VOLTAGE
DROP
CALCULATOR
Calculate wire size, cable length, current load and power loss for any electrical circuit — instantly and accurately
Voltage Drop Calculator
Enter circuit parameters to calculate voltage drop, wire gauge recommendations, and power loss
What Is Voltage Drop — And Why It Matters
Voltage drop is the reduction in electrical potential that occurs as current flows through a conductor. Every wire has resistance, and when current flows through that resistance, some of the electrical energy is converted to heat — causing the voltage at the end of the run to be lower than at the source. This is not a defect; it is a fundamental consequence of Ohm’s Law and the physical properties of conductive materials.
The consequences of excessive voltage drop range from annoying to dangerous: motors run hotter and fail prematurely, LED drivers flicker or malfunction, sensitive electronics behave erratically, and in worst cases, overloaded conductors become fire hazards. Understanding and calculating voltage drop is a core competency for electricians, electrical engineers, solar installers, and anyone designing or troubleshooting an electrical system.
The Voltage Drop Formula Explained
The core formula used by professional electricians and our calculator for single-phase AC and DC circuits is:
Where each variable represents:
- VD — Voltage drop in volts
- K — Resistivity constant (12.9 for copper; 21.2 for aluminum)
- I — Current in amperes (amps)
- L — One-way length of the conductor in feet
- CM — Cross-sectional area of the conductor in circular mils
- 2 — The multiplier accounts for the full circuit path (current travels to the load and back)
For three-phase AC circuits, the formula is modified to: VD = (1.732 × K × I × L) / CM, where 1.732 is the square root of 3, accounting for the phase relationship between the three conductors.
| AWG | Circular Mils (CM) | Max Amps (NEC) | Resistance (Ω/1000ft) | Common Use |
|---|---|---|---|---|
| 14 | 4,110 | 15A | 3.14 | Lighting circuits |
| 12 | 6,530 | 20A | 1.98 | General outlets |
| 10 | 10,380 | 30A | 1.24 | A/C units, dryers |
| 8 | 16,510 | 40A | 0.778 | Ranges, EV chargers |
| 6 | 26,240 | 55A | 0.491 | Sub-panels, feeders |
| 4 | 41,740 | 70A | 0.308 | Large feeders |
| 2 | 66,360 | 95A | 0.194 | Service entrance |
| 1/0 | 105,600 | 125A | 0.122 | Service entrance |
| 2/0 | 133,100 | 145A | 0.0967 | Service entrance |
How to Use This Voltage Drop Calculator
Select Mode
Choose Basic DC/AC, Wire Gauge Finder, Three-Phase, or Solar depending on your system.
Enter Voltage
Input your system voltage — 12V, 24V, 120V, 240V, 480V, or any custom value.
Enter Current
Enter the circuit current in amps. Use the actual load, not just the breaker rating.
Wire Length
Enter the ONE-WAY distance from source to load. The formula automatically doubles for the return path.
Wire Gauge
Select your wire gauge or use the Wire Gauge Finder mode to get a recommendation.
Read Results
See voltage drop in volts and percent, power loss in watts, and NEC compliance status.
Voltage Drop for Solar and Low-Voltage DC Systems
Solar photovoltaic and off-grid DC systems are particularly sensitive to voltage drop because low system voltages (12V, 24V, 48V) result in proportionally higher percentage losses for the same absolute voltage drop. A 1-volt drop on a 120V circuit is only 0.83% — on a 12V system, that same 1 volt represents an 8.3% loss, potentially pushing the system well beyond acceptable thresholds.
For solar DC wiring, professional installers typically target a maximum of 2–3% voltage drop between the solar array and charge controller, and another 1–2% between the battery bank and inverter. These tighter tolerances protect MPPT charge controllers and ensure maximum energy harvest from expensive PV modules.
Battery cable sizing is especially critical. A heavily underspecced battery cable doesn’t just cause voltage drop — it creates heat that degrades cable insulation and connectors, and represents a genuine fire risk in high-current DC systems where short-circuit currents can be enormous.
Three-Phase Systems and the √3 Factor
Three-phase electrical systems are the standard for commercial, industrial, and utility-scale applications. In a balanced three-phase system, the phase relationship between the three conductors means that the effective round-trip voltage drop is calculated using a factor of 1.732 (√3) rather than 2, because the return currents partially cancel each other through the neutral conductor.
