Voltage Drop Calculator
Calculate voltage drop, percent drop, load voltage, recommended wire size, or maximum run length for copper and aluminum conductors.
Calculator is for informational purposes only. Terms and Conditions
Choose what to solve for
Select the circuit type, conductor material, and calculation goal.
Enter the known values
Use one-way conductor length. The calculator applies the correct circuit factor.
Visual Check
See the source voltage, conductor run, load voltage, and voltage drop status.
Solution
Live result, quick checks, warnings, and full solution steps.
Quick checks
- Voltage drop—
Show solution steps See the equation, substitutions, assumptions, and result path
- Enter values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
This calculator uses the simplified circular-mil voltage drop equation for educational estimating and early conductor sizing.
- Final electrical design must be verified against applicable NEC requirements, local code, equipment data, and qualified engineering judgment.
On this page
Calculator Guide
How to Use the Voltage Drop Calculator
The Voltage Drop Calculator above estimates voltage lost in a conductor run, percent voltage drop, load voltage, minimum wire size, or maximum one-way run length. Enter the source voltage, load current, one-way length, circuit type, conductor material, and wire size to quickly check whether a circuit is likely to stay within a practical voltage drop target.
Voltage drop matters because conductors have resistance. As current flows through copper or aluminum wire, some voltage is lost before it reaches the equipment. Too much voltage drop can cause dim lights, weak motor starting, nuisance trips, overheated conductors, inefficient equipment operation, and poor performance on long low-voltage runs.
Voltage drop sizing and ampacity sizing are different checks. Voltage drop tells you whether enough voltage reaches the load. Ampacity tells you whether the conductor can safely carry the current without overheating. A final conductor size must satisfy both.
Quick Answer
For a DC or single-phase circuit, voltage drop can be estimated with \(V_d=\frac{2KIL}{CM}\). For a balanced three-phase circuit, use \(V_d=\frac{\sqrt{3}KIL}{CM}\). The percent voltage drop is \(\frac{V_d}{V_s}\times100\), and the load voltage is \(V_{load}=V_s-V_d\). Lower resistance, larger wire, shorter length, and lower current all reduce voltage drop.
Do not rely on the simplified result when…
Do not use this simplified calculator as the only basis for final electrical design, code compliance, conductor ampacity, motor-starting studies, harmonic analysis, voltage regulation, utility interconnection, or equipment protection. Voltage drop is only one design check. Final conductor selection also depends on ampacity, insulation temperature rating, terminals, ambient correction, conduit fill, local code, equipment data, and qualified review.
Inputs and Outputs Used by the Calculator
The calculator uses the values that control conductor resistance and voltage loss. The most common workflow is to calculate voltage drop from known voltage, current, length, material, circuit type, and wire size.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input | Source voltage | Nominal voltage at the start of the circuit, such as 12 V DC, 120 V, 240 V, or 480 V. | V |
| Input | Load current | Current flowing through the conductor. Voltage drop increases directly with current. | A |
| Input | One-way length | Distance from source to load. Do not double this value; the formula factor handles the circuit path. | ft or m |
| Input | Wire size | Conductor cross-sectional area. Larger conductors have lower resistance and lower voltage drop. | AWG or kcmil |
| Input | Conductor material | Copper and aluminum have different resistance constants. Aluminum usually needs a larger size for the same drop. | Copper or aluminum |
| Input | Circuit type | Determines whether the calculator uses the DC/single-phase factor or the balanced three-phase factor. | DC, single-phase, three-phase |
| Output | Voltage drop | Voltage lost in the conductor before power reaches the load. | V |
| Output | Percent drop | Voltage drop as a percentage of source voltage. This is often the easiest result to judge. | % |
| Output | Load voltage | Estimated voltage available at the equipment after conductor drop is subtracted. | V |
| Output | Minimum wire size or maximum length | Estimated conductor size or one-way run length needed to stay within a selected voltage drop target. | AWG, kcmil, ft, m |
Practical insight
On low-voltage systems, the same voltage loss is a much larger percentage of the source voltage. A 2 V drop is only 1.7% on a 120 V circuit, but it is 16.7% on a 12 V circuit. This is why solar, battery, LED, RV, landscape lighting, and control circuits often need larger conductors than expected.
Voltage Drop Formula
The simplified circular-mil method estimates conductor voltage drop from conductor material, load current, one-way distance, and wire area. It is useful for quick estimating and early wire sizing.
DC and Single-Phase Voltage Drop
Use this form for DC two-wire circuits and single-phase AC circuits when \(L\) is the one-way conductor length. The factor of 2 accounts for the outgoing and return conductor path.
