Wire size calculator.
Use this wire size calculator to find the smallest listed conductor that meets a voltage-drop target. Enter phase, volts, amps, one-way distance, copper or aluminum, and the percent drop limit.
This calculator solves the voltage-drop part of conductor sizing. It does not replace ampacity, terminal temperature, equipment, or local code checks.
Direct answer: wire size for voltage drop comes from solving for circular mil area. Single phase uses 2 x K x I x D divided by the allowed voltage drop. Three phase uses 1.732 x K x I x D divided by the allowed voltage drop. Then pick the next listed conductor size.
20 amps, 240 volts, 100 feet, copper, 3% target
Inputs
- Single phase
- 240 volts
- 20 amps
- 100 feet one way
- Copper conductor
- 3% voltage-drop target
Result
The allowed voltage drop is 7.2 volts. Required circular mil area is about 7,167 CM. The next listed size in this calculator is 10 AWG copper, which drops about 4.97 volts on this run.
Here is the trap: the voltage-drop answer may be larger than the ampacity answer. On an exam, solve the part the question asks for, then check whether another rule controls the final conductor.
Wire-size mistakes that move the answer
- Treating this as ampacity. Voltage drop and ampacity are different checks. A wire can pass ampacity and still miss the voltage-drop target on a long run.
- Doubling distance twice. Enter one-way distance. The single-phase formula already handles the return path.
- Using copper K for aluminum. Aluminum has a higher K value. Same load and distance, larger conductor.
- Rounding conductor size down. If the required area lands between two sizes, pick the larger listed conductor.
After you size the wire, drill the rule
Wire size calculator questions
Is this wire size calculator an ampacity calculator?
No. This tool sizes wire by voltage-drop target. A final code answer still needs ampacity, terminal temperature, conductor material, equipment instructions, and the article that controls the circuit.
Do I enter one-way distance or round-trip distance?
Enter one-way distance from source to load. For single phase, the calculator applies the 2 multiplier. Do not double the distance before entering it.
What voltage-drop target should I use?
Use the target given in the question. If no target is given, many exam prep problems use 3% for a branch circuit and 5% total feeder plus branch as design targets.
Why does aluminum need a larger wire size than copper?
The simplified formula uses K=12.9 for copper and K=21.2 for aluminum. A higher K means more voltage drop for the same size and run, so aluminum often needs more circular mil area.
Is wire gauge the same as wire size?
For common exam wording, yes. Wire gauge usually means AWG size. Larger physical conductors have smaller AWG numbers until the list moves into kcmil sizes.
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