Voltage Drop Calculator

Run AWG 12 copper 100 feet to a 15-amp, 120-volt load and you lose about 5.79 volts along the way - roughly a 4.8% drop, which is over the recommended limit. Step up to AWG 10 and the same run drops only about 3.6 volts (around 3%), and AWG 8 brings it comfortably under. That swing is the whole point of a voltage drop calculator: the wire that is "big enough" to carry the current safely can still deliver too little voltage to the device at the end of a long run. This tool shows the drop in volts, the percent drop, the voltage that actually reaches the load, and whether you are over the 3% line - plus the next copper gauge that fixes it.

How voltage drop is calculated

For a single-phase circuit, the calculator uses the standard formula:

where L is the one-way distance in feet, I is the load current in amps, and R is the conductor resistance in ohms per 1000 feet. The 2 accounts for current traveling out on one conductor and back on the other. For a balanced three-phase circuit, the 2 is replaced by the square root of 3 (about 1.732). The percent drop is simply the volts dropped divided by the system voltage, times 100.

A worked example, step by step

Take a 120 V, 20 A single-phase circuit running 150 feet on AWG 10 copper (R = 1.21 ohms per 1000 ft):

• Volts dropped: 2 × 150 × 20 × 1.21 ÷ 1000 = 7.26 V.
• Percent drop: 7.26 ÷ 120 × 100 = 6.05% - well over the 3% target.
• Voltage at the load: 120 − 7.26 = 112.74 V.

To pass, you would jump to AWG 6 copper (R = 0.491), which drops only about 2.95 V or roughly 2.45%. The calculator runs that same comparison automatically and tells you which gauge to use.

What counts as acceptable voltage drop

The National Electrical Code does not mandate a maximum in most cases, but its informational notes recommend keeping voltage drop to about 3% on a branch circuit and no more than 5% total for the feeder and branch circuit combined. Staying under 3% keeps motors, electronics, lighting and chargers operating at their rated voltage. This calculator uses the 3% branch-circuit figure as its pass/fail threshold.

Why voltage drop matters

Every foot of wire has resistance, and resistance turns electrical energy into heat. On a short run that loss is negligible, but it grows with length and current. The consequences of too much drop include:

• Wasted energy: the power lost in the wire is heat you pay for but never use.
• Underperforming equipment: motors lose torque and run hot, heaters and chargers slow down, and pumps move less water.
• Flicker and failures: LED drivers and sensitive electronics can flicker, reset, or wear out early on a sagging supply.
• Nuisance trips: motors drawing extra current to compensate can trip breakers or overloads.

How to use this voltage drop calculator

You only need four numbers and a phase selection to get a realistic answer. Work through the fields in order:

1. System voltage: enter the nominal circuit voltage. Common choices are 120 V and 240 V for homes, and 277 V or 480 V for commercial work - use the quick buttons or type your own.
2. Load current: enter the running amps of the load (not the breaker size). For motors, use the full-load amps from the nameplate.
3. One-way distance: measure from the panel to the load along the actual cable route, in feet. Enter the one-way length; the formula doubles it for you.
4. Wire gauge: pick the copper AWG you plan to use. Smaller numbers mean thicker wire and less drop.
5. Phase: choose single phase for typical residential circuits, or three phase for balanced three-phase loads.

Press Calculate and read the percent drop at the top. If it is highlighted red, it is over 3%; the calculator names the next gauge that passes and shows a full table so you can see the trade-offs across all sizes.

Who this calculator is for

This tool is built for anyone running a circuit far enough that the wire length matters. That includes:

• Homeowners and DIYers wiring a detached garage, shed, shop, or backyard outlet.
• EV-charger installers sizing a 240 V run from the panel to a parking spot.
• Solar and off-grid builders checking long DC and AC runs between panels, inverters and loads.
• Pump and irrigation installers feeding a well pump or pivot hundreds of feet away.
• Apprentices and estimators sanity-checking a design before pulling wire or buying it.