Balanced Three-Phase Voltage Drop
Use this form for a balanced three-phase circuit. The \(\sqrt{3}\) factor reflects the phase relationship in a three-phase system.
Percent Voltage Drop
Percent drop is often the most useful output because it normalizes the voltage loss against the source voltage.
Load Voltage
Load voltage is the estimated voltage available at the equipment terminals after conductor drop is subtracted.
Minimum Circular Mils for a Target Drop
Use \(F=2\) for DC or single-phase circuits and \(F=\sqrt{3}\) for balanced three-phase circuits. The calculator can compare the required circular mil area to common AWG and kcmil conductor sizes.
Maximum One-Way Length
This rearranged form estimates the longest one-way run that stays within a selected voltage drop target.
Conductor Power Loss
Voltage drop also represents power dissipated as heat in the conductors. This is especially important on long, high-current, or low-voltage runs.
What the Variables Mean
Each variable must use the correct unit system. The simplified circular-mil formula is traditionally used with length in feet, current in amps, and conductor area in circular mils.
| Symbol | Meaning | How to Enter It |
|---|---|---|
| \(V_d\) | Voltage drop in the conductor run. | Calculated by the tool or set from a target percent when solving for size or length. |
| \(V_s\) | Source voltage at the start of the circuit. | Enter the nominal system voltage, such as 120 V, 240 V, or 480 V. |
| \(V_{load}\) | Estimated voltage available at the load. | Calculated as source voltage minus voltage drop. |
| \(K\) | Conductor resistivity constant. | Common estimating values are about 12.9 for copper and 21.2 for aluminum in ohm-circular-mils per foot. |
| \(I\) | Load current flowing through the conductor. | Enter amperes. Use design load current, not breaker size, unless breaker size is intentionally being used as a conservative estimate. |
| \(L\) | One-way distance from source to load. | Enter one-way length. Do not double it for DC or single-phase circuits. |
| \(CM\) | Conductor area in circular mils. | Selected from AWG or kcmil wire size. Larger \(CM\) lowers voltage drop. |
| \(F\) | Circuit factor used in rearranged formulas. | Use \(F=2\) for DC or single-phase and \(F=\sqrt{3}\) for balanced three-phase. |
| \(P_{loss}\) | Approximate conductor power loss. | Calculated from current times voltage drop. The result is watts when current is in amps and voltage drop is in volts. |
Important conductor note
Voltage drop sizing is not the same as ampacity sizing. A conductor can have acceptable voltage drop but still be too small for ampacity, temperature, insulation, termination, or code requirements. Always check both.
How to Use the Calculator
Start by choosing what you want to solve for: voltage drop, minimum wire size, or maximum one-way length. Then enter the circuit values that match the selected solve mode.
Select the solve mode
Choose whether the calculator should find voltage drop, recommend a minimum wire size, or estimate maximum run length for a target drop.
Choose the circuit type
Select DC, single-phase AC, or balanced three-phase AC. This changes the formula factor used in the calculation.
Enter one-way length
Use the physical distance from source to load. Do not enter round-trip distance unless you are intentionally using a different custom method.
Set material and wire size
Select copper or aluminum, then choose the conductor size. Aluminum has higher resistance, so it often needs a larger conductor for the same current and length.
Review the result and quick checks
Compare voltage drop, percent drop, load voltage, warnings, and the solution steps. If the percent drop is high, try a larger conductor, shorter run, higher voltage, or lower current.
How to Interpret Voltage Drop Results
A lower voltage drop is usually better, but the acceptable value depends on the circuit, equipment, load type, voltage level, and design target. Percent drop is the fastest way to judge the result.
| Result Pattern | What It May Mean | What to Check Next |
|---|---|---|
| 0% to 2% | Low voltage drop for many circuits. Often acceptable from a performance standpoint. | Still verify ampacity, overcurrent protection, and installation conditions. |
| About 3% | Common branch-circuit design target used for satisfactory equipment performance. | Check whether there is also feeder voltage drop upstream. |
| About 5% total | Common feeder-plus-branch total design target in many low-voltage building circuits. | Split the drop between feeder and branch circuit and verify load voltage at the equipment. |
| Above 5% | May cause performance problems, especially for motors, lighting, electronics, and low-voltage systems. | Try a larger conductor, shorter run, higher voltage, lower current, or distributed power source. |
| Negative or impossible result | The input set is not physically meaningful or a unit/length error was entered. | Check source voltage, current, one-way length, target percent, and selected solve mode. |
What to do with the result
If the percent drop is acceptable, the next step is to verify conductor ampacity and equipment requirements. If the drop is too high, increase wire size, reduce the one-way length, reduce load current, use a higher system voltage where appropriate, or split the load into shorter circuits.
