A voltage drop calculator for wire computes the voltage lost between the source electrical panel and the load equipment based on wire gauge, material, current, distance, and voltage. NEC informational notes recommend voltage drop under 3 percent for branch circuits and feeders individually, with combined drop not exceeding 5 percent. This article provides an interactive voltage drop calculator, explains the formula and NEC recommendations, compares copper and aluminum voltage drop characteristics, provides circular mils reference charts, distinguishes voltage drop from ampacity requirements, includes distance tables for common wire sizes, and covers practical applications for wire size selection in residential and commercial installations.

Across thousands of Southwire and Cerrowire orders shipped from IB Lighting nationwide, our electrician-led team advises DIY installers and licensed electricians on wire sizing decisions including the critical voltage drop analysis that determines whether a code-minimum wire size is actually adequate for the specific installation distance. Founded in 2013 by a licensed Master Electrician with field experience across DTE Energy and Consumers Energy projects in Michigan, IB Lighting resells the full Southwire and Cerrowire electrical wire and cable lineup.

This article provides the voltage drop calculator for wire that we use internally to help buyers verify wire size decisions for their specific installations, using formulas and recommendations aligned with the current National Electrical Code (NFPA 70) and industry practice from the International Association of Electrical Inspectors.

The Fast Answer: What a Voltage Drop Calculator Does

A voltage drop calculator for wire takes five inputs (wire gauge, material, current, one-way distance, source voltage) and returns the voltage drop in both volts and percentage of source voltage. The calculation reveals whether the wire size is adequate for the distance under the specific load conditions. If voltage drop exceeds 3 percent, the wire size should be upgraded to the next larger size to maintain acceptable voltage at the load. Combined feeder plus branch circuit voltage drop should not exceed 5 percent total per NEC informational notes.

Why voltage drop matters

Voltage drop reduces the voltage available at the load equipment below the nominal source voltage. Equipment designed for 240 volts operating at 232 volts (3.3 percent drop) or 228 volts (5 percent drop) experiences reduced performance, increased current draw to maintain output, additional heating in windings, and potential protective function activation. Modern equipment including motors, EV chargers, HVAC compressors, and electronic devices all specify minimum operating voltage requirements.

When voltage drop calculation is critical

Voltage drop calculation is most critical for long circuit runs, high-current continuous loads, motor circuits with high inrush current, distant subpanels serving detached structures, and installations near the maximum ampacity of the selected wire size. Short residential branch circuits (under 30 feet) rarely have voltage drop issues, but longer runs to garages, sheds, well pumps, and outdoor equipment often approach or exceed NEC voltage drop recommendations.

Interactive Voltage Drop Calculator for Wire

Use the calculator below to check voltage drop for your specific installation. Enter your wire gauge, material, current in amps, one-way distance in feet, and source voltage. The calculator returns voltage drop in volts and percentage, with pass/fail indicators showing whether the installation meets NEC 3 percent voltage drop recommendations.

The Voltage Drop Formula Explained

Understanding the voltage drop formula clarifies how the voltage drop calculator for wire produces its results and helps electricians perform manual calculations when a calculator is not available. The formula is the mathematical foundation of every voltage drop calculator for wire.

The single-phase voltage drop formula

The single-phase voltage drop formula is: VD = (2 × K × I × L) / CM. VD is the voltage drop in volts. The factor of 2 accounts for the current flowing through both the hot conductor to the load and back through the return conductor (either neutral or the second hot in 240V circuits). K is the material constant (12.9 for copper, 21.2 for aluminum). I is the load current in amps. L is the one-way distance from source to load in feet. CM is the circular mils of the conductor cross-section.

The three-phase voltage drop formula

The three-phase voltage drop formula uses a multiplier of 1.732 (the square root of 3) instead of 2: VD = (1.732 × K × I × L) / CM. This reflects the different mathematical relationship between line and phase currents in three-phase systems. Three-phase voltage drop calculations apply primarily to commercial and industrial installations rather than typical residential single-phase circuits.

The K value for copper vs aluminum

The K value represents the resistance of one circular mil of the conductor material for a one-foot length at typical operating temperature. Copper K value is 12.9 ohm-cmil per foot. Aluminum K value is 21.2 ohm-cmil per foot. The higher aluminum K value reflects aluminum's higher electrical resistance compared to copper, which is why aluminum conductors have approximately 60 percent higher voltage drop than copper conductors of the same gauge.

