⚡ Voltage Drop Calculator
Find the voltage drop, drop percentage, and voltage reaching the load — from wire gauge, run length, and current. DC, single-phase, and three-phase, with a 3% NEC check. Free, no sign-up.
A voltage drop calculator finds how much voltage is lost in a wire run before it reaches the load. Every conductor has resistance, and current flowing through that resistance loses voltage along the way. The calculator returns the drop in volts, the drop as a percentage, and the voltage left at the load. The core formulas are:
How to calculate voltage drop in 3 steps:
- Pick a wire gauge and material, or enter a known resistance directly.
- Enter the voltage, load current, and one-way distance, and choose DC, single-phase, or three-phase.
- Click Calculate to see the drop in volts, the percentage, and the voltage at the load.
For example, 12 AWG copper carrying 15 A over a 50-foot single-phase run drops about 2.9 volts — 2.4% of a 120-volt supply, leaving 117 volts at the load. That passes the 3% recommendation.
Voltage Drop Calculator
Enter wire gauge, distance, and current — see the voltage drop and what reaches the load.
Know How Much Voltage Reaches the Load –
Before You Pull the Wire
Every foot of wire has resistance, and resistance steals voltage. On a long run, what arrives at the load can be noticeably less than what left the panel. This free calculator turns wire gauge, distance, and current into an exact voltage drop — and checks it against the 3% guideline.
⚡ Try the Calculator NowThe Voltage That Leaves the Panel Isn't the Voltage That Arrives
Plug something in and you assume it gets the full voltage. On a short run, near enough. On a long one, not really. Wire has resistance, and resistance quietly skims voltage off the top the whole way to the load — so what arrives can be meaningfully less than what left the breaker.
It comes down to one chain of cause and effect. Every conductor has resistance. Push current through resistance and you lose voltage — that is Ohm's law. The longer the run, the smaller the wire, or the higher the current, the bigger the loss. Too much of it and motors run hot, lights dim, and electronics misbehave.
This Voltage Drop Calculator turns that into hard numbers. Pick a wire gauge, or enter a resistance directly; add the voltage, current, distance, and phase; and it returns the drop in volts, the drop as a percentage, the voltage left at the load, and whether the circuit clears the 3% recommendation.
How the Voltage Drop Calculator Works
Pick a mode, fill in the circuit, click once. Here is what each field does.
Choose a Mode
Wire Gauge mode uses standard AWG resistance values — just pick a size and material. Custom Resistance mode lets you type in an ohms-per-distance figure straight from a spec sheet.
Set the Wire & Phase
In Wire Gauge mode, choose copper or aluminum and the AWG size. Then pick the system: DC, AC single-phase, or AC three-phase — each uses a different multiplier.
🟠 Copper has lower resistance than aluminum.Enter the Voltage
The system voltage — 12, 120, 240, 480, or whatever your circuit runs at. The drop is compared against this to give the percentage.
Enter the Load Current
The current the load draws, in amps. Voltage drop is directly proportional to current — a heavier load on the same wire means a bigger drop.
⚡ More amps, more drop.Enter the One-Way Distance
The distance from the source to the load — one direction only. The calculator doubles it for the return conductor on DC and single-phase circuits.
Calculate — Read the Drop
Instantly see the voltage drop in volts, the percentage, the voltage at the load, the power lost as heat, and a pass-fail check against your drop limit.
✅ Drop % = Voltage drop ÷ Source voltageThe Voltage Drop Formula, Explained
Voltage drop is Ohm's law applied to a wire run. The whole thing rests on V = I × R.
The base idea. Voltage drop equals current times resistance. The current is what your load draws; the resistance is the resistance of the wire it travels through. The only real work is figuring out how much wire the current actually passes through.
Why distance is doubled. On a DC or single-phase circuit, current leaves the source through one conductor and returns through another. So it travels through twice the one-way distance. That is why the formula is 2 × current × resistance-per-foot × one-way distance — the 2 accounts for the round trip.
The three-phase difference. A balanced three-phase circuit shares current across three conductors, and the math works out to a factor of √3 — about 1.732 — instead of 2. Same idea, different multiplier, because the current path is arranged differently.
A worked example. Take 12 AWG copper, which has about 1.93 ohms per 1,000 feet, carrying 15 amps over a 50-foot single-phase run. The drop is 2 × 15 × 1.93 × (50 ÷ 1000) = 2.9 volts. On a 120-volt supply that is 2.9 ÷ 120 = 2.4% — and 117 volts reaches the load.
Parallel conductor sets change one term. Running two identical sets side by side halves the effective resistance, which halves the drop. The calculator divides the wire's resistance by the number of sets before doing the rest.
