How is voltage drop calculated?

Voltage drop is calculated from current, conductor resistance, and a phase factor. For DC and AC single-phase circuits the formula is V_drop = 2 × I × R, where the 2 accounts for the round-trip path. For three-phase circuits the formula becomes V_drop = √3 × I × (R × PF + X × sin θ), where R is the one-way resistance, X is the reactance, PF is the power factor, and sin θ = √(1 − PF²). Resistance values come from NEC Chapter 9 Table 9 — this calculator uses those values automatically when you pick an AWG size and conduit type.

What causes voltage drop?

Three primary factors drive voltage drop: (1) conductor resistance, which rises with length and operating temperature and falls with cross-sectional area; (2) conductor reactance, which depends on cable geometry and conduit material (magnetic steel raises X; PVC lowers it); and (3) the load current itself. On AC circuits, power factor also affects the size of the reactive contribution. Higher temperature, longer runs, smaller wire, larger current, and lower power factor all increase drop.

What is an acceptable voltage drop?

NEC Informational Note 4 of Article 210 recommends voltage drop of no more than 3% on a branch circuit, and NEC Informational Note 2 of Article 215 recommends no more than 5% combined on feeder plus branch. NEC 647.4 imposes a stricter 1.5% target for sensitive electronic equipment, and NEC 690.45 typically holds solar PV circuits to 2% for efficiency. These are non-mandatory guidelines, but the industry treats them as a working ceiling — and most inspectors flag installations that exceed them.

Does wire size affect voltage drop?

Yes — substantially. Wire resistance is inversely proportional to cross-sectional area, so doubling the cross-section roughly halves voltage drop at constant current. Stepping from 12 AWG to 10 AWG cuts resistance by about 37%; 10 AWG to 8 AWG cuts another 37%, and so on. This is the most cost-effective lever for reducing drop on long runs — upsizing copper one or two AWG sizes is almost always cheaper than the lifetime energy loss of an undersized circuit.

Copper vs aluminum wiring?

Copper has about 1.6× the electrical conductivity of aluminum, so the same drop at the same current requires aluminum with roughly 1.6× the cross-section. Copper costs more per foot but terminates more reliably and resists thermal cycling, which is why it dominates branch circuits, motor feeders, and termination-dense panels. Aluminum becomes cost-competitive at large gauges (4/0 and up) and is standard for service entrances and overhead distribution where the savings outweigh the larger cable diameter.

Why is voltage drop important?

Excessive voltage drop wastes energy (loss = I² × R), reduces equipment performance (motors lose torque, lights dim, heaters under-perform, electronics can reset under transients), shortens the life of any device operated below its rated voltage, and can put life-safety branches below required minimums. In long-running circuits (24/7 motors, EV chargers, lighting loops) even modest drop translates into hundreds of dollars of lost energy per year — and proper sizing typically pays back the upsize cost in months.

What is NEC's recommendation for voltage drop?

NEC 210.19(A) Informational Note 4 recommends no more than 3% voltage drop on a branch circuit. NEC 215.2(A) Informational Note 2 recommends no more than 5% combined on feeder plus branch. NEC 647.4 limits sensitive equipment branches to 1.5%. NEC 690.45 (solar PV) typically targets 2% from the array to the inverter and 3% combined. These are guidelines, not enforceable rules — but they are universal in engineering practice and most inspectors hold installations to them.

How can I reduce voltage drop?

There are five practical levers: (1) upsize the wire one or two AWG steps — usually the most cost-effective change; (2) shorten the run by relocating the panel or sub-feeding closer to the load; (3) run parallel conductors per NEC 310.10(H) on high-amperage feeders; (4) raise the voltage — moving from 120 V to 240 V on the same circuit halves the current and quarters the percent drop; (5) reduce conductor temperature through cooler installation or higher-rated insulation (90°C THHN runs cooler than 60°C TW).

What happens if voltage drop is too high?

Loads receive less voltage than designed and perform poorly. Incandescent lamps dim noticeably below 95% of rated voltage; motor torque scales with the square of voltage so a 90% sag costs 19% torque; heaters lose output the same way; electronics may reset, restart, or refuse to switch on under sag; battery chargers run slow; LED drivers buzz. Continuous high-current circuits also heat the conductor, accelerating insulation aging — and over time, undersized wire is a fire risk. In safety-critical contexts (fire pumps, life-safety branches, generator feeders) excessive drop can prevent equipment from operating during an emergency.