Converting voltage drop to percentage

Voltage drop percentage is calculated as: VD% = (VD / Source Voltage) × 100. For a 240 volt source with 6 volts of voltage drop, the drop percentage is (6 / 240) × 100 = 2.5 percent. This calculation converts the absolute voltage drop into a proportional measure that is easier to compare against the 3 percent NEC recommendation regardless of the specific circuit voltage.

Why NEC Recommends 3 Percent Maximum Voltage Drop

The 3 percent voltage drop recommendation appears in NEC informational notes rather than mandatory code requirements, but it represents best practice for reliable equipment operation.

NEC 210.19(A)(1) informational note

NEC 210.19(A)(1) informational note recommends that branch circuit conductors be sized to prevent voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations thereof. The note specifies that combined feeder plus branch circuit voltage drop should not exceed 5 percent to provide reasonable efficiency of operation.

NEC 215.2(A)(1)(b) informational note

NEC 215.2(A)(1)(b) informational note provides similar guidance for feeder circuits: feeder conductors should be sized to prevent voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads. Combined with the branch circuit 3 percent recommendation, this produces the 5 percent combined limit that maintains adequate voltage at connected loads.

Why 3 percent is the target

The 3 percent recommendation balances practical wire sizing against equipment operation requirements. At 3 percent voltage drop, the load voltage remains within the operational range of virtually all electrical equipment. Higher voltage drops cause progressively worse performance, and drops exceeding 5 to 10 percent can cause equipment failure or protective function activation on modern electronics.

Why the recommendation is not mandatory

Voltage drop recommendations are informational rather than mandatory because they involve equipment operation performance rather than direct safety hazards. NEC mandatory requirements focus on preventing electrical fires and shock hazards. Voltage drop primarily affects equipment performance, energy efficiency, and equipment longevity, which are considered engineering considerations rather than fundamental safety issues.

Copper vs Aluminum Voltage Drop Differences

The voltage drop calculator for wire produces significantly different results depending on whether copper or aluminum conductors are selected because these materials have different electrical resistance characteristics.

Aluminum has higher resistance than copper

Aluminum has approximately 60 percent higher electrical resistance than copper for the same conductor gauge. This is reflected in the K values used in the voltage drop formula: copper K is 12.9 while aluminum K is 21.2. For identical wire size, current, and distance, aluminum voltage drop is approximately 64 percent higher than copper voltage drop.

Aluminum wire size compensation

To match copper voltage drop performance, aluminum conductors typically require the next larger wire size. For example, 6 AWG copper voltage drop approximately equals 4 AWG aluminum voltage drop for the same current and distance. This is why aluminum SER cable configurations at typical residential feeder sizes use one gauge larger conductors than the equivalent copper SER.

When aluminum voltage drop matters most

Aluminum voltage drop matters most for long installations where the cost savings of aluminum are significant enough to justify the material choice despite the voltage drop disadvantage. For short runs, the voltage drop difference between copper and aluminum may not be significant enough to affect wire size selection. For long runs, the required aluminum wire size upgrade partially offsets the material cost savings.

Practical cost-benefit analysis

Aluminum wire costs approximately 40-60 percent less per foot than equivalent copper wire, but the required larger wire size for aluminum installations reduces this savings. A 100 amp copper feeder using 3 AWG might cost equivalent to aluminum 1/0 to achieve the same voltage drop performance. The actual cost savings depends on specific wire prices and installation length.

Circular Mils Reference Chart for All Common Wire Sizes

The circular mils values used in voltage drop calculations correspond to specific wire gauge sizes according to the American Wire Gauge (AWG) system.

Circular mils by AWG size

Circular mils for common wire sizes: 14 AWG has 4,110 CM, 12 AWG has 6,530 CM, 10 AWG has 10,380 CM, 8 AWG has 16,510 CM, 6 AWG has 26,240 CM, 4 AWG has 41,740 CM, 3 AWG has 52,620 CM, 2 AWG has 66,360 CM, 1 AWG has 83,690 CM, 1/0 AWG has 105,600 CM, 2/0 AWG has 133,100 CM, 3/0 AWG has 167,800 CM, and 4/0 AWG has 211,600 CM.

The AWG numbering system

The AWG numbering system uses decreasing numbers for increasing wire size up to 1 AWG, then transitions to "aught" sizes (1/0, 2/0, 3/0, 4/0) that continue increasing in size beyond 1 AWG. Above 4/0 AWG, wire sizes are specified directly in circular mils or in thousand circular mils (kcmil) with common sizes including 250 kcmil, 350 kcmil, 500 kcmil, and larger.

