CSLB C-10 Electrical ContractorElectrical Theory and CalculationsHard
A load requires 50 amps at 240V, single-phase. If the circuit conductors are 2 AWG copper, and the total one-way length of the circuit is 150 feet, what is the approximate voltage drop, assuming a specific resistance of 0.162 ohms per 1000 feet for 2 AWG copper?
- A4.8V
- B2.4V
- C9.6V
- D1.2V
Show answer & explanationAnswer & explanation
Correct answer: B. 2.4V
First, calculate the total resistance of the conductors (R = ρ * (2L)/A or use ohms/foot). Then, use Ohm's Law (Vd = I * R) to find the voltage drop. Remember it's a single-phase circuit, so current flows through two conductors (out and back).
Why the other options are wrong
- A. This is a calculation error, likely doubling the resistance incorrectly or miscalculating total length.
- C. This value is too high, indicating a significant calculation error.
- D. This might be a calculation error, possibly using one-way length directly without doubling for return path.
Voltage Drop Calculation (Conductor Length)
Voltage drop over a conductor is calculated by determining the total resistance of the conductor over its full length (out and back) and then applying Ohm's Law (Vd = I * R).
- R_total = (resistance per 'X' feet) * (total length / 'X' feet).
- For single-phase, total length is 2 * one-way length.
- For three-phase, total length is √3 * one-way length (often simplified to just one-way length for resistance calculation if using K factor).
- Vd = I * R_total.
Memory trick: Resistance per Foot times Total Feet gives Total Resistance, then Ohm's Law!