Voltage Drop Interactive Calculator

Whenever you run current through a wire, you'll lose some voltage along the way. The farther you go, the more you lose. This adds up quickly in long circuits, low-voltage setups, or with motors that draw a big surge at startup. This Voltage Drop Calculator will let you check your voltage loss, decide on wire size, find your max safe run, or see how much current you can push based on conductor type, wire gauge, voltage, and length. You'll run into these issues in solar panels, boats running 12V, HVAC jobs, automation panels, and anywhere code says you need to keep branch circuits under 3% drop (or 5% total). Below you’ll find the formula, a step-by-step example, background notes on AC/DC, temp effects, and an FAQ for troubleshooting odd cases.

What is Voltage Drop?

Voltage drop is the amount of voltage lost as current moves through a wire’s resistance. Longer wires and higher currents cost you more voltage before it gets to your load.

Simple Explanation

If you’ve used a long garden hose, you know the pressure at the nozzle drops the longer the hose and the faster you flow water. Wires behave the same—some voltage is lost along the wire. If the drop is too great, motors don’t start, lights dim, or controls go haywire. If you ignore it, gear might overheat or not work at all.

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How to Use This Calculator

1. Pick the calculation mode—voltage drop, required wire size, max run, max current, or percent drop.
2. Fill in your circuit’s current (A), one-way length (ft), and system voltage (V).
3. Select wire gauge and material (copper or aluminum). If solving wire size or max run, enter the allowed voltage drop.
4. Hit Calculate for the answer.

Simple Example

A 120V branch circuit carries 20A through 14 AWG copper wire over a 100 ft one-way run.

Total resistance = (2 × 100 × 2.525) / 1000 = 0.505 Ω

Voltage drop = 20A × 0.505 Ω = 10.1 V

Percent drop = (10.1 / 120) × 100 = 8.4% — excessive. Upsize to 10 AWG or shorten the run.

Circuit Diagram

Voltage Drop Calculator

Voltage Drop Equations

The equation below will give you the voltage lost for a given conductor and load.

Here’s how you size wire if you know your voltage drop limit.

Theory & Practical Applications

Fundamental Physics of Voltage Drop

Every conductor resists current flow, and that resistance causes voltage loss. The further the electrons go, the more they bump into atoms in the wire, heating it up. This is basic Ohm’s Law (V = IR), and the resistance comes from the wire material, length, and cross-section—R = ρL/A. Don’t skip the “2” in the formula: voltage drop happens on both the supply and return path, so length doubles by default. In three-phase, the math shifts to using √3, not 2, for balanced loads—mixing these up leads to wire that's oversized or (worse) undersized by about 15%, which can become expensive or risky in commercial setups.

Temperature matters—a hot location (like an attic) can increase resistance 15–20% over book values. Copper’s resistance rises about 0.393% per °C above 20°C; that adds up quick if you’re wiring near heat sources or in spaces that bake in summer. The NEC lists correction factors, but real-world jobs—especially those facing repeated heavy current loads (like EV charging)—push insulation and metal well beyond steady-state tables, and can drive resistance higher over time.

Industry-Specific Applications

Solar Photovoltaic Systems: For DC solar work, you’re at the mercy of both string voltage dropping as modules get hot and the voltage drop in your DC wiring. Drop 3% on the wire, and if a string is already running toward the lower limit for the inverter, you might find yourself out of the MPPT (maximum power point tracking) window, throwing away more power than you saved by using cheaper cable. For large commercial jobs, even dropping losses by 1% can be worth tens of thousands in extra energy per MW-year—hard to ignore.

Marine and Automotive 12V/24V Systems: Voltage drop eats low-voltage systems alive. Lose 3V on a 12V pump circuit, and you’re 25% down before you start. Marine grade (tinned) wire adds even more resistance—expect 10–15% increase over bare copper. Factor in salt corrosion (which is inevitable on a boat) adding resistance at connections, and it’s easy to end up with pumps or lights working well below spec. When it matters—like a bilge pump—that’s a risk you don’t want to live with.

