Voltage drop calculator

What voltage drop is and why it matters

Every conductor carrying current dissipates some of the supply voltage as heat. The longer the wire and the higher the current, the more voltage you lose between the panel and the load. By the time the electricity reaches the motor, compressor, or LED driver at the far end, the voltage is lower than what the panel delivered. If it drops far enough, motors fail to start, heating elements underperform, and electronic ballasts cycle on and off chasing a moving voltage target.

The National Electrical Code does not enforce a voltage-drop limit as hard code, but it does publish recommended limits in two informational notes:

• NEC 210.19(A) Informational Note No. 4: branch circuits should not exceed 3 percent voltage drop.
• NEC 215.2(A)(4) Informational Note No. 2: the combined drop of a feeder plus its branch should not exceed 5 percent.

Informational notes do not have the legal force of "shall" code language, but every utility inspector treats them as standard of care, every insurance underwriter expects them on commercial work, and every motor manufacturer voids warranties for voltage outside +/- 10 percent of nameplate. Treat the 3 percent number as a hard ceiling for any permanent install.

The K-constant formula

The standard NEC voltage-drop formula uses the K-constant method from NFPA 70 (NEC) Chapter 9 Table 8. K represents the DC resistance of one circular mil of conductor across 1,000 feet, measured at 75 C. Two material values cover all standard residential and commercial wire:

• Copper: K = 12.9
• Aluminum: K = 21.2

Aluminum runs about 64 percent higher resistance per circular mil at the same gauge. That is why aluminum service-entrance cable must be upsized two gauges versus copper to carry the same load: 4/0 aluminum replaces 2/0 copper, 2/0 aluminum replaces #1 copper. The formula itself is straightforward:

Single phase: VD = 2 × K × I × L / CM
Three phase: VD = sqrt(3) × K × I × L / CM

Where I is current in amps, L is one-way length in feet, and CM is the conductor's cross-sectional area in circular mils. The factor of 2 in single phase accounts for the round-trip distance: current goes out on the hot, returns on the neutral. The sqrt(3) (about 1.732) in three phase comes from the geometric vector sum: three currents 120 degrees apart cancel in the neutral, so the effective return distance is shorter than two times one-way.

Wire gauge in circular mils

AWG gauges convert to circular mils per NEC Chapter 9 Table 8. Every two AWG sizes roughly doubles the circular mil area, which is why each gauge step represents about a 26 percent change in resistance:

• #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
• #4 AWG: 41,740 CM
• #2 AWG: 66,360 CM
• #1 AWG: 83,690 CM
• 1/0 AWG: 105,600 CM
• 2/0 AWG: 133,100 CM
• 4/0 AWG: 211,600 CM
• 250 kcmil: 250,000 CM
• 500 kcmil: 500,000 CM

Above 4/0 AWG, wire is sized in kcmil (thousand circular mils) rather than gauge numbers. Standard sizes jump 250, 300, 350, 400, 500, 600, 750, and 1,000 kcmil. Most residential work tops out at 4/0; commercial feeders run to 500 kcmil or are paralleled to handle larger loads.

Worked example: a 240 V condenser at the end of a long run

A typical 3-ton air conditioner condenser has an MCA (minimum circuit ampacity) of 26 amps and a MOCP (maximum overcurrent protection) of 40 amps. The wire-size calculator says #10 copper meets the 26 A ampacity at a 75 C terminal. But what if the panel is in the basement and the condenser is on a pad 90 feet away on the far side of the house?

Plug it in: #10 copper (10,380 CM), 90 ft one-way, 26 A load, 240 V, single phase. Voltage drop equals 2 x 12.9 x 26 x 90 / 10,380 = 5.81 V, or 2.42 percent. Passes the 3 percent NEC recommendation. If the same panel-to-pad distance were 130 feet, voltage drop hits 8.39 V or 3.50 percent, fails the limit, and you need to upsize to #8 copper. The calculator above does the math for you and surfaces the recommended gauge directly.

