Wire Size Calculator (Voltage Drop)
Find the minimum wire gauge for your circuit by voltage drop. Enter current, distance, voltage and material to get the recommended AWG and actual voltage drop.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
Voltage-drop sizing only. Always verify against NEC ampacity (Table 310.16), breaker rating and local code before wiring.
How the Wire Size Calculator Works
This tool finds the minimum conductor size that keeps voltage drop within your target over a given run. Choose the conductor material, the system type, then enter the load current, the one-way distance, the source voltage, and the maximum voltage drop. It returns the required circular-mil area, the smallest standard wire that meets it, and the actual drop you would see.
The Formula
The classic K-factor (circular-mil) voltage-drop method is:
Required circular mils = (factor × K × current × one-way distance) ÷ allowed voltage drop (volts)
Where factor is 2 for single-phase (accounting for the round-trip conductor length) or 1.732 (√3) for three-phase, K is 12.9 for copper or 21.2 for aluminum, and the allowed voltage drop in volts equals source voltage × allowed drop %. The calculator then selects the smallest standard AWG or kcmil size whose area is at least the required circular mils.
Worked Example
For a 20 A single-phase copper circuit running 100 ft at 120 V with a 3% drop limit:
- Allowed voltage drop = 120 × 3% = 3.6 V
- Required circular mils = (2 × 12.9 × 20 × 100) ÷ 3.6 = 51,600 ÷ 3.6 ≈ 14,333 cmil
- Smallest standard size = 8 AWG (16,510 cmil)
- Actual drop with 8 AWG = (2 × 12.9 × 20 × 100) ÷ 16,510 ≈ 3.13 V = 2.60%
So 8 AWG copper keeps the drop under the 3% target on this long run.
Why Voltage Drop Drives Wire Size
On short runs the breaker and ampacity set the wire size, but on long runs voltage drop usually controls:
- Performance — excessive drop dims lights, weakens motors, and wastes energy as heat.
- Code compliance — the NEC recommends 3% per branch and 5% total.
- Material choice — aluminum’s higher K means it always needs a larger gauge than copper.
| Material | Current | Distance | Recommended (3%, single-phase) |
|---|---|---|---|
| Copper | 20 A | 100 ft @ 120 V | 8 AWG |
| Copper | 30 A | 50 ft @ 240 V | 12 AWG |
| Aluminum | 50 A | 150 ft @ 240 V | 3 AWG |
Frequently asked questions
How do I calculate the wire size I need?+
Use the voltage-drop method: required circular mils = (factor × K × current × one-way distance) ÷ allowed voltage drop in volts, where K is 12.9 for copper or 21.2 for aluminum and the factor is 2 for single-phase or 1.732 for three-phase. Then pick the smallest standard wire whose area meets that figure.
What is the K factor in the voltage drop formula?+
K is the conductor resistivity in ohm-circular-mils per foot. The standard approximate values are about 12.9 for copper and 21.2 for aluminum at typical operating temperatures. A higher K means more resistance, so aluminum needs a larger cross-section than copper for the same circuit.
What is an acceptable voltage drop?+
The US National Electrical Code recommends a maximum of 3% voltage drop on a branch circuit and 5% total for feeder plus branch combined. This calculator defaults to 3%. Lower drop means better performance but larger, costlier wire, so 3% is a common practical target.
Does this calculator replace NEC ampacity tables?+
No. It sizes wire only for voltage drop. You must also confirm the conductor meets NEC ampacity (Table 310.16), matches your breaker rating, and satisfies derating for temperature and conduit fill. Always use the larger of the ampacity-based and voltage-drop-based sizes.
Why does distance matter so much for wire size?+
Voltage drop is proportional to wire length, so doubling the run distance doubles the drop for a given wire. Long runs often need a thicker conductor than the ampacity tables alone would suggest, which is exactly what the voltage-drop calculation captures.