Industrial Automation and Robotics: Today’s machines and controllers don’t like supply fluctuations. The spec might allow 5% drop, but in reality, torque drops and heat rises fast if the phases aren’t balanced or if voltage sags further. VFDs are sensitive—drop the AC voltage and you get less DC on the bus, which means your motor can run out of torque when you need it most.

LED Lighting Systems: Forget bulbs—LEDs with constant current drivers will try to compensate as long as possible, but if you fall below their minimum input voltage, the result is dropout or dimming. If your lighting runs are long and you have different branches, some fixtures end up visibly dimmer due to uneven wiring drops, even if you’re technically code compliant.

Worked Example: Commercial HVAC Rooftop Unit Installation

Scenario: A 5-ton rooftop A/C needs to start (LRA 142A) and run (RLA 28.7A) safely on a long circuit: 209 feet one-way, 208V three-phase. Wire needs to keep voltage drop below 2% under LRA for starting, 3% for running (NEC guidelines).

Step 1: Calculate Maximum Allowable Voltage Drop

At starting (LRA): V_drop,max = 208V × 0.02 = 4.16V

At running (RLA): V_drop,max = 208V × 0.03 = 6.24V

Step 2: Calculate Maximum Conductor Resistance (Three-Phase Formula)

For three-phase: R_total = V_drop / (I × √3)

At starting: R_max,start = 4.16V / (142A × 1.732) = 4.16 / 245.9 = 0.01692Ω

At running: R_max,run = 6.24V / (28.7A × 1.732) = 6.24 / 49.71 = 0.1255Ω

Step 3: Calculate Required Resistance per 1000 Feet

R_Ω/kft = (R_max × 1000) / (2 × L)

At starting: R_Ω/kft,max = (0.01692Ω × 1000) / (2 × 209ft) = 16.92 / 418 = 0.0405Ω/kft

At running: R_Ω/kft,max = (0.1255Ω × 1000) / (2 × 209ft) = 125.5 / 418 = 0.300Ω/kft

Step 4: Select Wire Size

Standard wire tables: 4/0 copper is 0.0490Ω/kft—too much resistance for starting. 250 kcmil is 0.0431Ω/kft—just over the limit. 300 kcmil is 0.0360Ω/kft, which fits.

Step 5: Verify Actual Voltage Drops

Using 300 kcmil copper (0.0360Ω/kft):

R_total = (2 × 209ft × 0.0360Ω/kft) / 1000 = 0.01505Ω

At starting: V_drop = 142A × 1.732 × 0.01505Ω = 3.70V (1.78% - acceptable)

At running: V_drop = 28.7A × 1.732 × 0.01505Ω = 0.748V (0.36% - excellent)

Step 6: Ampacity Check

300 kcmil THHN copper (75°C) is rated for 285A, which clears the NEC’s 125% of running load easily. Breaker is usually sized for starting, so might use a 100A unit. But you’re really only forced that big for voltage drop—the ampacity itself is nowhere near that high; the extra copper just keeps the drop in check. The cost leap—several thousand dollars in cable alone—is unavoidable if you want reliable starts and NO nuisance tripping.

Engineering Note: On long motor runs, voltage drop—not ampacity—almost always decides wire size. The extra copper is a cost, but it’s cheaper than service calls and equipment failures.

Advanced Considerations and Edge Cases

Harmonic Current Effects: Got VFDs or lots of electronics? The extra harmonic current, especially the 3rd, piles up in the neutral, sometimes doubling the expected neutral current and making voltage drops larger where you least expect. In big buildings with nonlinear loads, always check the neutral sizing.

Voltage Drop in Parallel Conductors: When paralleling wires, the math says you split the load, but with small differences in length, tightness, or routing, current hogs through the path of least resistance. This causes one set to get hotter than the rest. Best practice: make all runs the same length (within an inch or two if possible), and verify sharing at commissioning.

Cold Temperature Effects: Resistance drops in cold weather (about 0.393%/°C for copper), so your wires get “better” in the cold. For outdoor or mixed-temperature runs, expect the numbers to shift, and keep an eye on connections—aluminum especially can loosen with freeze/thaw cycles.

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Frequently Asked Questions

📹 Video Walkthrough — How to Use This Calculator

📹 Video Walkthrough — How to Use This Calculator