Three calculator modes: drop check, min size, max distance

Voltage drop is one equation with four unknowns: drop percent, wire gauge, load current, and one-way length. Fix any three of those and the fourth falls out. The mode toggle at the top of the calculator picks which one you want to solve for:

• Voltage drop check. Given gauge, length, current, and voltage, returns the drop in percent and volts and tells you whether it passes the NEC 3 percent or 5 percent recommendation. The default mode and what most people want.
• Min conductor size. Given length, current, and voltage, returns the smallest standard AWG that holds drop within the NEC limit. Useful when you know the run length and load but want to pick the cheapest wire that works. Skips the gauge input entirely.
• Max circuit distance. Given gauge, current, and voltage, returns the longest one-way run that still meets the NEC limit. Useful when you are running a single gauge of wire on a spool and want to know how far you can go before voltage drop forces an upsize. Common question for EV chargers, well pumps, and outbuilding feeders.

Each mode solves the same underlying equation (VD = 2 * K * I * L / CM for single phase, sqrt(3) * K * I * L / CM for three phase), just rearranged. The results panel changes based on the mode: drop check shows PASS or FAIL, min size shows the recommended gauge, max distance shows the maximum length in feet.

Power factor for motor loads

The standard NEC voltage-drop formula assumes a purely resistive load (power factor of 1.0). Resistive heating elements, incandescent bulbs, and tungsten loads all behave this way. Motor loads do not. The current drawn by a motor leads or lags the voltage by an angle phi, and the effective resistive component of the voltage drop is reduced by a factor equal to the cosine of phi: the power factor.

Typical residential and light commercial power factors:

• 1.00: resistive loads (heating coils, baseboard heat, incandescent lighting)
• 0.95: modern ECM blower motors, variable-speed compressors with internal PFC, LED lighting drivers
• 0.85: typical residential air conditioning compressor at design load. This is the value the calculator uses when you select "typical HVAC compressor."
• 0.75: older induction motors at part load, fluorescent ballasts without correction caps

Applying a 0.85 power factor to a 5 percent resistive-only drop yields 4.25 percent actual drop on the motor circuit. The calculator multiplies the resistive voltage drop by the power factor per the IEEE Std 141 simplified form, which is the standard approach when conductor reactance is not separately modeled. For very large feeders above 1/0 AWG with significant cable reactance, NEC Chapter 9 Table 9 provides effective Z (impedance) values that combine both resistance and reactance; the K method understates drop by 5 to 8 percent at those sizes.

Terminal temperature rating: residential vs commercial

The K-constant in the voltage-drop formula is the conductor's resistance at a specific operating temperature. NEC Chapter 9 Table 8 publishes resistance at 60, 75, and 90 C. The right value depends on the terminal rating of the equipment at both ends of the wire:

• Residential 60 C: Most residential breakers, receptacles, and standard distribution panels are rated for 60 C conductor termination per NEC 110.14(C)(1). The K-factor is 12.1 for copper and 19.9 for aluminum.
• Commercial 75 C: Most commercial equipment terminals are listed for 75 C, including HVAC condensers, motor starters, and panelboards above 100 A. K = 12.9 for copper, 21.2 for aluminum. The most-cited reference values.
• Commercial 90 C: Equipment rated for higher operating temperatures, plus continuous-duty conditions where the conductor is expected to run hot. K = 13.6 for copper, 22.5 for aluminum.

The difference between 60 C and 90 C terminal ratings shifts the K value by about 6 percent each way from the 75 C reference. For typical residential and HVAC work, the effect on voltage drop is small (well under 1 percentage point in most cases), but the building mode dropdown lets you match the actual terminal rating of your equipment when accuracy matters.

