A master electrician is supervising the rough-in wiring for a new commercial office space. A 20A, 120V general-purpose receptacle branch circuit has a total calculated load of 15A. The circuit will use 12 AWG THHN copper conductors. What is the maximum voltage drop permitted for this branch circuit, assuming a total circuit length (feeder + branch) of 100 feet, according to NEC recommendations?
- A6.0V
- B4.8V
- C1.2V
- D3.6V
Show answer & explanationAnswer & explanation
Correct answer: D. 3.6V
The NEC recommends a maximum total voltage drop of 5% for the combined feeder and branch circuit (NEC 210.19(A)(1) FPN No. 4 and 215.2(D)(3) FPN No. 2). For a 120V circuit, 5% of 120V is 120V * 0.05 = 6V. However, the question asks for the branch circuit, and general recommendations are 3% for the branch circuit. If the question implies a separate calculation for the branch circuit, 3% of 120V = 3.6V. If it asks for the *total* allowed for *this branch circuit* from the *total* circuit length, then the 5% applies to the whole length. However, the question asks for the 'maximum voltage drop permitted for this branch circuit', implying the branch circuit's portion of the drop. NEC FPNs recommend 3% for the branch circuit to the farthest outlet. So, 120V * 0.03 = 3.6V.
Why the other options are wrong
- A. Incorrect. This represents 5% of 120V, which is the total recommended voltage drop for feeder AND branch, not just the branch.
- B. Incorrect. This is 4% of the voltage, not a standard recommendation.
- C. Incorrect. This might be 1% of the voltage, which is too low for the recommended maximum.
NEC Voltage Drop Recommendations
The NEC recommends, but does not mandate, specific maximum voltage drops to ensure efficient operation of equipment and lighting. It suggests a maximum of 3% for branch circuits to the farthest outlet and a total of 5% for the combined feeder and branch circuit.
- Recommended, not required, by NEC (FPNs).
- 3% max for branch circuits (NEC 210.19(A)(1) FPN No. 4).
- 5% max total for combined feeder and branch (NEC 215.2(D)(3) FPN No. 2).
- Higher voltage drop leads to increased power loss, dimmer lights, and motor overheating.
Memory trick: VD: VOLTAGE DROPS, DON'T OVERLOOK PERCENTS.