division.
- D. Incorrect. This would be the result if the current was 1A.
Impedance in AC Circuits
Impedance (Z) is the total opposition to current flow in an AC circuit, combining resistance and reactance. It is measured in Ohms (Ω) and can be calculated using Ohm's Law for AC circuits: Z = V / I.
- Z = V / I (Ohm's Law for AC)
- Includes resistance (R), inductive reactance (XL), and capacitive reactance (XC).
- Units are Ohms (Ω).
- For purely resistive circuits, Z = R.
Memory trick: AC OHM'S: IMPEDANCE is 'I'n V's' Way.