Voltage Drop Calculator

Calculate electrical voltage drop instantly using wire size, current, distance, voltage, phase type, conductor material, and NEC standards.

Circuit Inputs

Use NEC Chapter 9 reference values for wire resistance and reactance. The most common workflow for sizing real-world feeder and branch circuits.

Wire Material

Wire Size

Conduit Material

Phase Type

Source Voltage

Load Current

One-Way Distance

Length Unit

Power Factor0.1 – 1.0

Real-world templates

What Is Voltage Drop?

Voltage drop is the reduction in electrical potential as current flows through a conductor. Every wire has some resistance (and AC wires also carry reactance), so the voltage you measure at the load is always lower than the voltage delivered from the source. The longer the run, the smaller the wire, and the larger the current, the bigger the drop. Excessive voltage drop wastes energy, weakens equipment, dims lighting, and can prematurely fail motors.

This calculator uses NEC Chapter 9 reference data to compute drop for DC, single-phase, and three-phase circuits. You can also estimate resistance from first principles, enter custom impedance values from a datasheet, or have the recommender pick the smallest wire size that meets both your voltage-drop and ampacity targets — useful tools for any engineer, electrician, solar installer, or DIY-er. Pair it with our unit converter when working in metric, or our scientific calculator for related electrical math.

How Voltage Drop Works

Ohm's Law in long wires

Voltage drop = I × R for each foot of wire. Multiply by the round-trip path length (2× for DC and single-phase, √3 for three-phase) and you have the total drop seen at the load.

AC adds reactance

On AC circuits, conductor reactance (X) also stores and releases energy each cycle. The effective voltage drop becomes I × (R × PF + X × sin θ) — power factor and the type of conduit (magnetic vs not) both matter.

Temperature increases R

Hotter wire = higher resistance. Resistance scales by roughly 0.39% per °C for copper and 0.40% per °C for aluminum above 20°C — substantial across the 60°C–90°C operating range for THWN insulation.

Size dominates everything

Doubling the cross-sectional area halves resistance. That's why upsizing from 12 AWG to 10 AWG (a ~60% area increase) is the most cost-effective way to cut voltage drop on a long run.

Core Voltage Drop Formulas

Every result this calculator produces comes from one of three closed-form equations. R is the one-way conductor resistance, X is the reactance, I is the load current, and PF is the power factor (≤ 1.0). sin θ is automatically derived as √(1 − PF²).

V_drop = 2 × I × R

Round-trip resistance × current. No reactance, no phase factor — pure Ohm's Law.

AC Single Phase

V_drop = 2 × I × (R · PF + X · sin θ)

Adds the reactive term. Conduit type changes X (steel raises it; PVC lowers it).

AC Three Phase

V_drop = √3 × I × (R · PF + X · sin θ)

Three balanced legs share the load; the √3 factor produces ~13% less drop than equivalent single-phase.

How to Use This Calculator

1
Pick a calculation mode
Default to NEC Data for everyday wiring. Use Estimated Resistance to model unusual temperatures or non-standard conductors, Custom Resistance for manufacturer-supplied values, and Wire Size Recommendation when you want the smallest legal gauge.
2
Enter circuit basics
Choose wire material (copper or aluminum), AWG size, conduit type, and phase. Then fill in source voltage, load current, and one-way distance. The unit toggle handles metric.
3
Set power factor if AC
Most resistive loads (heaters, incandescent lighting) run at PF = 1.0. Motors and electronics typically run 0.8–0.95. Lower PF means proportionally more drop on the reactive term.
4
Open Advanced for fine control
Add parallel conductors for high-current feeders, change conductor temperature, override the conductor area or resistivity for non-standard cable, or paste in manufacturer R and X figures.
5
Calculate and review
Read the drop in volts and percent, check NEC compliance, scan the distance and wire-size charts, and use the report export to keep a clean PDF for your panel schedule.

Key Electrical Concepts

Electrical resistance

Resistance opposes current flow and dissipates energy as heat. Copper has roughly 1.7× the conductivity of aluminum, which is why copper conductors deliver the same current at smaller diameters and lower drop.