What changes the result most?
Voltage drop changes directly with current and length, and inversely with conductor area. Doubling the current doubles the drop. Doubling the one-way length doubles the drop. Doubling the conductor circular mil area roughly cuts the drop in half. Source voltage does not reduce voltage lost in the wire, but it reduces the percentage that loss represents.
Voltage drop also means power loss
The power lost in the conductor is approximately \(P_{loss}=I \times V_d\). In the worked example below, \(15\,A \times 2.96\,V \approx 44.4\,W\) is lost as heat in the circuit conductors. This is why long, high-current runs can waste energy even when the load still operates.
Quick sanity check
For a 120 V circuit, a 3% drop is \(120\times0.03=3.6\,V\), so the load voltage is about \(116.4\,V\). If your result says a normal 120 V branch circuit loses 15 V or more, recheck the length, current, wire size, and whether you accidentally entered round-trip distance.
Input Quality Checklist
Voltage drop errors usually come from simple input mistakes. Check these items before using the result for design decisions.
Use one-way length
Enter the distance from source to load, not the round-trip conductor distance. The formula already applies the correct path factor.
Use actual load current
Voltage drop depends on current. A lightly loaded circuit drops less voltage than a circuit loaded to the breaker rating.
Match material correctly
Copper and aluminum use different \(K\) values. Selecting copper for an aluminum run will understate voltage drop.
Confirm circuit type
DC and single-phase calculations use a different factor than balanced three-phase calculations. The wrong selection can shift the result significantly.
Separate feeder and branch drop
For a full installation, calculate voltage drop through the feeder and branch circuit, then add them for the total path to the load.
Check equipment voltage range
Some equipment tolerates voltage variation better than others. Motors, LEDs, controls, and inverters can be sensitive to low terminal voltage.
Step-by-Step Worked Example
This example calculates voltage drop for a common 120 V single-phase branch circuit using copper wire.
Start with the single-phase formula
Substitute the values
Calculate percent voltage drop
Calculate load voltage
Estimate conductor power loss
Result
Voltage drop: about 2.96 V. Percent drop: about 2.47%. Load voltage: about 117.0 V. Conductor loss: about 44.4 W.
Is the result reasonable?
Yes. A 50 ft one-way 12 AWG copper run carrying 15 A on a 120 V circuit lands below a 3% target in this simplified estimate. If the same load were much farther away or on aluminum conductors, the drop would increase and a larger conductor may be needed.
Voltage Drop Circuit Diagram
A voltage drop diagram helps separate the source voltage, conductor run, voltage lost in the wire, and load voltage. The key idea is that voltage is not consumed by distance alone; it is lost because current flows through conductor resistance.
Reference Values for Wire Size, Constants, and Targets
These values are useful for checking whether a result looks plausible. Exact values can vary by conductor type, temperature, insulation rating, and calculation method, so use them as estimating references rather than final design values.
| Reference Item | Typical Value | How It Affects the Result |
|---|---|---|
| Copper \(K\) | About 12.9 | Lower resistance constant, so copper usually has less voltage drop than aluminum for the same size. |
| Aluminum \(K\) | About 21.2 | Higher resistance constant, so aluminum usually requires a larger conductor for the same drop target. |
| 12 AWG area | About 6,530 circular mils | Common branch-circuit wire size; voltage drop can become noticeable on longer 120 V runs. |
| 10 AWG area | About 10,380 circular mils | Larger area reduces drop compared with 12 AWG at the same current and length. |
| 3% target | Common branch-circuit design reference | Often used as a practical limit for satisfactory performance at the load. |
| 5% total target | Common feeder-plus-branch reference | Used to check total voltage drop from source through the final branch circuit. |
Temperature matters
Conductor resistance increases as temperature rises. The simplified \(K\) values are useful for estimating, but actual conductor resistance depends on conductor temperature, installation conditions, material, and published conductor data.
Design Ranges and Practical Voltage Drop Checks
A mathematically correct voltage drop result is not always a complete design answer. The best target depends on equipment sensitivity, load type, system voltage, starting current, and total circuit path.
Branch Circuits
A 3% target is commonly used for branch circuits where maintaining load voltage helps lighting, motors, electronics, and receptacle loads operate properly.
Feeder + Branch
A 5% total target is commonly used for the combined feeder and branch circuit path. Calculate both segments when the source is far upstream.
