What Is the Voltage Drop Calculator?
The Voltage Drop Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your system voltage, phase, conductor material, you get instant results including voltage drop, drop percentage, voltage at load. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting voltage drop right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your system voltage and need to find the right voltage drop. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
Voltage Drop Calculator
Voltage Drop by Wire Gauge at 100 ft (Copper, Single Phase, 240V)
Voltage drop percentage for common loads and wire sizes.
| Wire Gauge | 10A | 20A | 30A | 40A |
|---|---|---|---|---|
| 14 AWG | 1.57% | 3.14% | 4.71% | 6.28% |
| 12 AWG | 0.99% | 1.98% | 2.97% | 3.96% |
| 10 AWG | 0.62% | 1.24% | 1.87% | 2.49% |
| 8 AWG | 0.39% | 0.78% | 1.17% | 1.56% |
| 6 AWG | 0.25% | 0.49% | 0.74% | 0.98% |
| 4 AWG | 0.16% | 0.31% | 0.47% | 0.62% |
| 2 AWG | 0.10% | 0.19% | 0.29% | 0.39% |
How to Use This Calculator
- Enter Your System Voltage: Start by entering your system voltage — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: phase, conductor material, wire gauge, one-way wire length, load. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your voltage drop, drop percentage, voltage at load. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
Voltage drop occurs when electrical current flows through a wire's resistance. Excessive drop reduces equipment performance and can violate NEC requirements.
The basic rule:
- Single-phase formula: VD = (2 × K × I × L) / CM, where K is resistivity (12.9 copper, 21.2 aluminum), I is amps, L is one-way length in feet, and CM is circular mils of the wire
- Three-phase formula multiplies by 1.732 (√3) instead of 2
- NEC recommends max 3% voltage drop for branch circuits and 5% total for feeder + branch combined
Keeping voltage drop within NEC limits ensures motors start properly, lights don't flicker, and sensitive electronics receive adequate power. For long runs, upsizing wire gauge is the standard solution.
Tips & Considerations
- Double-check your system voltage before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both voltage drop and drop percentage — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to plan your projects.
- If you're unsure about your load, start with a conservative estimate and adjust from there.
Frequently Asked Questions
What is an acceptable voltage drop per NEC?
NEC recommends no more than 3% voltage drop on branch circuits and no more than 5% total when combining feeder and branch circuits. For a 120V circuit, that means no more than 3.6V drop on the branch and 6V total. These are recommendations in NEC 210.19(A) and 215.2(A), not hard requirements.
How do I reduce voltage drop on a long wire run?
The most common solution is to increase wire gauge size. Going from 12 AWG to 10 AWG roughly cuts voltage drop in half. Other options include increasing voltage (240V vs 120V), shortening the run, or reducing the load. For very long runs, consider a sub-panel closer to the load.
What is the K factor in voltage drop calculations?
K is the resistivity constant of the conductor material measured in ohm-circular mils per foot. For copper, K = 12.9 and for aluminum, K = 21.2 at 75°C. Aluminum requires larger wire gauges to achieve the same voltage drop as copper due to its higher resistivity.
Does wire length mean one-way or round-trip distance?
Enter one-way distance only — the distance from your electrical panel to the load. The formula uses a multiplier of 2 (single-phase) or 1.732 (three-phase) to account for the return path automatically.
Why is voltage drop worse on 120V circuits than 240V?
A 240V circuit carries half the current of a 120V circuit for the same wattage (P=V×I). Since voltage drop is proportional to current, the 240V circuit has roughly half the voltage drop. This is why long runs to workshops, outbuildings, and wells often use 240V.
What circular mil values does this calculator use?
This calculator uses NEC Chapter 9 Table 8 values: 14 AWG = 4,110 CM, 12 AWG = 6,530 CM, 10 AWG = 10,380 CM, 8 AWG = 16,510 CM, 6 AWG = 26,240 CM, up to 4/0 AWG = 211,600 CM. These are standard values used throughout the electrical trade.