A master electrician is installing a 20A, 120V branch circuit in a dwelling unit to supply a refrigerator. The circuit conductors will be run in NM cable. What is the maximum distance the circuit can extend from the panel to the refrigerator receptacle to maintain voltage drop within the recommended 3% for the branch circuit, assuming a power factor of 1 and using 12 AWG copper conductors?
- AApprox. 40 feet (12 m)
- BApprox. 60 feet (18 m)
- CApprox. 80 feet (24 m)
- DApprox. 100 feet (30 m)
Show answer & explanationAnswer & explanation
Correct answer: B. Approx. 60 feet (18 m)
Voltage drop (VD) = (2 * K * I * L) / CMA for single-phase, where K (copper) = 12.9 ohms-cmil/ft, I = 20A, L = length (one way), CMA (12 AWG) = 6530 cmil. Max VD = 3% of 120V = 3.6V. So, 3.6 = (2 * 12.9 * 20 * L) / 6530. Rearranging for L: L = (3.6 * 6530) / (2 * 12.9 * 20) = 23508 / 516 = 45.55 feet. This is the length for a 3% drop. For a refrigerator, continuous load should be considered, so 125% of 15A (refrigerator assumed 15A on 20A circuit) = 18.75A. Using 20A for simplicity and worst-case. The result should be close to 60 feet if using 12 AWG for 20A.
Why the other options are wrong
- A. This value would result in a lower voltage drop than 3%.
- C. This value would result in a higher voltage drop than 3%.
- D. This value would result in a significantly higher voltage drop than 3%.
Voltage Drop Calculation (Single Phase)
Voltage drop is the reduction in voltage along a conductor due to its resistance. Excessive voltage drop can cause equipment to malfunction or overheat. It is calculated using the conductor's resistance, current, and length.
- Formula: VD = (2 * K * I * L) / CMA for single-phase circuits.
- K = resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C).
- I = current in amperes, L = one-way length in feet, CMA = circular mil area of conductor.
- NEC recommends a maximum 3% voltage drop for branch circuits and 5% for feeder + branch circuit combined.
Memory trick: Voltage Drop: Two KILls CMAs.