Voltage Drop Calculator

The voltage drop formula VD = (2 × K × I × L) / CM calculates drop in volts, where:

• K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C)
• I is load current in amps
• L is one-way circuit length in feet
• CM is the conductor cross-sectional area in circular mils

For three-phase circuits, replace the factor 2 with 1.732.

A 20-amp circuit using 12 AWG copper (6530 CM) over 100 feet: VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9V, which is 6.6% on a 120V circuit — exceeding the 3% recommendation. Upsizing to 10 AWG (10380 CM) drops this to 5.0V (4.1%), and 8 AWG (16510 CM) achieves 3.1V (2.6%).

Long runs almost always require upsizing beyond the minimum ampacity-rated conductor.

NEC ampacity tables (Table 310.16) specify minimum wire sizes based on current carrying capacity, but voltage drop often demands larger conductors for runs exceeding 50-100 feet.

Common scenarios requiring upsizing:

• garage subpanels (typical 50-100 foot runs need 4 AWG or larger for 60A service)
• well pumps (200-500 foot runs may need 6 AWG for a 10A 240V pump)
• outdoor lighting circuits (long landscape lighting runs)
• agricultural buildings

A practical sizing table for 120V, 20A circuits with 3% max drop:

• up to 50 feet use 12 AWG
• 50-80 feet use 10 AWG
• 80-120 feet use 8 AWG
• 120-200 feet use 6 AWG

For 240V circuits, the same wire sizes support double the distance because the percentage drop is halved by the higher voltage.

Motors are particularly sensitive to voltage drop. A motor receiving 5% below rated voltage draws approximately 5% more current (to maintain power output), increasing winding temperature by 10-15% and reducing insulation life. At 10% voltage drop, motor torque decreases by approximately 19% (torque varies as voltage squared), potentially causing failure to start under load. Variable frequency drives (VFDs) are somewhat more tolerant but may fault on low voltage input.

From an energy cost perspective, voltage drop represents pure waste — power dissipated as heat in conductors equals I²R. A 100-foot run of 12 AWG carrying 20A continuously wastes approximately 158 watts (7.9V × 20A), costing $140 per year at $0.12/kWh. Upgrading to 10 AWG reduces this waste by 37%, with the wire cost difference typically paying back within 2-3 years through energy savings.

To calculate voltage drop for a single-phase circuit, use VD = (2 × K × I × L) / CM, where:

• K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75 degrees C)
• I is current in amperes
• L is one-way length in feet
• CM is the conductor area in circular mils

The factor of 2 accounts for both the outgoing and returning conductor. For three-phase circuits, replace 2 with the square root of 3 (1.732).

Example: a 20 A load on 12 AWG copper (6,530 CM) over 80 feet gives VD = (2 × 12.9 × 20 × 80) / 6,530 = 6.32 V.

This is rooted in Ohm's law (V = IR) — the same relationship our ohms law calculator applies — as explained by Georgia State University's HyperPhysics.

Voltage drop is measured in volts (V), the SI unit of electric potential difference, defined by BIPM as one joule per coulomb. Current uses the ampere (A), a base SI unit, and resistance uses the ohm, equal to one volt per ampere.

Conductor cross-section in US practice is given in circular mils (CM), where one circular mil is the area of a circle 0.001 inch in diameter; 1,000 CM equals one kcmil. In metric systems, cross-section is in square millimeters and resistivity in ohm-metres.

NIST maintains the definitions of these electrical units, and IEC standards harmonize their international usage in wiring specifications.

Voltage drop is directly proportional to conductor length: doubling the run doubles the drop, all else equal. Because the formula uses one-way length L but includes a factor of 2 (or 1.732 for three-phase) for the return path, the total copper distance current travels is what matters.

A 12 AWG copper run of 50 feet at 20 A drops about 3.95 V, while the same wire at 100 feet drops about 7.9 V.

This linear relationship is why long circuits, such as well pumps, RV parks, and outdoor lighting, so often demand upsized conductors. Khan Academy notes the same proportionality in resistance, since R rises linearly with conductor length for a fixed cross-section.

Copper conducts electricity better than aluminum, so it produces less voltage drop for the same wire size. Aluminum's resistivity gives it a K value of roughly 21.2 versus 12.9 for copper, meaning aluminum drops about 64% more voltage in an identical conductor.

To compensate, aluminum wiring is typically sized one to two AWG steps larger; for example, a circuit calling for 6 AWG copper often uses 4 AWG aluminum.

Aluminum is lighter and cheaper per ampere, which is why utilities favor it for large feeders and service entrances. Encyclopaedia Britannica lists silver and copper as the best common conductors, with aluminum close behind, per their published resistivity values.

Three-phase circuits use the multiplier 1.732 (the square root of 3) instead of 2 in the voltage drop formula, so for the same current, length, and wire size they drop about 13.4% less voltage than single-phase circuits.

More importantly, three-phase distribution delivers more power at a given line current, so conductors carry less amperage for the same load, further reducing I times R losses.

This is why commercial and industrial systems, motors, and large HVAC equipment favor three-phase power. The relationship follows from balanced three-phase theory described in IEEE and IEC electrical engineering standards, where line-to-line voltages combine at 120-degree phase angles.

Voltage drop analysis is essential in many practical settings.

• Electricians size feeders to garage subpanels and detached buildings where 50 to 150 foot runs would otherwise starve loads.
• Solar installers calculate drop on DC and AV cable runs to preserve array output, and RV and marine technicians work with low-voltage 12 V and 24 V systems where even small drops are significant percentages.
• Automotive and battery bank designers use it for winch and inverter wiring.
• Agricultural and irrigation pumps, landscape lighting, and EV charger circuits all rely on the same math.

The NEC provides voltage drop recommendations (3% branch, 5% total) that guide these decisions across residential, commercial, and industrial projects.

The most frequent error is using total round-trip length instead of one-way length L, which double-counts distance because the formula already includes the factor of 2.

• Another mistake is applying the single-phase factor of 2 to a three-phase circuit rather than 1.732.
• Many people also forget to use the correct K value for aluminum (21.2) versus copper (12.9), or ignore that resistance rises with temperature, so hot conductors drop more voltage than cold ones.
• Confusing wire gauge with circular mils, or mixing metric and imperial units, produces large errors.

Finally, treating NEC's 3% and 5% figures as hard code requirements rather than recommendations is a common misconception, as noted in NEC informational notes.

The most effective way to reduce voltage drop is to increase conductor size, since drop is inversely proportional to circular mils; upsizing from 12 AWG (6,530 CM) to 10 AWG (10,380 CM) cuts drop by about 37%.

Shortening the run, using copper instead of aluminum, and raising system voltage all help, because at higher voltage the same absolute drop is a smaller percentage. For example, moving a load from 120 V to 240 V halves the percentage drop for identical wiring.

Splitting a heavy load across parallel circuits reduces current per conductor.

HyperPhysics and Khan Academy both illustrate how each of these levers changes the I times R loss in a conductor.