Voltage Drop Calculator
Estimate the voltage drop of an electrical circuit based on wire size, distance, and load current.
How it works
Voltage drop is the loss of electrical potential as current travels through a wire. Using Ohm's Law, the drop is calculated from the wire's resistance, the one-way distance, and the load current. NEC recommends keeping drop at or below 3% for branch circuits and 5% combined for feeders and branch circuits.
Voltage Drop Calculator |Complete Guide
What Is a Voltage Drop Calculator?
A Voltage Drop Calculator is an essential tool for electricians, electrical engineers, contractors, and DIY enthusiasts who need to determine how much voltage is lost as electrical current travels through a wire. When electricity flows through a conductor, resistance in the wire causes a reduction in voltage by the time it reaches the end of the run. This reduction is called voltage drop.
Our free online Voltage Drop Calculator helps you instantly calculate:
- The total voltage drop (in volts) across a wire run
- The voltage drop as a percentage of your source voltage
- The actual voltage available at the load (end of the run)
Whether you are wiring a home, running power to an outbuilding, designing an industrial circuit, or sizing cables for a solar installation, this voltage drop calculator gives you fast, accurate results — no manual math required.
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Why Does Voltage Drop Matter?
Voltage drop is one of the most overlooked factors in electrical design, yet it has a direct impact on the performance, safety, and efficiency of any electrical system. When too much voltage is lost along a wire run, several problems can occur: \[https://www.calculator.net/concrete-calculator.html]
- Lights flicker or dim — Lighting circuits with excessive drop cause bulbs to burn dimly or flash unexpectedly
- Motors overheat and fail — Motors running on reduced voltage draw higher current to compensate, causing them to run hotter and burn out prematurely
- Heating elements underperform — Electric heaters, ovens, and water heaters produce less heat when they receive less voltage
- Electronic equipment malfunctions — Computers, control panels, and sensitive electronics may behave erratically or sustain damage from under-voltage
- Energy waste — Resistive losses in undersized wires generate heat that is wasted energy and a potential fire hazard
For these reasons, the National Electrical Code (NEC) provides clear recommendations on acceptable voltage drop limits for electrical installations.
NEC Voltage Drop Recommendations
The National Electrical Code (NEC) in the United States provides the following recommended limits:
| Circuit Type | Max Recommended Voltage Drop |
|---|---|
| Branch circuits (outlets, lights) | 3% or less |
| Feeders (from panel to sub-panel) | 3% or less |
| Feeders + Branch circuits combined | 5% or less |
Our voltage drop calculator displays a color-coded status indicator so you can immediately see whether your circuit meets NEC recommendations:
- ✔ Green — Drop is 3% or less. Excellent. Your wire size is well suited for the run.
- ⚠ Yellow — Drop is between 3% and 5%. Acceptable but consider upgrading wire size for longer-term reliability.
- ✖ Red — Drop exceeds 5%. You should upsize the wire or shorten the run to stay within safe limits.
How to Use the Voltage Drop Calculator
Using the voltage drop calculator is straightforward. Simply fill in the following fields and click Calculate:
1. Wire Material
Select the conductor material — Copper or Aluminum. Copper has lower resistance and is the most common choice for general wiring. Aluminum is often used for large feeder runs and service entrances due to its lower cost, but it has roughly 1.6× the resistance of copper for the same wire size.
2. Wire Size (AWG / kcmil)
Select the wire gauge from the dropdown. Sizes range from 14 AWG (the smallest common household wire) up to 1000 kcmil (used for large industrial feeders). In the American Wire Gauge (AWG) system, a smaller number means a thicker wire with lower resistance. For kcmil sizes, a larger number means a bigger wire.
3. Phase
Choose the circuit type:
- DC — Direct current circuits such as solar panels, battery banks, and automotive wiring
- AC Single-Phase — Standard household and commercial circuits (120V, 240V)
- AC 3-Phase — Industrial and commercial circuits (208V, 480V three-phase)
4. Source Voltage (V)
Enter the voltage at the source (the panel or power supply). Common values are 12V, 24V, 48V (DC), 120V, 240V (single-phase AC), or 208V, 480V (3-phase AC).
5. Load Current (A)
Enter the load current in amperes — this is the amount of current the connected equipment will draw. You can find this on the equipment nameplate, or calculate it using P ÷ V (watts divided by volts).
6. One-Way Distance
Enter the length of the wire run from the source to the load (one way). The voltage drop calculator accounts for the return path automatically. You can enter the distance in feet, meters, miles, or kilometers.
7. Power Factor (AC circuits)
For AC circuits, enter the power factor of the load (a value between 0.0 and 1.0). Resistive loads such as heaters and incandescent lights have a power factor of 1.0. Inductive loads such as motors and transformers typically have a power factor between 0.7 and 0.95. For DC circuits, leave this at 1.
Voltage Drop Formula Explained
The voltage drop calculator uses standard Ohm’s Law-based formulas:
DC and AC Single-Phase Circuits
V_drop = 2 × I × R × L
The factor of 2 accounts for the full round-trip length of the circuit (current travels to the load and back through the return conductor).