The modified three-phase formula — VD = (1.732 × K × I × L) / CM — gives line-to-line voltage drop. For a 480V three-phase system, a 3% voltage drop tolerance allows up to 14.4V of drop, making longer runs more feasible than on single-phase 120V circuits where 3% is only 3.6V.
Motor circuits are the most common three-phase voltage drop concern. Motors are sensitive to low voltage: below 90% of rated voltage, motor torque drops significantly and winding temperatures rise sharply, dramatically reducing motor life and efficiency.
📊 Voltage Drop % vs. Wire Length — 12 AWG Copper, 20A, 120V
Copper vs. Aluminum Conductors
The choice between copper and aluminum conductors significantly affects voltage drop calculations. Aluminum has approximately 61% of the conductivity of copper, meaning an aluminum conductor has about 1.64× the resistance of the same-sized copper conductor — represented by the different K values (12.9 for copper, 21.2 for aluminum).
To compensate for aluminum’s higher resistivity, aluminum conductors must be upsized. As a practical rule, aluminum conductors should be upsized by approximately two AWG sizes to match the current-carrying capacity and voltage drop performance of copper — though the exact upsize needed depends on the specific application and should be calculated rather than estimated.
The tradeoff: aluminum is significantly cheaper and lighter than copper, making it the standard choice for large service entrance cables, utility feeders, and overhead distribution lines where weight and cost are significant factors. Copper remains preferred for branch circuit wiring due to its smaller size and better termination reliability at smaller gauges.
Common Voltage Drop Problems and Solutions
Dimming Lights at the End of Long Runs
The most visible symptom of excessive voltage drop in lighting circuits. Solving it involves increasing wire gauge, reducing circuit length by adding a sub-panel closer to the loads, or splitting the circuit into shorter runs from the panel.
Motor Starting Problems
Motors draw 6–10× their rated current at startup. Even a circuit with acceptable voltage drop at running current may have severe voltage drop on startup, preventing the motor from reaching operating speed. Size motor circuit wiring for the locked-rotor current, not the full-load current.
EV Charger Installations
Level 2 EV chargers (240V, 30–50A) on long runs to garages or external charging locations are a common source of voltage drop problems. A 50A circuit running 75 feet in 6 AWG copper has approximately 2.3% voltage drop — acceptable but tight. Upgrading to 4 AWG or running a sub-panel closer to the charging location are the professional solutions.
Generator and Inverter Connections
Transfer switch connections and inverter output cables are frequently undersized. High-current inverters (3000W+ at 24V or 48V) require very heavy gauge DC cabling to prevent both voltage drop and heat generation. Always calculate the actual current — a 3000W inverter at 24V draws 125A, requiring 2/0 AWG or larger cable for any run over a few feet.
Frequently Asked Questions
Common questions from electricians, DIYers, solar installers, and engineers using voltage drop calculators.
Voltage drop is the reduction in electrical potential (voltage) as current flows through a conductor’s resistance. By Ohm’s Law, V = I × R — any resistance in the path will result in a voltage reduction proportional to the current flowing through it.
Excessive voltage drop causes equipment to receive less voltage than it was designed for, leading to: motors running hotter, reduced motor torque, LED lights flickering or dimming, electronic equipment malfunctioning, and in severe cases, fire risk from overheated conductors. The NEC recommends keeping voltage drop under 3% for branch circuits and 5% combined for feeders plus branch circuits.
The National Electrical Code (NEC) provides voltage drop recommendations in Fine Print Notes (FPNs), which are informational rather than mandatory requirements. The recommendations are:
- Branch circuits: Maximum 3% voltage drop from panel to outlet/load
- Feeders: Maximum 3% voltage drop from service entrance to branch circuit panel
- Combined total: Maximum 5% voltage drop from service entrance to furthest outlet
Note that local codes may make these mandatory, and some applications (hospitals, data centers, sensitive equipment) use tighter tolerances of 1–2%.
For single-phase AC or DC circuits, use the formula: VD = (2 × K × I × L) / CM
- K = 12.9 for copper, 21.2 for aluminum
- I = current in amps
- L = one-way length in feet
- CM = circular mils of the conductor (look up by AWG size)
To get voltage drop percentage: VD% = (VD / System Voltage) × 100
For three-phase: replace the factor 2 with 1.732 (√3).
