Voltage drop calculator.
Round trip math. The 2 multiplier on single phase comes from current going out and coming back. K = 12.9 for copper, 21.2 for aluminum. CM from Chapter 9 Table 8.
Uses K=12.9 for copper, K=21.2 for aluminum, from NEC Chapter 9 Table 8. Some prep books use different K values. Numbers may differ slightly.
Voltage drop is the voltage lost across a conductor run. Enter phase, system voltage, current, one-way distance, material, and conductor size. The calculator uses K values from NEC Chapter 9 Table 8 and shows volts dropped plus percent drop for single-phase or three-phase circuits. If you need to choose the conductor size from a voltage-drop target, use the wire size calculator after this page.
20 amps, 240 volts, 100 feet, 12 AWG copper
Inputs
- Single phase
- 240 volts
- 20 amps
- 100 feet one way
- Copper, 12 AWG
Result
Formula: 2 x 12.9 x 20 x 100 / 6,530 circular mils. That is about 7.9 volts dropped. On a 240 volt circuit, that lands near 3.3%.
The exam trap is distance. If the question gives 100 feet from panel to load, enter 100. Do not enter 200. The single-phase formula already accounts for the out-and-back path.
The three places voltage-drop questions steal points
- Doubling distance twice. Enter one-way distance. The formula handles the return path for single phase.
- Using the wrong phase. Single phase uses 2. Three phase uses 1.732. Same numbers, different answer.
- Mixing copper and aluminum. K changes by material. If you miss the material in the question, the rest of the math can be clean and still wrong.
After you run the calculator, drill the setup
Voltage drop calculator questions
What voltage drop is allowed by the NEC?
For most branch-circuit and feeder work, NEC informational notes point to 3% on the branch circuit and 5% total feeder plus branch as a design target. That is not a blanket enforceable rule for every circuit. Always check the article that applies to the equipment in the question.
Do I enter one-way distance or round-trip distance?
Enter the one-way distance from source to load. For single phase, the calculator applies the 2 multiplier because current goes out and comes back. Do not double the distance yourself or you will double-count the run.
Why does three phase use 1.732?
Three phase voltage-drop math uses the square root of 3 multiplier. On the exam, this is one of the easiest places to lose a point because the same job-site numbers produce different answers depending on whether the circuit is single phase or three phase.
Do electrician exams test voltage drop?
Yes. Voltage drop shows up because it tests three skills at once: reading the circuit type, pulling conductor circular mil area from the right table, and running clean arithmetic under time pressure.
Is this a wire size calculator too?
This page checks voltage drop for a conductor size you choose. If the question asks which conductor size meets a voltage-drop target, use the wire size calculator instead, then come back here to verify the selected conductor.
See where you actually stand on the exam.
15 questions. 15 minutes. No signup. You get a topic-by-topic weakness map showing exactly which NEC sections to focus on before test day.
Take the free diagnostic