Voltage Drop Calculator

A voltage drop calculator finds how many volts a wire loses over its run using Vd = 2 × L × I × R ÷ 1000 (single phase). Example: AWG 12 copper carrying 15 A at 120 V over 100 feet drops 5.79 V - a 4.8% loss, over the recommended 3% limit - so you upsize to AWG 10. This tool shows the volts dropped, the percent drop, the voltage reaching the load, and the next gauge that passes.

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.

Voltage drop by wire gauge and distance

This matrix shows the percent voltage drop for a 240 V, 20 A single-phase circuit as the copper gauge and one-way run change. Red cells are over the recommended 3% limit - read down a column to see how far each gauge reaches before it fails.

Assumes stranded copper at ~75°C. At 120 V the same volts lost are double the percent, so halve the reach; at other currents, drop scales in direct proportion.

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.

How far can you run each wire gauge?

This quick reference gives the maximum one-way run in feet that keeps a single-phase copper circuit at or under 3% for common voltage and current pairs. Beyond these lengths, size up a gauge.

Computed from the one-way limit L = 0.03 × V × 1000 ÷ (2 × I × R). These lengths satisfy voltage drop only - always confirm the conductor's ampacity for the load first.

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).

3-phase voltage drop calculator: single vs three phase

A three-phase balanced load uses √3 (1.732) in place of the 2, so for the same current and run it shows less line-to-line drop. This table compares both for a 480 V, 30 A load over a 200 ft one-way run - switch the phase selector above to reproduce either column.

Three-phase percentages are about 13.4% lower than single-phase for the same inputs (√3 ÷ 2). Three-phase drop uses the line-to-line voltage and assumes a balanced load.

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:

Wiring is just one part of a build - the rest of our construction tools estimate materials rather than electrical loads. Use the Concrete Calculator for a slab or footing, the Roofing Calculator for shingle squares, the Roof Pitch Calculator to turn rise-over-run into an angle, and the Paint Calculator for gallons of coverage. Reach for this voltage drop tool only when you are sizing conductors for a circuit run.

• To work out the underlying volts, amps, ohms or watts on a circuit, use the Ohm's Law Calculator.
• To size heating or cooling output for a space, use the BTU Calculator.
• To estimate concrete for a pad, slab or footing under a panel or pad-mount, use the Concrete Calculator.
• To measure the area of a room, shop or yard you are wiring, use the Square Footage Calculator.
• To figure tons or yards of gravel for a trench base or pad, use the Gravel Calculator.
• To convert dimensions of any material into yards, use the Cubic Yard Calculator.
• To estimate gallons for painting the finished space, use the Paint Calculator.

Voltage drop and Ohm's law

Voltage drop is just Ohm's law applied to the wire itself. Ohm's law says V = I × R: the voltage across any resistance equals the current through it times that resistance. The conductor between your panel and the load is a resistor, so the voltage it "eats" is the current times the round-trip wire resistance. That is exactly what the formula computes - the × 2 (or √3) and the ÷ 1000 simply convert one-way feet and ohms-per-1000-ft into the total resistance in the circuit. If you want to explore the V = I × R relationship on its own, or solve for power and resistance, the Ohm's Law Calculator handles the general case while this tool specializes it for long wire runs.

DC and low-voltage runs (12V, 24V, 48V)

Low-voltage DC systems - RV and marine wiring, solar arrays, landscape lighting, and 12V or 24V battery banks - are where voltage drop bites hardest, because the percentage is measured against a small number. A 0.5-volt drop is trivial on a 240 V circuit (0.2%) but is a painful 4.2% on a 12 V system. That is why off-grid and automotive installers often target a tighter 2-3% on DC and run surprisingly thick cable for modest currents. The single-phase formula in this calculator (with the × 2 factor for the out-and-back conductors) applies directly to two-wire DC circuits; just enter your nominal DC system voltage, the load current in amps, and the one-way run. The copper resistance values are the same whether the current is AC or DC at these gauges.

Sources

• National Fire Protection Association (NFPA) - NFPA 70, National Electrical Code (NEC): branch-circuit and feeder voltage-drop informational notes (3% / 5%) and Chapter 9, Table 8 conductor properties.
• NEC Chapter 9, Table 8 - direct-current resistance values for copper and aluminum conductors in ohms per 1000 ft, the basis for the resistances used here.
• NEC Article 210.19 and 215.2 informational notes - recommended voltage drop on branch circuits and feeders.