Calculate wire voltage loss, percent drop, and voltage at load for AC, DC, and three-phase circuits so you can choose a safer conductor size before you install.
Follow these steps to estimate voltage loss, compare wire sizes, and review results for NEC-style design checks.
Input the current flowing through the circuit in amperes.
Specify the total length of the cable run in feet.
Select the appropriate wire gauge (AWG) from the dropdown.
View calculated voltage drop and percentage values instantly.
Learn what the calculator is showing you and how to decide whether a circuit design is healthy, marginal, or ready for a larger conductor.
Voltage drop is the difference between the source voltage and the voltage that actually arrives at the load. Every conductor has resistance, so every circuit loses some electrical pressure while current travels through the wire. The goal is not to force the drop to zero. The goal is to keep the loss small enough that your lights, heaters, chargers, electronics, and motors still operate as intended.
The first number to watch is voltage drop in volts. This shows the raw amount of voltage lost in the run. The second number is percentage drop, which is usually the most useful design metric because it tells you how large the loss is relative to your system voltage. A 3-volt loss is minor on a 480V feeder, but it is serious on a 12V battery circuit.
Your third key output is voltage at load. This is the voltage your equipment actually sees after wire losses are taken out. If that value falls too low, performance can change quickly. Motors can overheat or struggle to start, LED drivers can flicker, and sensitive electronics can reset under heavy load. That is why electricians check both ampacity and conductor resistance before locking in wire size.
Use this when you want to know the actual lost volts across the one-way distance and return path of the circuit.
Use this for design decisions. It helps you compare a 12V circuit, a 120V branch circuit, and a 480V feeder on the same scale.
Use this to estimate what your equipment receives at the end of the run, especially on motors, chargers, and low-voltage systems.
As a practical rule, many designers aim for 3% or less on a branch circuit and 5% or less for the feeder plus branch circuit together. Those targets are popular because they balance performance, energy efficiency, and cost. If your result lands between 3% and 5%, you may still be workable, but you should think about the type of load, startup current, future expansion, and the consequences of voltage sag. If you are over 5%, the circuit is usually a good candidate for upsizing, shortening the run, or changing the design.
Also remember that the calculator is only part of the design picture. You still need to verify ampacity, insulation rating, ambient temperature, conduit fill, parallel conductor rules, local code amendments, and equipment manufacturer instructions. Voltage drop explains how well the circuit delivers power. Ampacity tells you whether the conductor can carry the load safely. You need both.
See how to calculate voltage drop manually so you can check a field estimate, understand the math, or explain the result to a customer or inspector.
The simplest voltage drop relationship comes from Ohm's Law: voltage loss equals current multiplied by resistance. For a DC or single-phase run, the conductor path includes the outgoing wire and the return path, so the common field formula is: Vdrop = 2 x I x R x L when resistance is expressed per unit length. For a balanced three-phase circuit, the factor of 2 is replaced by 1.732, which is the square root of 3.
DC or single-phase AC: Vdrop = 2 x I x R x L
Three-phase AC: Vdrop = 1.732 x I x R x L
I = load current, R = conductor resistance or impedance per unit length, L = one-way distance
In this calculator, the resistance values come from NEC-style wire data. For AC circuits, the tool also considers conduit type and power factor so you can estimate impedance instead of relying only on DC resistance. That makes the result more useful for motor loads, commercial circuits, and longer runs where reactance starts to matter.
Say you are planning a 120V, 20A branch circuit to a detached workshop that sits 150 feet away. You want to know whether 12 AWG copper is good enough.
Using a common copper resistance value of 1.98 ohms per 1000 feet, the calculation is:
Vdrop = (2 x 150 x 20 x 1.98) / 1000 = 11.88 volts
Percentage drop = 11.88 / 120 x 100 = 9.9%
That is far above the common 3% design target for a branch circuit. If you upsize the same run to 6 AWG copper at roughly 0.491 ohms per 1000 feet, the result becomes:
Vdrop = (2 x 150 x 20 x 0.491) / 1000 = 2.95 volts
Percentage drop = 2.95 / 120 x 100 = 2.46%. That revised design is much closer to what you would normally want for reliable tool performance.
Manual math is useful, but you still need judgment. A heater may tolerate more drop than a motor starting under load. A short feeder in a cool mechanical room behaves differently from a long low-voltage landscape run outdoors. That is why this calculator lets you test conductor material, phase, power factor, and distance quickly before you buy cable or pull wire.
These examples show where voltage drop calculations matter most and how a small design choice can change the result.
Long residential runs are one of the most common reasons people exceed acceptable voltage drop. A 120V, 20A circuit that travels 150 feet one way will often need much more than 12 AWG if you want strong motor starting and stable voltage at the load. If you are feeding saws, compressors, or dust collection equipment, treat branch circuit drop seriously. A design that looks fine on breaker size can still perform badly at the receptacle.
Electric vehicle charging is a high-value use case because it combines continuous current with a fixed equipment location. A 240V charger drawing 48A over an 80-foot run on 6 AWG copper can stay near 1.34% drop, which is excellent. For EV projects, check both the continuous-load ampacity rules and the voltage drop. A slightly larger conductor can improve charging performance and reduce wasted heat over years of daily use.
Low-voltage systems punish small design mistakes. A 12V lighting run carrying 8A over 75 feet on 14 AWG copper can lose more than 30% of its source voltage. That is why fixtures near the transformer can look bright while the far end looks weak or uneven. On 12V and 24V systems, heavier cable, shorter runs, or split circuits are often necessary from the start.
