CSLB C-10 Electrical ContractorElectrical Theory and CalculationsMedium
A client requests a 120V circuit to power a new load drawing 1500 watts. If the circuit conductors have a total resistance of 0.2 ohms, what is the approximate voltage drop across the conductors?
- A2.5 V
- B3.0 V
- C1.5 V
- D4.0 V
Show answer & explanationAnswer & explanation
Correct answer: A. 2.5 V
First, calculate the current (I) using the power formula: I = P / V = 1500W / 120V = 12.5A. Then, calculate the voltage drop (Vd) using Ohm's Law: Vd = I × R = 12.5A × 0.2Ω = 2.5V.
Why the other options are wrong
- B. This might result from a calculation error or using an incorrect current value.
- C. This could be a miscalculation of either current or the voltage drop itself.
- D. This may arise from using a higher incorrect current or resistance value.
Voltage Drop Calculation
The reduction in voltage along the length of a conductor due to its resistance, calculated using Ohm's Law after determining circuit current.
- Requires knowing circuit power and voltage to find current.
- Then, Vd = I × R (current times total conductor resistance).
- Excessive voltage drop can cause poor equipment performance.
Memory trick: First find Current, then know Resistance, then drop the Voltage.