What happens when voltage drop is ignored

Equipment manufacturers design for nameplate voltage plus or minus 10 percent. NEMA MG-1 (the motor standard) tightens that further for motors specifically: voltage at the motor terminals must stay between 90 percent and 110 percent of nameplate. Three things go wrong when voltage drop pushes the load voltage below 90 percent of nominal:

• Motor starting current spikes. A compressor that draws 100 LRA at nameplate voltage draws closer to 120 LRA at 90 percent voltage, because torque is proportional to voltage squared. The starter winding heats up faster, the overload kicks out, and the unit fails to start.
• Running current increases. Same compressor running at low voltage pulls more amps to deliver the same mechanical horsepower. Operating temperature climbs, winding insulation degrades 50 percent faster per 10 C of overtemperature (Arrhenius rule), and the motor dies years before its rated life.
• Electronic ballasts and drivers cycle. LED drivers, ECM blower modules, and electronic ballasts have a minimum bus voltage below which they shut down for self-protection. When inrush load briefly drops voltage below the threshold, the driver dies, comes back, dies again. The user sees blinking lights or a furnace blower that hunts.

The 3 percent NEC limit exists because at 3 percent drop you still have motor terminal voltage within 5 percent of nominal (panel typically runs at 95 to 100 percent of nominal; subtract 3 percent and you are at 92 to 97 percent, still inside the NEMA window). Push past 5 percent and you are out of the NEMA window even with a healthy supply.

How this calculator pairs with the wire-size calculator

Wire sizing for HVAC and electrical work is a two-step process:

1. Ampacity first. The wire-size calculator picks the smallest gauge that meets NEC 310.16 ampacity at the terminal temperature rating. For an HVAC condenser with 26 A MCA and 75 C terminals, that is #10 copper.
2. Voltage drop second. This calculator verifies the ampacity-selected wire still meets the 3 percent limit at the install distance. If not, upsize until it does.

The wire-size calculator on this site does both steps in one pass for HVAC condensers. The voltage-drop calculator here is more general: any branch or feeder, any voltage, any phase. Use this one when you are designing electric vehicle charging circuits, sub-panels, well pumps, long-run lighting circuits, or anything that does not fit the HVAC-specific assumptions in the wire-size tool.

Three-phase voltage drop and why sqrt(3) shows up

Three-phase electrical systems carry power on three conductors that are 120 electrical degrees apart. In a balanced load, the current returning on each phase is offset enough in time that the geometric vector sum at the neutral cancels. The result: voltage drop on a three-phase circuit is lower than on a single-phase circuit at the same gauge, length, and line current, by a factor of sqrt(3) / 2 (about 86 percent).

A 480 V three-phase 100 A feeder of 2/0 copper across 300 feet drops 5.04 V or 1.05 percent. The same wire and length carrying 100 A single-phase 240 V drops 8.72 V or 3.63 percent (and fails the 3 percent limit). Three-phase is the standard delivery method for commercial and industrial work because it allows smaller conductors for the same load and distance. Residential is single-phase because most residential transformers are single-phase 120/240 V split-phase.

Common voltage-drop pitfalls

• Using nominal voltage instead of measured voltage. The 240 V on the spec sheet is nominal. Actual service voltage at the panel can range 230 V to 250 V depending on transformer loading. Run the calculation at nominal for design; verify at install with a meter.
• Forgetting voltage drop on the neutral. Single-phase voltage drop counts both hot and neutral conductors (the factor of 2). Three-phase balanced loads have minimal neutral current. Unbalanced three-phase or shared-neutral multiwire circuits need separate analysis.
• Mixing AC reactance into DC resistance. The K-constant method uses DC resistance, which is accurate enough for residential 60 Hz on copper or aluminum at wire sizes below 1/0. For larger conductors above 1/0 AWG with significant reactance, NEC Chapter 9 Table 9 gives effective Z (impedance) values that are slightly higher than pure resistance. For typical HVAC and feeder work, the DC method understates drop by less than 5 percent and is the standard the trade uses.
• Ignoring temperature. Conductor resistance climbs about 0.4 percent per degree C above the 75 C reference. A wire run that passes voltage drop at room temperature may fail at 90 C operating temperature inside a hot conduit. NEC ampacity tables already bake in 75 C; voltage drop calculations conservatively use the same reference.