In a solar or battery installation, voltage drop becomes lost energy. If your 48V solar segment drops 5.1%, that is 5.1% of output turned into heat instead of useful charging power. Many installers target tighter numbers on DC renewable systems than they would on a normal branch circuit. If you are planning panels, combiner runs, charge controllers, or battery-to-inverter links, keep one-way distance short and conductor resistance low.
A 480V three-phase motor run can look forgiving because the percentage drop stays lower at higher voltage, but motor loads still need careful planning. Startup current is much higher than running current, so a circuit that seems acceptable on paper can still be unpleasant during acceleration. For compressors, pumps, and process equipment, keep an eye on power factor, conductor impedance, and the actual voltage at load during heavy operation.
One of the biggest content gaps on the old page was practical distance guidance. This section gives you a fast way to sanity-check long runs before you pull wire.
Designers often talk about the NEC 3% and 5% recommendations because they are useful planning targets. In everyday terms, 3%-or-less on the branch circuit usually keeps equipment happy, while 5% total for the feeder plus branch circuit is a common upper design boundary. The exact enforcement can vary by local rules and by the authority having jurisdiction, but these targets are still a strong starting point for good electrical work.
Distance matters because voltage drop grows in direct proportion to one-way run length. Double the run and, all else equal, you double the drop. That is why outbuildings, parking lot lights, pumps, EV chargers, and rooftop equipment regularly need larger conductors than a quick glance at the breaker rating would suggest.
| Circuit | Approx. max one-way distance at 3% | Notes |
|---|---|---|
| 120V / 20A / 12 AWG copper | About 55 feet | A common residential branch circuit that gets tight fast on longer runs. |
| 120V / 20A / 10 AWG copper | About 87 feet | A practical upsizing move for workshops, garages, and shed circuits. |
| 120V / 20A / 8 AWG copper | About 138 feet | Often needed when distance is the real design problem. |
| 240V / 20A / 12 AWG copper | About 109 feet | Higher voltage lowers the percentage drop for the same conductor resistance and load. |
| 240V / 30A / 10 AWG copper | About 116 feet | Useful for water heaters, small equipment, and some EV circuits. |
| 12V / 10A / 12 AWG copper | About 9 feet | Shows why low-voltage systems need short runs or heavier wire. |
Treat those distances as design-level guidance, not a substitute for a full review. Ambient temperature, aluminum wire, conduit choice, power factor, parallel conductors, and actual voltage can all shift the result. If you are close to the limit, run the exact numbers in the calculator instead of relying on a shortcut table.
If your percentage drop is too high, your usual fixes are straightforward: choose a larger wire size, shorten the one-way distance, reduce the load current, or use a higher system voltage when the equipment allows it. In many projects, moving from 120V to 240V or from aluminum wire to copper wire can make a bigger difference than people expect.
Keep your electrical planning moving with related LiteCalc tools for power, current, unit conversion, and supporting calculations.
Calculate watts, amps, and volts when you need a quick power check before choosing a circuit design.
Review voltage, current, resistance, and power relationships with a broader electrical calculator.
Solve for volts, amps, ohms, or watts when you need a fast companion calculation.
Convert temperature values when ambient heat matters to conductor resistance and derating decisions.
Convert feet, meters, kilometers, and other units when you are checking one-way distance and project measurements.
Measure long runs more confidently when you need a quick distance estimate for outbuildings and site equipment.
Quick answers to the voltage drop questions people ask most before sizing wire, planning a long run, or checking NEC design targets.
For a single-phase AC or DC run, voltage drop is commonly estimated as 2 x current x one-way distance x conductor resistance. For balanced three-phase systems, use 1.732 x current x one-way distance x conductor resistance. The exact resistance depends on wire size, material, temperature, and whether you use DC resistance or AC impedance data.
A common NEC design target is no more than 3% voltage drop on the branch circuit and no more than 5% total on the feeder plus branch circuit together. These are widely used design recommendations because they help equipment run correctly and reduce wasted energy.
The right wire size depends on your current, one-way distance, voltage, conductor material, phase, and the maximum drop you will allow. Start with a wire that meets ampacity rules, then check whether the voltage drop stays inside your design limit. Long runs often need a larger conductor than ampacity alone would suggest.
As a rule of thumb, 12 AWG copper on a 120-volt, 20-amp circuit reaches about 55 feet one way before it gets close to a 3% drop. At 240 volts, the same wire can go roughly twice as far for the same load because the percentage drop is lower.
Yes. Aluminum has higher resistance than copper at the same gauge, so it will produce more voltage drop over the same length and load. To get similar performance, you usually need to upsize aluminum conductors compared with copper.
For a balanced three-phase circuit, use the square-root-of-three version of the formula: 1.732 x current x one-way distance x conductor impedance. This reflects the way current returns through the other phases rather than through a separate return conductor.
The NEC commonly points designers toward a 3% maximum drop on branch circuits and 5% total on the feeder plus branch circuit to the farthest outlet. Many electricians treat those limits as best-practice design targets even where they are not enforced as mandatory code language.
The most common fixes are using a larger wire size, shortening the run, reducing the load current, or increasing system voltage where the equipment allows it. In some projects, splitting the load or using copper instead of aluminum also helps.
Low-voltage systems lose a much larger percentage of their supply when even a small number of volts is lost in the wire. A 1-volt drop on a 120-volt circuit is minor, but a 1-volt drop on a 12-volt circuit is a major performance hit.