Voltage drop calculator
Estimate circuit voltage drop and percent drop from AWG size, run length, current, and conductor material.
What this calculator covers
Use this calculator to estimate how much voltage a branch or feeder run will lose before the load sees it.
The result is a simplified engineering screen, not a code-compliance decision. It is most useful for comparing conductor options and seeing how length, current, and material affect the drop percentage.
Frequently asked questions
- What does percent voltage drop mean in practice?
- Percent drop is the voltage lost in the conductors expressed as a fraction of the supply voltage. A 3% drop on a 120 V circuit means the load sees roughly 116 V instead of 120 V. Keeping drop low helps equipment run closer to its rated voltage.
- Why does the formula use one-way length if the current travels both ways?
- The formula doubles the one-way length internally to account for the full round-trip conductor path — current flows out through one conductor and returns through the other. Enter only the one-way distance from panel to load.
- What is the difference between copper and aluminum conductors in the calculation?
- Copper and aluminum have different resistivity constants (K values). Copper has a lower resistivity, meaning less voltage drop for the same wire size. Selecting the correct material keeps the estimate accurate for the conductor you actually plan to install.
- Is this result sufficient for a code-compliance decision?
- No. This calculator provides a simplified engineering estimate for comparison and planning purposes. Ampacity ratings, insulation temperature ratings, bundling derating, and local electrical code requirements must all be verified separately, ideally by a licensed electrician.
Tool
Run the calculation
Result
RESULT · VOLTAGE DROP
â„–191
Primary result
2.49 V
10 AWG copper conductors over 50 ft one way drop 2.4855 V, or 2.07% of a 120 V system.
- Voltage drop
- 2.4855 V
- Percent drop
- 2.07%
- Estimated delivered voltage
- 117.5145 V
- Conductor lookup
- 10 AWG = 10,380 cmil
Step-by-step solution
- 1.Look up 10 AWG as 10,380 circular mils and use K = 12.9 ohm-cmil/ft for copper.
- 2.Apply the simplified two-conductor formula Vdrop = 2 x K x I x L / cmil with 20 A and 50 ft.
- 3.Divide the voltage drop by 120 V to read percent drop and subtract it from system voltage to estimate delivered voltage.
Walkthrough
Visual walkthrough
Voltage drop depends on current, conductor length, material, and wire area. The calculator uses the standard K and circular-mil lookup path.
01
Choose the conductor constants
10 AWG -> 10,380 cmil; K = 12.9
AWG size sets the conductor area, while copper versus aluminum changes the resistivity constant.
02
Run the drop formula
Vdrop = 2 x K x I x L / cmil
The length term is one-way distance, so the formula doubles it to cover the full round-trip conductor path.
03
Convert drop into a percentage
Percent drop makes it easier to compare the result against common branch-circuit targets such as 3%.
2.4855 V (2.07%)