Voltage Drop Calculator
Calculate voltage drop, wire size, cable voltage drop, DC voltage drop, and resistor voltage drop.
How It Works
Enter circuit type, voltage, current, wire length, wire size, conductor material, or resistor ohms. The calculator estimates voltage drop, percent drop, circuit resistance, load voltage, and whether the result needs review.
Voltage Drop Calculator Guide
How It Works
The Voltage Drop Calculator helps USA users estimate voltage loss in DC, single-phase, three-phase, and resistor scenarios using current, length, conductor size, material, and resistance assumptions. The main inputs influence the estimate because small changes in cost, time, rate, or revenue can move the result enough to change a decision.
What Is Voltage Drop Calculator?
A voltage drop calculator is an electrical planning tool for estimating how much voltage is lost along conductors or across a resistor. Electricians, engineers, solar installers, RV owners, low-voltage installers, and DIY planners use it before selecting wire size or checking circuit performance.
When Should You Use It?
| Situation | Why Use It |
|---|---|
| Long branch circuit | Check whether wire size is adequate. |
| DC solar or battery run | Estimate voltage loss over cable distance. |
| Three-phase load | Model industrial or commercial feeder drop. |
| Resistor circuit | Use Ohm’s law across a component. |
| Low-voltage lighting | Avoid dim output on long runs. |
| Wire size comparison | Compare copper and aluminum conductors. |
Key Factors That Affect Results
| Factor | How it affects the result | Practical note |
|---|---|---|
| Current | Higher amperage increases voltage drop. | Use expected load current. |
| Length | Longer circuits add resistance. | Round-trip length often matters. |
| Wire size | Larger conductors reduce resistance. | AWG choice affects drop. |
| Material | Copper and aluminum have different resistance. | Use the actual conductor material. |
| Circuit type | DC, single-phase, and three-phase formulas differ. | Select the right mode. |
Use this quick visual to see which assumptions usually deserve the most attention before acting on the result.
Calculation Method
Formula: Voltage drop = current x resistance. For conductor runs, resistance depends on wire size, material, and circuit length.
| Variable | Meaning |
|---|---|
| Current | Amps flowing through the circuit. |
| Resistance | Ohms in conductor or resistor. |
| Length | One-way or circuit length depending on formula. |
| Voltage drop | Voltage lost before the load. |
| Percent drop | Voltage drop divided by source voltage. |
Example Calculation
| Example | Inputs | Result |
|---|---|---|
| Simple | 12 V DC, 10 amps, long small-gauge wire | Voltage drop may be large enough to affect equipment. |
| Intermediate | 120 V branch circuit, 20 amps, 75 ft copper run | Percent drop helps decide whether larger wire is worth checking. |
| Advanced | Three-phase feeder with aluminum conductors | Formula and conductor data must match the installation. |
Common Mistakes
- Using one-way length when round-trip length is needed.
- Selecting copper when the run is aluminum.
- Ignoring temperature, conduit fill, and code requirements.
- Treating NEC informational notes as the only design requirement.
- Using voltage drop to size overcurrent protection.
- Skipping professional review for permanent wiring.
How to Use These Results
Use the result to decide whether to shorten the run, increase conductor size, reduce load, or ask a qualified electrician to review the installation. High voltage drop should trigger further design review.
Electrical planning may also require load, breaker, and conductor checks outside this calculator. For cost planning on materials or service work, a Markup Calculator or Profit Margin Calculator can help price the job separately.
Comparison Scenarios
| Scenario | Inputs | Result |
|---|---|---|
| Smaller AWG | Higher resistance | More voltage drop. |
| Larger AWG | Lower resistance | Less voltage drop. |
| Longer run | More resistance | Higher percent drop. |
| Higher voltage system | Same drop is lower percent | Often more efficient for distance. |
Assumptions and Limitations
Voltage drop estimates do not replace electrical code compliance. Temperature, conductor type, terminals, conduit, load type, motor starting current, local code, and equipment instructions can change design requirements.
Methodology
The method uses Ohm’s law and conductor resistance concepts. NEC informational notes are widely used as design guidance for voltage drop targets, commonly 3% for branch circuits and 5% total feeder plus branch circuit.
Author Review
Disclaimer
This calculator is for educational and planning use only. It is not tax, legal, investment, accounting, payroll, or financial advice. Verify important decisions with official records and qualified professionals.
Formula Explanation
Voltage drop formula: Vdrop = I x R. For two-wire DC or single-phase circuits, round-trip length is used. For three-phase circuits, voltage drop is estimated with sqrt(3) x current x conductor resistance.
Trust and disclaimer
This calculator provides estimates for informational planning only. It is not tax, legal, payroll, accounting, investment, or professional advice. For exact figures, compare the result with your official documents, employer payroll portal, tax agency guidance, lender quote, or a qualified professional.
Last updated: May 2026. Reviewed by Editorial Team.
FAQ
How do you calculate voltage drop?
Voltage drop is calculated from current and circuit resistance. For a basic DC or single-phase circuit, Vdrop = current x total circuit resistance. Three-phase circuits use a different multiplier.
What is an acceptable voltage drop?
The NEC includes informational notes commonly used as design guidance: about 3% for branch circuits and 5% total for feeder plus branch circuit. These are design recommendations, not a substitute for code review.
How do I calculate DC voltage drop?
For DC, use the round-trip conductor length, current, conductor resistance per foot, and material. Longer runs, smaller wire, and higher current increase voltage drop.
How do you calculate voltage drop across a resistor?
Use Ohm’s law: voltage drop equals current multiplied by resistance, or V = I x R. If current is 2 amps and resistance is 5 ohms, the drop is 10 volts.
Does wire size affect voltage drop?
Yes. Larger conductors have lower resistance, so they usually reduce voltage drop. Copper also has lower resistance than aluminum for the same size.
Can this replace an electrician or code calculation?
No. It is a planning tool. Electrical installations must follow applicable codes, equipment requirements, and qualified professional judgment.
Why does distance matter so much?
Longer conductor runs add resistance. For DC and single-phase circuits, the current travels out and back, so round-trip length is usually considered.
What happens if voltage drop is too high?
Equipment may run inefficiently, lights may dim, motors may overheat, and performance can suffer. The right fix may involve larger conductors, shorter runs, or load changes.
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