Three common scenarios

Voltage drop problems almost always come down to long runs at meaningful current. Here are three typical cases:

• Backyard shed, 120 V / 15 A / 120 ft: on AWG 12 the drop is about 5.8% - too much. AWG 8 brings it to roughly 2.3%.
• EV charger, 240 V / 40 A / 150 ft: on AWG 8 the drop is about 3.8% - over the line; moving to AWG 6 lands near 2.5% and passes.
• Well pump, 240 V / 10 A / 250 ft: on AWG 10 the drop is about 2.5% - acceptable - but at 350 ft the same wire climbs to roughly 3.5% and exceeds 3%, so distance alone can force an upsize.

Factors that change the result

Adjust the inputs and you will see a handful of factors dominate the outcome:

• Distance: drop is directly proportional to length - double the run, double the drop.
• Current: also directly proportional - a heavier load on the same wire drops more voltage.
• Wire gauge: the biggest lever you control. Each step up in size cuts resistance by roughly 20-37%.
• System voltage: higher voltage means a smaller percent drop for the same volts lost, which is why 240 V tolerates longer runs than 120 V.
• Phase: three-phase uses √3 instead of 2, so it shows less line-to-line drop than single phase at the same current.
• Conductor material and temperature: aluminum and hotter conductors have higher resistance and more drop (this tool assumes copper near 75 degrees C).

Key electrical terms

• AWG (American Wire Gauge): the U.S. sizing system for wire. Counterintuitively, a smaller number is a thicker wire with lower resistance.
• Ampacity: the maximum current a conductor can carry safely. It sets the minimum legal wire size before you even consider voltage drop.
• Resistance (ohms/1000 ft): how much a conductor opposes current per thousand feet. It is the R in the voltage-drop formula.
• One-way vs. circuit length: one-way is panel-to-load; circuit length is the full out-and-back path. The formula multiplies one-way by 2.
• Branch circuit vs. feeder: a branch circuit supplies the final loads; a feeder supplies a subpanel. The 3% target is for the branch; 5% is the combined total.
• Single vs. three phase: single phase is the typical residential supply; three phase is common in commercial and industrial settings and uses the √3 factor.

Tips to keep voltage drop in check

• Size up one gauge on long runs. If you are near the 3% line, the next size down in AWG number buys a comfortable margin for cheap insurance.
• Use the higher voltage where you can. A 240 V circuit drops half the percent of a 120 V circuit at the same wattage, so it reaches much farther.
• Route smart, but don't cheat the math. Measure the real cable path including vertical drops and bends; "as the crow flies" underestimates length.
• Buy about 10% extra wire. Slack at both ends, splices, and routing around obstacles eat into a spool faster than you expect - order ~10% more than the straight-line estimate.
• Check ampacity first, then drop. Pick the smallest wire that is legal for the current, then upsize only if the drop is too high.

Limitations and assumptions

This calculator is a fast planning estimate, not a finished design. Keep these assumptions in mind:

• It assumes copper conductors at about 75 degrees C; aluminum and hotter conductors drop more.
• It uses simple DC-style resistance and does not model reactance, power factor, or harmonics, which matter on large or inductive loads.
• It does not check ampacity, derating, conduit fill, or termination temperature limits - all of which can require a larger wire than voltage drop alone.
• It assumes a balanced load for three-phase and a single steady current, not motor inrush or varying demand.
• Final sizing must follow the current edition of the National Electrical Code and any local amendments, verified by a licensed electrician for permitted work.

How it compares to related calculators

This page answers "how much voltage will I lose on this run, and is the wire big enough?" If you have a different project question, a sister tool fits better:

• To estimate concrete for a pad, slab or footing, use the Concrete Calculator.
• To measure the area of a room or yard, use the Square Footage Calculator.
• To figure tons or yards of gravel for a base or driveway, use the Gravel Calculator.
• To size mulch for landscaping beds, use the Mulch Calculator.
• To convert dimensions of any material into yards, use the Cubic Yard Calculator.
• To estimate gallons for a paint job, use the Paint Calculator.