Wire Gauge & Resistance Tables
The resistance of common wire sizes, and the drop limits worth knowing. These are the numbers the calculator works from.
| Wire Size | Copper (Ω/1000 ft) | Aluminum (Ω/1000 ft) |
|---|---|---|
| 14 AWG | 3.07 | 5.06 |
| 12 AWG | 1.93 | 3.18 |
| 10 AWG | 1.21 | 2.00 |
| 8 AWG | 0.764 | 1.26 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 2 AWG | 0.194 | 0.319 |
| 1/0 AWG | 0.122 | 0.201 |
| 2/0 AWG | 0.0967 | 0.159 |
| 4/0 AWG | 0.0608 | 0.100 |
The core formula in plain words: voltage drop equals the phase factor — 2 for DC and single-phase, 1.732 for three-phase — times the current, the resistance per foot, and the one-way distance. Divide by source voltage for the percentage.
Voltage Drop Limits
| Circuit | Recommended Limit | Why |
|---|---|---|
| Branch circuit | 3% | The NEC recommendation for the final run to a load |
| Feeder plus branch | 5% | The combined total the NEC suggests as a ceiling |
| Sensitive electronics | 2% | Tighter, where stable voltage matters most |
| Long DC / low-voltage runs | varies | 12V systems are very sensitive to drop |
How Far 12 AWG Copper Carries 15 A Before 3% Drop
| System Voltage | Approx. Max One-Way Run | Note |
|---|---|---|
| 120 V | ~62 ft | A typical household branch circuit |
| 240 V | ~124 ft | Higher voltage tolerates a longer run |
| 12 V DC | ~6 ft | Low voltage drops out very fast |
The same wire and current behave completely differently by voltage. This is why low-voltage DC runs need surprisingly heavy wire over even short distances.
Factors That Affect Voltage Drop
Five things move the drop. Change any one and the number changes with it.
Copper vs Aluminum & Phase Types
Two choices shape the result before you touch a number: the conductor metal, and the type of electrical system.
| Choice | Effect on Drop | Typical Use |
|---|---|---|
| Copper | Lower resistance, less drop per gauge | Branch circuits, most residential wiring |
| Aluminum | Higher resistance; needs a larger size to match copper | Service entrances and large feeders |
| DC | Factor of 2 for the return conductor | Solar, automotive, low-voltage systems |
| AC single-phase | Factor of 2, same as DC | Homes and light commercial |
| AC three-phase | Factor of √3 (1.732) | Commercial and industrial power |
How to Size Wire for a Run: A Phase-by-Phase Roadmap
Picking a wire size is two questions answered in order — can it carry the current, and can it keep the drop in check. Here is the path.
Find the load's current draw in amps and measure the one-way run from the source to the load. These two figures, with the system voltage, define the whole problem.
Every wire size has an ampacity — the current it can safely carry. Start with the smallest gauge rated for your load current. This is the safety floor, set by code, before drop is even considered.
Enter the ampacity-sized wire here. If the drop comes in under 3%, the gauge works on both counts. If it fails, the run is too long for that size — move to the next phase.
If the drop is too high, go up one wire size and calculate again. Repeat until the drop clears the limit. The final choice is whichever gauge satisfies both ampacity and voltage drop.
The Real Cost of Voltage Drop
Voltage drop is not just a number on a form. It costs money in two ways — wasted energy now, and shortened equipment life later.
Every volt dropped in the wire is energy turned into heat instead of useful work. On a long, heavily loaded run, that lost power adds up on the electricity bill, year after year, doing nothing but warming the cable. Upsizing the wire trades a one-time material cost for a permanent reduction in that loss.
The second cost is harder to see. Equipment fed low voltage works harder to do its job. Motors draw extra current and run hotter, which shortens their life. Lighting dims and electronics can behave unpredictably. The price of excessive drop shows up later as premature failures and service calls.
Example Voltage Drop Calculations
Three circuits, three outcomes — each worked through with verified math so you can check your own.
Keeping Voltage Drop Under Control
Good wire sizing is mostly a few habits applied before the cable is ever pulled. These are the ones that matter.
The first habit is checking drop on every run that is even moderately long. Short circuits rarely have a problem, so they get a pass — but a run of fifty feet or more, especially at 120 volts, deserves a quick calculation. Catching a marginal circuit on paper is free; catching it after the wall is closed is not.
The second is treating ampacity and voltage drop as two separate hurdles, both of which the wire must clear. A gauge can be perfectly safe for the current and still drop too much voltage over distance. When the two rules disagree, the larger wire wins — that is the size that satisfies both.
Six Habits for Controlling Voltage Drop
When This Calculator Is the Wrong Tool
The arithmetic here is exact, but a real electrical installation has factors a quick calculation does not capture. Here is where the output needs an expert's eye.
1. It uses resistance only, not full impedance. On AC circuits, conductors also have reactance, and in steel conduit that reactance is significant. This calculator works from DC resistance, which is accurate for most planning but slightly optimistic for large AC conductors in metallic conduit.
2. It does not size for ampacity. Voltage drop is only half of wire sizing. The wire must also be rated to carry the current safely without overheating. Always confirm the gauge against code ampacity tables — this tool does not.