The doubling relationship

Wire cross-sectional area (measured in circular mils) approximately doubles for every three AWG size steps. For example, 14 AWG (4,110 CM), 11 AWG (approximately 8,200 CM), and 8 AWG (16,510 CM) form a doubling sequence. This means voltage drop is halved when upgrading three AWG sizes for the same load and distance.

Using CM in the formula

The circular mils value substitutes directly into the voltage drop formula. Larger CM values produce smaller voltage drop results because the CM appears in the denominator. Doubling the CM (going up three AWG sizes) halves the voltage drop. This mathematical relationship is why upgrading from 10 AWG (10,380 CM) to 8 AWG (16,510 CM) reduces voltage drop by approximately 37 percent for the same load and distance.

Voltage Drop vs Circuit Ampacity: Two Different Concepts

Voltage drop and circuit ampacity are two separate concepts that must both be satisfied when selecting wire size, and understanding the distinction prevents common wire sizing mistakes.

Circuit ampacity determines minimum wire size

Circuit ampacity is the maximum current the wire can safely carry continuously without exceeding its temperature rating. NEC Table 310.16 provides ampacity values for various wire sizes and insulation temperature ratings. For a 30 amp circuit, the wire must have at least 30 amp ampacity. This is a mandatory NEC requirement based on safety.

Voltage drop determines whether wire size is adequate for distance

Voltage drop is calculated separately from ampacity to determine whether the code-minimum wire size is adequate for the specific installation distance. A wire size that meets ampacity requirements may not provide acceptable voltage drop for a long installation. This is a performance consideration rather than a safety requirement, but it significantly affects equipment operation.

Both must be satisfied for good installation

Good wire selection satisfies both ampacity (safety) and voltage drop (performance) requirements. The wire size that meets both requirements is the correct selection. In short installations, ampacity typically drives wire size selection because voltage drop is negligible. In long installations, voltage drop often drives wire size selection above what ampacity alone would require.

Example: 10 AWG copper for different distances

10 AWG copper wire has 30 amp ampacity (satisfies ampacity for a 30 amp circuit). For a 30 amp load at 240 volts, 10 AWG copper provides adequate voltage drop performance up to approximately 100 feet one-way. Beyond 100 feet, voltage drop exceeds 3 percent and upgrade to 8 AWG becomes appropriate even though ampacity does not require the upgrade.

Common Wire Distance Tables at 3 Percent Voltage Drop

Distance tables provide quick reference for common wire size and load combinations to determine maximum acceptable run length at 3 percent voltage drop.

240 volt copper wire distance at 3 percent drop

Maximum one-way distance for 3 percent voltage drop at 240 volts with copper wire: 14 AWG at 15 amps approximately 50 feet, 12 AWG at 20 amps approximately 60 feet, 10 AWG at 30 amps approximately 65 feet, 8 AWG at 40 amps approximately 80 feet, 6 AWG at 50 amps approximately 100 feet, 4 AWG at 70 amps approximately 110 feet, 2 AWG at 100 amps approximately 120 feet.

120 volt copper wire distance at 3 percent drop

At 120 volts, voltage drop percentage is doubled for the same volts of drop, so acceptable distances are halved compared to 240 volts. Maximum one-way distance for 3 percent voltage drop at 120 volts with copper wire: 14 AWG at 15 amps approximately 25 feet, 12 AWG at 20 amps approximately 30 feet, 10 AWG at 30 amps approximately 32 feet, 8 AWG at 40 amps approximately 40 feet, 6 AWG at 50 amps approximately 50 feet.

240 volt aluminum wire distance at 3 percent drop

Aluminum wire distances at 3 percent drop are approximately 60 percent of copper distances for the same wire size due to higher aluminum resistance. Maximum one-way distance for 3 percent voltage drop at 240 volts with aluminum wire: 6 AWG at 50 amps approximately 60 feet, 4 AWG at 70 amps approximately 68 feet, 2 AWG at 100 amps approximately 75 feet, 1/0 AWG at 125 amps approximately 100 feet, 4/0 AWG at 200 amps approximately 130 feet.

Using distance tables for planning

Distance tables provide quick approximations for wire size selection during initial planning. For precise voltage drop verification, use the calculator with actual installation values. The tables assume standard operating conditions and provide approximate values that may vary slightly with different wire temperature ratings and installation conditions.

Practical Applications of the Voltage Drop Calculator

The voltage drop calculator for wire has practical applications across all residential and commercial electrical installations where wire size decisions must balance code compliance, cost, and performance. Using a voltage drop calculator for wire before purchasing prevents costly wire size mistakes.