Reactance

Reactance is the AC-specific component of opposition caused by the conductor's inductance. In magnetic conduit (steel) X is higher than in PVC, which is why NEC Table 9 reports different values per conduit type.

Impedance |Z|

Impedance is the vector sum of resistance and reactance: |Z| = √(R² + X²). For most cable runs at 60 Hz, R dominates — but for very high currents or long runs, the X contribution is non-trivial.

Power factor

Power factor is the cosine of the angle between current and voltage on an AC circuit. A PF of 1.0 means current and voltage are aligned (resistive load). Lower PF increases the reactive part of voltage drop.

NEC voltage drop

NEC 210.19(A) Informational Note 4 and 215.2(A) Informational Note 2 recommend voltage drop of 3% or less on the branch, 5% combined on feeder + branch. They are guidelines, not hard rules — but inspectors and engineers treat them as a working ceiling.

Ampacity

Ampacity is the maximum continuous current a conductor can carry without exceeding insulation temperature. NEC Table 310.16 lists ampacity by AWG, temperature rating, and conductor count. Always check both voltage drop AND ampacity.

Voltage Drop in Real-World Wiring

🏠

Home wiring

Standard 120/240 V residential branch circuits should stay under 3%. 12 AWG copper is fine for 20 A within 50 ft; longer runs (kitchen islands, detached garages) often need 10 AWG or paralleled conductors.

☀️

Solar PV systems

PV strings often run on long DC cables before reaching the inverter — every percent of voltage drop is direct yield lost. NEC 690.45 (and most installer guides) recommend ≤ 2% drop from array to inverter, ≤ 3% from inverter to point of interconnection.

🚐

RV electrical

RV 12 V DC systems are the worst-case scenario — low voltage, high current, long runs. Even small drops are huge in percent. Most RV electricians upsize wires one or two AWG sizes beyond the ampacity minimum.

⚙️

Industrial motors

Motors require >95% of nameplate voltage to start under load. NEMA MG 1 allows operation at 90–110% of rated voltage, but starting torque drops with the square of voltage — under-sized feeders are the #1 cause of repeated motor failures.

🔋

Battery banks

Inverter-to-battery cables run hundreds of amps over a few feet. Even at 24 or 48 V DC, voltage drop in the battery cabling can starve the inverter under surge loads — 2/0 or larger is standard for serious off-grid installs.

🚗

EV chargers

Level-2 chargers pull 40–80 A continuously. Long runs from panel to garage can easily push beyond 3% on undersized wire. Most installers default to 6 AWG copper for 48 A circuits unless the run is exceptionally short.

🧱

Underground runs

Direct-burial or PVC conduit runs to outbuildings, well pumps, or yard panels are often 100–300 ft. Aluminum 2 AWG and 4 AWG are common because cost-per-foot drops fast at large diameters — but the metal needs upsizing vs copper.

⚡

Generator feeders

Standby generator feeders are sized for full nameplate current with low drop because voltage sag during transfers can trip sensitive electronics. Engineers typically design for ≤ 2% drop from genset to main panel.

Best Practices to Reduce Voltage Drop

✓Upsize wire one AWG step for any run over 100 ft, then verify with this calculator. The marginal copper cost is far smaller than the lifetime energy loss.
✓Prefer copper for long runs unless cost is the dominant constraint. Aluminum requires roughly 1.6× the cross-section to match copper's voltage drop.
✓Run parallel conductors on high-amperage feeders (per NEC 310.10(H)) rather than oversizing a single huge cable — termination is cheaper and easier.
✓Choose PVC over steel conduit when reactance matters (long three-phase runs) — magnetic conduit substantially raises X.
✓Boost voltage where possible — running 240 V instead of 120 V on the same circuit halves the current and quarters the voltage drop in percentage terms.
✓Locate the panel centrally in new builds so runs are short and balanced; long radial wiring designs are the #1 cause of avoidable drop.

Common Voltage Drop Mistakes

Sizing for ampacity only

Ampacity tells you the wire won't melt under steady current — it says nothing about whether the load receives enough voltage. Always check both.

Forgetting the round-trip

Distance is one-way; the formula doubles it for DC and single-phase (×2) or applies the √3 factor for three-phase. Skipping this halves your computed drop.