Use the Wire Gauge Finder mode above — enter your voltage, current, length, and maximum acceptable drop percentage, and the calculator will find the minimum AWG that meets your requirement.
As a general rule: longer runs, higher currents, lower voltages, and tighter voltage drop tolerances all require larger wire. Doubling wire length requires moving to the next wire gauge up. Doubling current also requires approximately two gauge sizes larger to maintain the same voltage drop percentage.
Related but not identical. Voltage drop is the reduction in voltage; power loss is the actual energy wasted as heat in the conductors. They are linked by the formula: Power Loss (W) = I² × R, or equivalently, Power Loss = VD × I.
A 2V drop on a 20A circuit loses 40W as heat in the conductors. Over time, this adds up: a circuit running continuously wastes 40W × 8760 hours/year = 350 kWh/year — about $35–70 in electricity depending on your rate. For large industrial systems, properly sized conductors pay for themselves in energy savings.
Because voltage drop percentage is calculated relative to system voltage. The same 1-volt drop represents very different percentages on different systems:
- 120V system: 1V drop = 0.83% (within limits)
- 24V system: 1V drop = 4.2% (potentially problematic)
- 12V system: 1V drop = 8.3% (almost certainly unacceptable)
This is why solar panels, battery systems, and automotive wiring require very heavy gauge cables for even short runs — the absolute resistance is the same, but the percentage impact is dramatically higher at low voltages.
Yes — conductor resistance increases with temperature. Copper has a temperature coefficient of resistance of approximately 0.393% per °C above 20°C (68°F). At elevated installation temperatures (inside conduit in hot environments, in attics, direct burial), wire resistance can be 10–25% higher than at standard conditions.
For critical applications, use the temperature-corrected formula: R(T) = R₂₀°C × [1 + α(T – 20)], where α = 0.00393 for copper. For most practical residential applications, the standard formula provides sufficient accuracy with a reasonable safety margin built into the next-gauge-up approach.
Aluminum wiring for branch circuits (15A, 20A outlets and lighting) has a troubled history — aluminum expands and contracts more than copper with temperature changes, which caused connection failures and fires in homes wired with aluminum in the 1960s–70s.
Modern aluminum wiring for branch circuits requires special CO/ALR rated receptacles and devices. For larger conductors (6 AWG and larger) used in service entrances, sub-panel feeders, and high-ampacity branch circuits, aluminum is widely accepted and used. Always use anti-oxidant compound on aluminum connections and ensure all terminations are rated for aluminum.
Motors are particularly sensitive to voltage drop because their torque is proportional to the square of the applied voltage. A 10% voltage drop reduces available torque by approximately 19%. Common effects:
- Reduced starting torque: Motor may fail to start under load
- Higher current draw: Motor draws more current to compensate, increasing heat
- Overheating: Winding temperatures increase, shortening insulation life
- Reduced efficiency: Power factor and efficiency both degrade
NEMA standards specify motors must operate satisfactorily within ±10% of nameplate voltage. Professional practice is to keep voltage drop under 3% for motor branch circuits.
These are two distinct (though sometimes related) problems. Voltage drop is the reduction in voltage magnitude from source to load, caused by conductor resistance and current flow. Voltage unbalance (or imbalance) is a difference in voltage magnitude or phase angle between the three phases of a three-phase system.
Voltage unbalance can cause negative-sequence currents in motors that produce a braking torque opposed to the motor’s rotation — even a 2% voltage unbalance can cause 10°C+ temperature rise in motor windings. NEMA MG-1 requires motors to be de-rated when voltage unbalance exceeds 1%. Voltage drop tends to affect all three phases equally if the wiring is balanced; voltage unbalance typically indicates a different problem such as an unbalanced load distribution or a utility supply issue.
Conclusion: Design Your Circuits Right the First Time
Voltage drop is one of those engineering realities that is easy to overlook during design but expensive and inconvenient to correct after installation. Running slightly larger wire — often just one gauge size up — during initial installation adds minimal material cost while eliminating future problems, improving energy efficiency, and providing headroom for future load growth.
Use the calculator above as part of every circuit design process, not just for long runs. Verify your design against NEC recommendations, consider future load expansion, factor in the actual operating conditions your conductors will experience, and you will create electrical installations that perform reliably for decades.