3. Resistance values are at a reference temperature. Conductor resistance rises as the wire heats under load. The table values assume a standard temperature, so a hot circuit will drop slightly more voltage than calculated.
4. It assumes a balanced, steady load. The math is for a constant current draw. Motors drawing heavy startup current, or unbalanced three-phase loads, behave differently and need a more detailed analysis.
Where to go instead: For code-critical work, a licensed electrician and the full NEC conductor tables are the authority — including Chapter 9 Table 9 for AC impedance in conduit. This calculator is a fast, accurate planning tool for estimating drop and comparing wire sizes, not a substitute for a code-compliant design.
Electrical Terms You'll See On This Page
Quick reference for the electrical terms used throughout this calculator.
- Voltage Drop
- The voltage lost in a conductor as current flows through its resistance, between the source and the load.
- Resistance
- A conductor's opposition to current flow, measured in ohms. It rises with length and falls with wire size.
- AWG
- American Wire Gauge — the standard sizing system for wire. A smaller AWG number means a thicker wire.
- Ampacity
- The maximum current a conductor can carry continuously without overheating, set by code.
- Ohm's Law
- The relationship V = I × R — voltage equals current times resistance — the basis of the drop formula.
- One-Way Distance
- The length of the run measured in a single direction, from the source to the load.
- Phase Factor
- The multiplier in the drop formula — 2 for DC and single-phase, 1.732 for three-phase.
- Conductor
- The metal wire that carries current — usually copper or aluminum.
- NEC
- The National Electrical Code, the US standard for electrical installation, including voltage drop guidance.
- Parallel Sets
- Two or more identical conductor runs wired side by side to share current and lower effective resistance.
- Branch Circuit
- The final wiring run between the last overcurrent device and the load it serves.
- Feeder
- A circuit between the service equipment and a branch-circuit panel further down the line.
- Load Current
- The current, in amps, that the connected equipment draws from the circuit.
- Voltage at Load
- The voltage actually reaching the equipment — the source voltage minus the voltage drop.
Frequently Asked Questions
The most common questions about calculating and controlling voltage drop.
How do I calculate voltage drop?
Voltage drop equals the current multiplied by the wire's total resistance. For a DC or single-phase circuit, the formula is 2 × current × resistance per foot × one-way distance, because the current travels out to the load and back. For three-phase, the factor is the square root of 3 instead of 2.
What is an acceptable voltage drop?
The National Electrical Code recommends keeping voltage drop at or below 3 percent on a branch circuit, and no more than 5 percent total across the feeder and branch combined. These are recommendations, not hard rules — but staying within them protects equipment performance and efficiency.
Why does voltage drop matter?
Voltage that is lost in the wire never reaches the device. Excessive drop makes motors run hot, lights dim, and electronics behave unreliably, and the lost energy is wasted as heat in the cable. Sizing the wire to limit drop keeps equipment running as designed.
How does wire length affect voltage drop?
Voltage drop is directly proportional to the length of the run. Double the distance and you double the drop. This is why long circuits often need a larger wire gauge than a short run carrying the same current — the extra copper offsets the added resistance.
Does a bigger wire reduce voltage drop?
Yes. A larger wire has a lower resistance per foot, so it loses less voltage for the same current and distance. Going up one or two gauge sizes is the standard fix for a circuit that fails the voltage drop check on a long run.
What is the difference between copper and aluminum wire for voltage drop?
Aluminum has a higher resistance than copper for the same gauge, so an aluminum conductor produces more voltage drop than a copper one of the same size. To match copper's performance, aluminum wire generally needs to be one or two sizes larger.
Why is the distance doubled in the voltage drop formula?
In a DC or single-phase circuit, current flows from the source to the load through one conductor and returns through another. The total wire the current passes through is twice the one-way distance, so the formula multiplies the one-way length by two.
How do I calculate voltage drop for three-phase?
For a balanced three-phase circuit, voltage drop equals the square root of 3 (about 1.732) multiplied by the current, the resistance per foot, and the one-way distance. The factor is 1.732 rather than 2 because of how current is shared across the three conductors.
What is voltage drop percentage?
Voltage drop percentage is the voltage lost in the wire divided by the source voltage, expressed as a percent. A 120-volt circuit that drops 3.6 volts has a 3 percent voltage drop. The percentage is the figure usually compared against code recommendations.
Does adding parallel conductor sets reduce voltage drop?
Yes. Running two identical sets of conductors in parallel halves the effective resistance, which halves the voltage drop. Parallel sets are common on large feeders where a single conductor would otherwise need to be impractically large.
What causes high voltage drop?
The main causes are a long wire run, a wire gauge that is too small for the current, a high load current, and using aluminum where copper would do better. High voltage drop is almost always solved by shortening the run or increasing the wire size.
How accurate is this voltage drop calculator?
The calculation is exact arithmetic from the figures you enter, using standard conductor resistance values. Real circuits also have temperature effects and, on AC, a small reactance component. For most planning it is highly accurate — for code-critical work, confirm against NEC conductor tables.
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