AC 3-Phase Circuits
V_drop = √3 × I × R × L × PF
The √3 (approximately 1.732) factor accounts for the phase relationship between the three conductors in a balanced three-phase system. \[https://purecalculations.com/square-footage-calculator/]
Where:
- I = Load current in amperes (A)
- R = Resistance of the wire in ohms per foot (Ω/ft), taken from the NEC AWG resistance table
- L = One-way length of the wire run in feet
- PF = Power factor of the load
AWG Wire Size Reference Table
The American Wire Gauge (AWG) system is the standard for wire sizing in North America. Here is a quick reference for common wire sizes and their copper resistance values:
| Wire Size | Diameter (mm) | Area (mm²) | Copper Resistance (Ω/1000 ft) | Common Use |
|---|---|---|---|---|
| 14 AWG | 1.628 | 2.08 | 3.14 | Lighting circuits (15A) |
| 12 AWG | 2.053 | 3.31 | 1.98 | General outlets (20A) |
| 10 AWG | 2.588 | 5.26 | 1.24 | Appliances, A/C (30A) |
| 8 AWG | 3.264 | 8.37 | 0.778 | Dryers, ranges (40–50A) |
| 6 AWG | 4.115 | 13.3 | 0.491 | Sub-panels, EV chargers |
| 4 AWG | 5.189 | 21.2 | 0.308 | Large feeders |
| 2 AWG | 6.544 | 33.6 | 0.194 | Service entrances |
| 1/0 AWG | 8.252 | 53.5 | 0.122 | Main service |
| 4/0 AWG | 11.68 | 107 | 0.0608 | Large service entrances |
| 250 kcmil | — | 127 | 0.0515 | Large industrial feeders |
Rule of thumb: Every 3 AWG decrease in gauge number doubles the wire’s cross-sectional area and roughly halves its resistance.
Practical Examples
Example 1 — Lighting Circuit (120V, Single-Phase)
You want to run a 15-amp lighting circuit 80 feet from the panel to a garage.
- Wire: 14 AWG Copper
- Voltage: 120V
- Phase: AC Single-Phase
- Current: 15A
- Distance: 80 ft
Result: Voltage drop ≈ 3.77V (3.14%) — Just at the NEC 3% threshold. Consider upgrading to 12 AWG for a comfortable margin.
Example 2 — Well Pump (240V, Single-Phase)
Running a 10-amp well pump 200 feet from the panel.
- Wire: 10 AWG Copper
- Voltage: 240V
- Phase: AC Single-Phase
- Current: 10A
- Distance: 200 ft
Result: Voltage drop ≈ 4.96V (2.07%) — Well within the 3% NEC recommendation.
Example 3 — Industrial Motor (480V, 3-Phase)
A 60-amp motor load 300 feet from a sub-panel.
- Wire: 4 AWG Copper
- Voltage: 480V
- Phase: AC 3-Phase
- Current: 60A
- Distance: 300 ft
- Power Factor: 0.85
Result: Voltage drop ≈ 8.12V (1.69%) — Excellent. Comfortably within NEC limits.
Tips for Reducing Voltage Drop
If your calculation shows excessive voltage drop, here are the most effective ways to bring it within acceptable limits:
- Upsize the wire — The most common solution. Moving from 12 AWG to 10 AWG or 10 AWG to 8 AWG roughly halves the resistance and voltage drop.
- Shorten the run — Voltage drop is directly proportional to distance. Repositioning the panel or sub-panel closer to the load is highly effective.
- Add a sub-panel — For long runs to outbuildings or remote areas of a facility, installing a sub-panel closer to the load reduces individual circuit run lengths.
- Use copper instead of aluminum — If you are already using aluminum wire, switching to copper of the same gauge reduces resistance by about 38%.
- Use parallel conductors — For very large loads, running two or more conductors per phase in parallel reduces the effective resistance proportionally.
- Increase voltage — Where possible, using 240V instead of 120V for the same wattage halves the current, which halves the voltage drop.
Frequently Asked Questions (FAQs)
What is an acceptable voltage drop percentage?
The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% for the combined total of feeders and branch circuits. For sensitive equipment, some engineers design for 2% or less.
Does voltage drop affect power consumption?
Yes. The power lost to resistance in the wire (I² × R) is wasted as heat. This increases your electricity bill over time and is a primary reason why undersized wiring is poor practice in permanent installations.
Is voltage drop the same for DC and AC circuits?
The calculation is slightly different. For DC circuits, only resistance matters. For AC circuits, impedance (resistance plus reactance) affects the drop, which is why power factor is included in 3-phase calculations.
How do I measure actual voltage drop on a circuit?
Use a multimeter. Measure the voltage at the source (panel terminals) and then at the load (outlet or equipment terminals) while the circuit is under full load. The difference is the actual voltage drop.
What wire size should I use for a 100-foot 20-amp circuit?
For a standard 120V, 20-amp circuit at 100 feet, 12 AWG copper gives approximately 2.38% drop — comfortably within NEC limits. For 200 feet, upsize to 10 AWG to stay under 3%.
Can I use this voltage drop calculator for solar and battery systems?
Yes. Select DC as the phase type and enter your system voltage (typically 12V, 24V, or 48V), the expected current, and the wire run length. Voltage drop is particularly important in low-voltage DC systems because even a small absolute drop represents a large percentage of the source voltage.
Conclusion
Proper wire sizing is one of the most important factors in safe, efficient, and code-compliant electrical installations. Too small a wire creates excessive voltage drop, wastes energy, shortens equipment life, and can create a fire hazard. Our free Voltage Drop Calculator takes the guesswork out of wire sizing — enter your circuit details and get instant, accurate results with NEC compliance status.
Use the calculator above for your next project and bookmark it for future reference. If you found it helpful, share it with a fellow electrician or contractor who could use it too.
Disclaimer: This calculator is provided for estimation purposes only. Always consult a licensed electrician and follow all applicable local electrical codes and NEC requirements for your installation.