Low-Voltage Circuits
12 V, 24 V, and 48 V systems are more sensitive to the same voltage loss. Even a few volts can be a large percentage drop.
Motor Loads
Motor starting can draw several times running current, which can cause temporary voltage sag beyond the running voltage drop estimate.
LED and Electronic Loads
Drivers, controls, and electronics may have input-voltage limits. Check manufacturer voltage range, not just a generic percent-drop target.
Long Outdoor Runs
Landscape lighting, gates, pumps, sheds, and outbuildings often need larger conductors because length dominates the calculation.
High Current Circuits
EV chargers, welders, heaters, and large motors can produce significant voltage drop even when the run does not look unusually long.
When voltage drop controls wire size
On short circuits, ampacity often controls conductor size. On long circuits, voltage drop may require a larger conductor than ampacity alone would require. This is why a breaker-size wire table can be insufficient for long runs to pumps, gates, lighting, sheds, motors, or remote equipment.
Unit Conversion Notes
Unit consistency is critical. The calculator can handle common unit selectors, but the underlying circular-mil method must be interpreted correctly.
| Quantity | Common Units | Conversion Reminder |
|---|---|---|
| Voltage | V | Percent drop is based on source voltage: \(\%V_d=V_d/V_s\times100\). |
| Current | A | Use amps. Do not enter milliamps unless the calculator specifically supports mA conversion. |
| Length | ft, m | \(1\,m=3.28084\,ft\). The circular-mil formula is commonly based on feet. |
| Wire area | circular mil, kcmil | \(1\,kcmil=1000\,circular\,mils\). AWG sizes map to circular mil areas. |
| Voltage drop target | % | A 3% target on 120 V equals \(3.6\,V\); a 3% target on 480 V equals \(14.4\,V\). |
Most common unit mistake
The most common mistake is entering round-trip length instead of one-way length. The second most common mistake is using meters while assuming feet, or selecting copper when the installed conductor is aluminum.
Circular-Mil Method vs. Ohm’s Law Method
Voltage drop can be calculated with the circular-mil method or by using conductor resistance and Ohm’s law. Both methods are based on the same physical idea: voltage drop equals current times resistance.
| Method | Best For | Formula Idea | Main Caution |
|---|---|---|---|
| Circular-mil method | Fast wire sizing with AWG/kcmil conductors and one-way length. | \(V_d=\frac{FKIL}{CM}\) | Depends on the chosen \(K\) value and assumes a simplified conductor model. |
| Ohm’s law method | When conductor resistance per foot or per meter is known. | \(V_d=IR\) | Requires the correct total circuit resistance at the operating temperature. |
| Impedance method | Larger AC feeders, power factor effects, and more detailed electrical studies. | Uses resistance, reactance, current, and phase angle. | More accurate for some AC systems but requires more inputs and engineering judgment. |
| Manufacturer or software study | Final design, motor starting, complex distribution, or sensitive equipment. | Uses detailed conductor, load, and system data. | Requires validated inputs and should be reviewed by qualified personnel. |
Why another calculator may not match exactly
Different voltage drop calculators may use different conductor temperature assumptions, \(K\) values, resistance tables, rounding, or AC impedance methods. Small differences are normal. Large differences usually indicate a unit, length, material, or circuit-type mismatch.
Common Mistakes That Cause Wrong Voltage Drop Results
These mistakes can make a voltage drop result look reasonable while still being wrong for the actual circuit.
Common Mistakes
- Entering round-trip conductor length instead of one-way source-to-load length.
- Using breaker size instead of actual load current without understanding the conservative assumption.
- Selecting copper when the installed or planned conductor is aluminum.
- Assuming voltage drop sizing automatically satisfies ampacity requirements.
- Ignoring upstream feeder drop when checking a final branch-circuit load.
- Using a 3% or 5% target as a hard universal rule without checking equipment needs.
Better Practice
- Measure or estimate the one-way distance from the source to the load.
- Use realistic design load current and check continuous-load requirements separately.
- Select the actual conductor material and conductor size.
- Check voltage drop, ampacity, terminal ratings, and protection requirements as separate steps.
- Add feeder and branch voltage drop when evaluating the total voltage at equipment.
- Review manufacturer voltage limits for motors, lighting drivers, electronics, and controls.
