Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingHard
A client's single-phase AC circuit has an RMS voltage of 120V and a peak current of 17A. Assuming a purely resistive load, what is the approximate RMS current in this circuit?
- A17A
- B8.5A
- C12A
- D24A
Show answer & explanationAnswer & explanation
Correct answer: C. 12A
For a sinusoidal AC waveform, the RMS value is the peak value divided by √2 (approximately 1.414). So, RMS current = Peak current / √2 = 17A / 1.414 ≈ 12.02A, which is approximately 12A.
Why the other options are wrong
- A. This is the peak current, not the RMS current.
- B. This is half the peak current, which is incorrect for RMS calculation.
- D. This might be a result of multiplying by √2 instead of dividing, or other incorrect factor.
RMS vs. Peak Values (AC)
Root Mean Square (RMS) values for AC voltage and current represent the effective value that would produce the same heating effect as a DC equivalent. For sinusoidal waveforms, RMS = Peak / √2.
- RMS = Peak / √2 (for sine wave).
- Peak = RMS * √2 (for sine wave).
- Most AC meters display RMS values.
- RMS is used for power calculations (P = V_rms * I_rms).
Memory trick: RMS is the 'real' power, peak is the 'pointy' power. Divide by root 2 to get real from pointy.