Long branch circuits to detached structures

Feeder circuits from the main electrical panel to detached garages, sheds, and workshops often approach or exceed voltage drop limits at standard NEC minimum wire sizes. Use the calculator to verify wire size adequacy for the specific distance before purchasing. For our 100 amp garage subpanel wire sizing article, voltage drop typically drives wire size selection above ampacity minimum for runs over 100 feet.

Well pump circuits with long submersible cable runs

Well pump circuits are notable for voltage drop being the primary wire sizing consideration rather than ampacity. Deep wells and remote well locations often require wire sizes significantly larger than motor ampacity alone would suggest. See our well pump wire sizing article for distance-driven sizing tables specific to well pump applications.

EV charger installations at distant locations

EV charger installations at 48 amp continuous current experience meaningful voltage drop on long runs. The 6 AWG copper minimum wire size for 48 amp charging typically supports runs up to 80-90 feet at 3 percent voltage drop, with upgrades to 4 AWG needed for longer runs. Our Level 2 EV charger wire sizing article covers voltage drop considerations for typical EV charger installations.

HVAC compressor circuits with motor starting inrush

Central AC and heat pump circuits require voltage drop analysis that accounts for motor starting inrush current, not just running current. Excessive voltage drop during motor starting can cause motor stall and nuisance breaker tripping. Our 4 ton central AC wire sizing article addresses voltage drop considerations for HVAC installations.

Voltage Drop Impact on Motor Starting

Motor starting represents the most challenging voltage drop scenario because motors draw significantly higher current during startup than during normal operation.

Motor starting inrush current

Electric motors typically draw 4 to 8 times their normal running current for the first few seconds of startup as the motor accelerates from stopped to running speed. This inrush current causes proportionally higher voltage drop during starting than during normal running operation. A motor drawing 20 amps running might draw 100 to 160 amps briefly at startup.

Starting voltage drop calculation

Voltage drop during motor starting is calculated using the inrush current instead of running current in the standard formula. For a 20 amp running motor with 6 times inrush (120 amp starting), the voltage drop during starting is 6 times the running voltage drop. A running voltage drop of 2 percent becomes 12 percent during starting.

Why starting voltage drop matters

Excessive starting voltage drop can cause the motor to stall (fail to reach running speed), trip protective devices due to prolonged high current, generate excessive heat in the motor windings, and shorten motor lifespan through repeated starting difficulties. Motors typically require voltage at the motor terminals above 85-90 percent of nameplate voltage during starting to reliably reach running speed.

Wire sizing for motor circuits

NEC Article 430 provides specific requirements for motor circuit conductor sizing that account for both ampacity and voltage drop considerations. For long motor circuits, use the voltage drop calculator with both running current and estimated inrush current to verify acceptable voltage drop under both conditions. Well pump circuits are the most demanding residential motor circuit application because of the combination of high inrush and long distances.

Effects of Excessive Voltage Drop on Electrical Equipment

Excessive voltage drop causes measurable performance degradation across different types of electrical equipment.

Motor performance degradation

Motors operating at reduced voltage produce less torque, run at slower speed, draw higher current to maintain output, generate more heat in windings, and experience reduced operating lifespan. A motor operating at 90 percent of nameplate voltage produces approximately 81 percent of nameplate torque (torque scales with voltage squared). Prolonged operation at reduced voltage causes progressive motor damage.

Electronic equipment protection functions

Modern electronic equipment including EV chargers, HVAC electronic controls, and computer power supplies include undervoltage protection functions that shut down or reduce output when supply voltage drops below rated minimums. EV chargers may reduce charging current when voltage drops significantly. Some electronic equipment produces error codes or refuses to operate at low voltage.

Incandescent and older lighting

Incandescent lighting output decreases significantly with reduced voltage. A 5 percent voltage reduction causes approximately 16 percent light output reduction. Modern LED and fluorescent lighting are less affected by voltage variation but still perform best at nominal voltage.

Resistance heating elements

Resistance heating elements (electric water heaters, electric dryer elements, oven heating elements) produce heat proportional to voltage squared. A 5 percent voltage reduction causes approximately 10 percent heat output reduction. This affects appliance performance including longer heating times and reduced maximum temperatures.

Selecting the Right Wire Size Based on Voltage Drop Calculation

Understanding voltage drop calculator results helps buyers select the right wire products for their specific installation requirements.

