Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A master electrician is troubleshooting a 120V AC circuit that includes a capacitive load. Which of the following statements accurately describes the relationship between current and voltage in this circuit?
- AVoltage lags current by 90 degrees.
- BCurrent lags voltage by 90 degrees.
- CCurrent leads voltage by 90 degrees.
- DCurrent and voltage are in phase.
Show answer & explanationAnswer & explanation
Correct answer: C. Current leads voltage by 90 degrees.
In a purely capacitive AC circuit, the current reaches its peak value 90 degrees before the voltage reaches its peak value. This is described as current leading voltage.
Why the other options are wrong
- A. This is an incorrect way to phrase the relationship in a capacitive circuit; voltage lags current, or current leads voltage.
- B. This describes an inductive AC circuit, where current lags voltage.
- D. This is true for purely resistive AC circuits, not capacitive.
Capacitive AC Circuits
In a purely capacitive AC circuit, the current leads the voltage by 90 electrical degrees due to the capacitor's ability to store charge.
- Capacitors oppose changes in voltage.
- Current leads voltage by 90 degrees.
- Capacitive reactance (Xc) is inversely proportional to frequency and capacitance.
Memory trick: ELI the ICE man: Voltage (E) leads Current (I) in Inductors (L); Current (I) leads Voltage (E) in Capacitors (C).