Troubleshooting Unexpected Results
If the calculator result looks too high, too low, or physically impossible, start with the inputs before changing the formula.
| Problem | Likely Cause | Fix |
|---|---|---|
| Voltage drop is much higher than expected | Length is too long, current is too high, wire is too small, or round-trip length was entered. | Check one-way length, increase wire size, or reduce current/load distance. |
| Load voltage is too low | Conductor resistance is too high for the source voltage and load current. | Use a larger conductor, shorten the run, reduce load current, or use a higher distribution voltage where appropriate. |
| Minimum wire size seems extremely large | Low-voltage circuit, high current, long distance, or very tight voltage drop target. | Recheck target percent and consider whether the circuit should be redesigned rather than oversized. |
| Maximum length seems too short | The selected conductor is too small for the load current and voltage drop target. | Try a larger wire size or reduce the design current. |
| Copper and aluminum results look similar | Material selection may not have changed, or a unit/input error is masking the difference. | Confirm the conductor material selector and compare the \(K\) values used. |
| Result disagrees with a field measurement | Actual load current, conductor temperature, splices, terminals, harmonics, or utility voltage may differ from assumptions. | Measure source voltage, load voltage, and current at the same operating condition. |
Suspicious result check
A calculated voltage drop greater than the source voltage means the input set is not practical for that circuit. It usually points to a very long run, very high current, very small conductor, or unit entry error.
Assumptions, Sources, and Limitations
This calculator is intended for educational use, preliminary estimating, and early conductor sizing. It uses a simplified conductor voltage drop method and does not replace a complete electrical design.
Formula Assumption
The calculation uses the simplified circular-mil voltage drop method with conductor \(K\) values for copper or aluminum.
Length Assumption
The length input is one-way distance from the source to the load. The formula applies the circuit path factor.
Load Assumption
The load current is assumed to be the current used for the voltage drop check. Starting current and variable loading are not automatically modeled.
Temperature Limitation
Actual conductor resistance changes with temperature. Hot conductors have higher resistance and may produce more voltage drop.
AC Limitation
The simplified method does not fully model conductor reactance, power factor, harmonics, or detailed AC impedance effects.
Final Design Note
Final electrical work should be checked against applicable code, utility requirements, equipment data, conductor ampacity, and qualified professional judgment.
Source and standards note
The commonly cited 3% branch-circuit and 5% total feeder-plus-branch voltage drop values are design recommendations often associated with NEC informational notes, not a universal substitute for enforceable code requirements in every application. Some special systems, sensitive electronic equipment, local amendments, utility rules, or project specifications may impose different requirements. For code context, refer to the official NFPA 70 National Electrical Code page, then verify the adopted code edition, local amendments, equipment data, and project-specific requirements.
Glossary of Voltage Drop Terms
These terms help explain the calculator output and the formulas used in voltage drop checks.
Voltage Drop
The voltage lost as current flows through conductor resistance between the source and the load.
Percent Voltage Drop
Voltage drop divided by source voltage, multiplied by 100. It is the easiest way to compare results across different voltages.
Load Voltage
The estimated voltage available at the equipment after conductor voltage drop is subtracted from source voltage.
Circular Mil
A unit of conductor area used in wire sizing. Larger circular mil area means lower resistance and lower voltage drop.
AWG
American Wire Gauge, a standard wire size system where smaller gauge numbers generally mean larger conductors.
kcmil
Thousand circular mils, used for larger conductors such as 250 kcmil, 500 kcmil, or 1000 kcmil.
Conductor \(K\)
A material resistance constant used in the simplified circular-mil voltage drop formula.
One-Way Length
The distance from source to load. The voltage drop formula applies the appropriate circuit factor for the return path or three-phase relationship.
Frequently Asked Questions
What does a voltage drop calculator calculate?
It estimates voltage lost in a conductor run, percent voltage drop, voltage available at the load, minimum conductor size for a target drop, or maximum one-way run length.
What is the voltage drop formula?
For DC and single-phase circuits, use \(V_d=\frac{2KIL}{CM}\). For balanced three-phase circuits, use \(V_d=\frac{\sqrt{3}KIL}{CM}\).
Do I enter one-way length or round-trip length?
Enter one-way length from the source to the load. The formula factor accounts for the circuit path, so entering round-trip length will usually overstate voltage drop.
What is an acceptable voltage drop?
A common design target is about 3% for a branch circuit and about 5% total for feeder plus branch circuit. The correct target depends on the equipment, application, local requirements, and engineering judgment.
Why is voltage drop worse on 12 V or 24 V circuits?
The same voltage loss is a larger percentage of a low-voltage source. For example, a 1 V loss is only 0.83% on 120 V, but it is 8.33% on 12 V.
Can I use voltage drop to choose the final wire size?
Voltage drop can help estimate conductor size, but final wire selection must also satisfy ampacity, overcurrent protection, insulation, terminal temperature, installation conditions, local code, and equipment requirements.