Ignoring temperature

75°C resistance values are NEC's mid-band default. Wire running in a hot attic, near machinery, or fully loaded can sit at 90°C+, with R about 6% higher than nameplate.

Using DC formulas for AC

On long AC runs, ignoring reactance and power factor under-estimates drop by 5–20% on inductive loads. Use the right formula for the phase.

Skipping voltage drop on motor feeders

Motors start at 5–8× their running current. The feeder must hold ≥ 85% of nameplate voltage during start — verify drop using starting current, not just running current.

Trusting nameplate alone

Real installed voltage at the panel can be 5–10 V below nominal. Measure with a meter under load before sizing critical feeders.

Why Voltage Drop Matters

Voltage drop is wasted energy — every percent below the source voltage is current dissipated as heat in the wire rather than delivered to the load. On a 50 A circuit running 24/7, even 3% drop wastes hundreds of dollars per year. On long runs it can cumulatively cost more than the wire itself.

Beyond energy, drop affects performance: lights dim, electronics reset under surge, motors stall, heaters under-perform, and battery chargers run slow. In safety-critical contexts (fire pumps, life-safety branches, generator feeders) it can even prevent equipment from operating during an emergency. Correct sizing protects the equipment, the wallet, and ultimately the people.

Built for electricians, engineers, solar installers, electrical inspectors, and serious DIYers.

Reference values cross-checked against NEC Chapter 9 Tables 8 and 9 (2023 edition) — see our methodology and editorial policy. Educational only — always have an installation reviewed by a licensed electrician and your local inspector.

Frequently Asked Questions

Voltage drop is the difference between the voltage at the source and the voltage at the load, caused by the electrical resistance and reactance of the wire connecting them. Every wire dissipates a portion of the supplied energy as heat — that portion is the voltage drop.

Voltage drop = current × conductor resistance × a phase factor. For DC and single-phase circuits, drop = 2 × I × R (round trip). For three-phase, drop = √3 × I × (R × PF + X × sin θ). The calculator above derives R and X from NEC Chapter 9 Table 9 reference values.

Three primary factors: conductor resistance (which rises with length and temperature, falls with cross-section), conductor reactance (a function of geometry and conduit material), and the load current itself. Power factor affects the magnitude of the reactive component on AC circuits.

NEC Informational Notes recommend ≤ 3% drop on a branch circuit and ≤ 5% combined on feeder plus branch. These are non-mandatory guidelines — but they are the industry standard threshold for satisfactory equipment operation and are typically enforced by good engineers and inspectors.

Yes — dramatically. Wire resistance is inversely proportional to cross-sectional area, so doubling the area roughly halves voltage drop at constant current. Going from 12 AWG to 10 AWG cuts resistance by about 37%; from 10 AWG to 8 AWG by another 37%, and so on.

Copper has about 1.6× the conductivity of aluminum, so the same drop at the same current requires aluminum with roughly 1.6× the cross-section. Aluminum is cheaper per foot at large gauges (4/0 and up) and dominant in service entrances and overhead transmission; copper is preferred for branch circuits, motor feeders, and anywhere terminations are dense.

Excessive voltage drop wastes energy (loss = I²R), reduces equipment performance (motors stall, lights dim, electronics fail), shortens component life from operating below design voltage, and can be a code violation when it pushes life-safety or fire-pump branches below required minimums.

NEC 210.19(A) Informational Note 4 and 215.2(A) Informational Note 2 recommend voltage drop of no more than 3% on a branch circuit, and no more than 5% combined on feeder plus branch. NEC 647.4 (for sensitive electronic equipment) and 690.45 (for solar PV) impose stricter ≤ 1.5% and ≤ 2% targets in their respective contexts.

Five practical levers: (1) upsize the wire — usually the most cost-effective change; (2) shorten the run — relocate the panel or the load; (3) run conductors in parallel; (4) raise the voltage (240 V instead of 120 V halves the current and quarters the percent drop); (5) reduce conductor temperature through cooler installation or higher-rated insulation.

Loads receive less voltage than designed: incandescent lamps dim noticeably below ~95%, motors lose torque (which scales with the square of voltage), heaters lose output (also square), electronics reset or restart under transient drops, and continuous high-amperage circuits heat the conductor — accelerating insulation aging and creating a real fire risk if the wire is too small.