CSLB C-10 Electrical ContractorPlanning and EstimatingMedium
A client is planning to install a new central air conditioning unit requiring a 240V, 40A dedicated circuit. The unit will be located 100 feet from the panel. The maximum allowable voltage drop is 3%. According to NEC recommendations, what is the minimum AWG copper conductor size required for this branch circuit?
- ANo. 6 AWG
- BNo. 8 AWG
- CNo. 4 AWG
- DNo. 10 AWG
Show answer & explanationAnswer & explanation
Correct answer: B. No. 8 AWG
First, calculate the maximum allowable voltage drop in volts. Then, use the single-phase voltage drop formula with the K-factor for copper and the given current and length to find the required circular mil area. Finally, select the appropriate AWG conductor size from NEC Table 8.
Why the other options are wrong
- A. Incorrect. This is an oversized conductor for the given parameters.
- C. Incorrect. This is significantly oversized, leading to unnecessary expense.
- D. Incorrect. This size will result in excessive voltage drop.
Single-Phase Voltage Drop Calculation
A calculation performed to ensure that the voltage drop in a single-phase circuit does not exceed recommended limits (typically 3% for branch circuits), preventing inefficient operation and equipment damage.
- Uses the formula CM = (2 * K * I * L) / VD.
- K-factor for copper is 12.9 at 75°C.
- Crucial for proper equipment function and energy efficiency.
Memory trick: Voltage Drop: 'V'ery 'D'ifficult if you don't 'K'now the formula.