When ampacity-minimum wire is adequate

For short residential branch circuits under 30 feet with typical residential loads, the NEC ampacity-minimum wire size is typically adequate for voltage drop performance. Standard NM-B cable in the appropriate size (14 AWG for 15 amp, 12 AWG for 20 amp, 10 AWG for 30 amp) provides good voltage drop performance for typical distances.

When wire size upgrade is beneficial

For circuits approaching 100 feet or longer, calculate voltage drop specifically before purchasing wire. If the calculator shows voltage drop above 3 percent, upgrading to the next larger wire size typically brings the drop back within acceptable range. The wire cost difference between adjacent gauges is modest compared to the performance improvement.

Southwire and Cerrowire product selection

Southwire and Cerrowire manufacture the full range of wire sizes needed for various voltage drop scenarios. Common sizes for residential installations include NM-B in 14, 12, 10, 8, and 6 AWG; THHN and THWN-2 in the same sizes plus larger; UF-B for direct burial applications; and SER for feeder circuits. All sizes are available through IB Lighting for nationwide delivery.

Copper vs aluminum selection

Copper is preferred for most residential branch circuits due to smaller wire size and easier termination. Aluminum can provide cost savings for large residential feeder circuits (100 amp subpanel feeders and larger) where the voltage drop disadvantage is manageable with the appropriate size upgrade. The voltage drop calculator helps compare copper and aluminum options for specific installations.

5 Common Mistakes When Using a Voltage Drop Calculator

The mistakes below cause the most frequent inaccurate voltage drop calculations that lead to installation problems when using a voltage drop calculator for wire.

Mistake 1: Using round-trip distance instead of one-way distance

The voltage drop formula includes the factor of 2 (or 1.732 for three-phase) to account for the return conductor path. This means the L value in the formula is the one-way distance from source to load, not the round-trip distance. Some users incorrectly enter round-trip distance, which produces a voltage drop calculation twice the actual value.

Mistake 2: Ignoring motor inrush current for motor circuits

Voltage drop for motor circuits must be calculated with both running current (for continuous operation) and inrush current (for starting) to verify adequate performance in both scenarios. Some users only calculate running voltage drop and miss potential motor starting issues due to excessive starting voltage drop.

Mistake 3: Confusing voltage drop with ampacity requirements

Voltage drop and ampacity are separate wire sizing considerations. Passing voltage drop calculation does not mean the wire size meets ampacity requirements. Both must be verified independently. Some users select smaller wire based on voltage drop calculation but fail to meet minimum ampacity requirements for the circuit breaker size.

Mistake 4: Using nominal voltage instead of actual source voltage

Real-world voltage from utility supplies varies from nominal. The 240 volt nominal residential supply may actually deliver 235 to 245 volts depending on distance from utility transformer and time of day. Using actual measured voltage provides more accurate voltage drop calculations than assuming nominal values, though nominal values are acceptable for planning purposes.

Mistake 5: Not accounting for temperature and installation conditions

The voltage drop formula uses simplified K values that assume standard operating temperature. Wire resistance increases with temperature, so hot conductors have higher resistance and higher voltage drop than cold conductors. For extreme installation conditions (high ambient temperatures, tight conduit fills, direct sunlight exposure), voltage drop may exceed the calculator estimates.

Conclusion

A voltage drop calculator for wire is an essential tool for verifying that the selected wire size provides acceptable voltage at the load equipment based on the specific installation distance and current. The calculator uses the formula VD = (2 × K × I × L) / CM for single-phase circuits with different K values for copper (12.9) and aluminum (21.2) conductors. NEC informational notes recommend voltage drop under 3 percent for branch circuits and feeders individually with combined drop not exceeding 5 percent.

Voltage drop and circuit ampacity are two separate wire sizing considerations that must both be satisfied. Ampacity determines the minimum wire size for safe current handling per NEC Table 310.16, while voltage drop determines whether that wire size is adequate for the distance. For short circuits with typical loads, ampacity-minimum wire is usually adequate for voltage drop. For long circuits, high-current continuous loads, motor circuits, and feeder circuits to detached structures, use the voltage drop calculator to verify wire size adequacy before purchase.

This article was last reviewed and updated in June 2026 to reflect current National Electrical Code recommendations and voltage drop calculation methodology. The voltage drop calculator uses simplified K values that assume standard operating temperature and provides approximations suitable for typical installations. For critical installations or unusual conditions, consult a licensed electrician or perform detailed engineering analysis. NEC voltage drop recommendations are in informational notes rather than mandatory code requirements.