Voltage Drop Calculator

A voltage drop calculator finds how many volts a wire loses over its run with Vd = 2 × L × I × R ÷ 1,000 (single phase). Example: 12 AWG copper carrying 15 A at 120 V over 100 feet drops 5.94 V, a 4.95% loss, well over the 3% the National Electrical Code recommends. 10 AWG still drops 3.10%, so the run needs 8 AWG (1.95%). The calculator shows the volts dropped, the percent drop, the voltage at the load and the first size that passes, for copper or aluminum.

How voltage drop is calculated

For a single-phase circuit, or a two-wire DC 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 1,000 feet from NEC Chapter 9, Table 8. 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 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 10 AWG copper (R = 1.24 ohms per 1,000 ft):

• Volts dropped: 2 × 150 × 20 × 1.24 ÷ 1,000 = 7.44 V.
• Percent drop: 7.44 ÷ 120 × 100 = 6.20%, about twice the 3% guideline.
• Voltage at the load: 120 − 7.44 = 112.56 V.

8 AWG (R = 0.778) still drops 4.67 V or 3.89%, so the run needs 6 AWG copper (R = 0.491): 2.95 V, or 2.46%. The calculator runs the same comparison automatically and names the first size that passes.

Voltage drop by wire size and distance

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

Stranded copper at 75°C (NEC Chapter 9, Table 8). At 120 V the same volts lost are twice the percent, so halve the reach; at other currents the drop scales in direct proportion.

What counts as acceptable voltage drop

For ordinary branch circuits and feeders the National Electrical Code does not set a mandatory maximum (a few special systems, such as fire pumps under Article 695, do). Its informational notes to 210.19(A) and 215.2(A) recommend keeping voltage drop to about 3% on a branch circuit and no more than 5% total for the feeder and branch circuit combined, which the code describes as providing reasonable efficiency of operation. Staying under 3% keeps motors, electronics, lighting and chargers close to their rated voltage. This calculator uses the 3% branch-circuit figure as its pass/fail line.

How far can you run each wire size?

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, rounded down to the whole foot. Beyond these lengths, size up.

Computed from the one-way limit L = 0.03 × V × 1,000 ÷ (2 × I × R). These lengths satisfy voltage drop only; always confirm the conductor's ampacity for the load first.

Copper vs. aluminum conductors

Aluminum conductors cost less and weigh less, which is why they are common for services and large feeders, but they have about 64% more resistance than copper of the same size. Table 8 lists 12 AWG aluminum at 3.25 ohms per 1,000 ft against 1.98 for copper, and 4/0 aluminum at 0.100 against 0.0608. In practice an aluminum run needs a conductor about two sizes larger to match copper's voltage drop. For example, a 240 V, 100 A feeder 150 ft long drops 1.53% on 1/0 copper and 1.99% on 2/0 aluminum; 4/0 aluminum brings it down to 1.25%. Switch the calculator's material to aluminum to run your own numbers. Aluminum terminations must be rated for it, a separate code requirement this tool does not check.

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 need four numbers, a material and a phase. Work through the fields in order:

1. System voltage: enter the nominal circuit voltage. Common choices are 120 V and 240 V for homes, 208, 277 or 480 V for commercial work and 12 V for low-voltage DC; 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. The formula doubles it for you.
4. Material and size: choose copper or aluminum, then the AWG size you plan to use. Smaller numbers mean thicker wire and less drop.
5. Phase: single phase for typical residential circuits and for DC, three phase for balanced three-phase loads.

The result updates as you type. If the headline turns red, the drop is over 3%; the calculator names the first larger size that passes and lists the drop for every size so you can see the trade-offs.

Who this calculator is for

Anyone running a circuit far enough that the wire length matters, including:

• 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 hundreds of feet away.
• Apprentices and estimators checking a design before pulling or buying wire.

Three common scenarios

Voltage drop problems almost always come down to long runs at meaningful current. Three typical copper cases:

• Backyard shed, 120 V, 15 A, 120 ft: 12 AWG drops 5.94%, too much; 10 AWG still drops 3.72%, and 8 AWG brings it to 2.33%.
• EV charger, 240 V, 40 A, 150 ft: 8 AWG drops 3.89%, over the line; 6 AWG lands at 2.46% and passes.
• Well pump, 240 V, 10 A, 250 ft: 10 AWG drops 2.58%, acceptable, but at 350 ft the same wire reaches 3.62%, so distance alone can force an upsize.

Factors that change the result

• Distance: drop is directly proportional to length: double the run, double the drop.
• Current: also directly proportional, so a heavier load on the same wire drops more voltage.
• Wire size: the biggest lever you control. Each two-size step (14 to 12, 12 to 10 AWG) cuts resistance by about 37%, and each step between 1/0 and 4/0 by about 21%.
• 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.
• Material and temperature: aluminum and hotter conductors have higher resistance and more drop. Table 8 values are at 75°C.

3-phase voltage drop: single vs. three phase

A balanced three-phase 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 copper 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 = 0.866). 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. A smaller number is a thicker wire with lower resistance; sizes above 1 AWG are written 1/0, 2/0, 3/0 and 4/0.
• Ampacity: the maximum current a conductor can carry continuously under its conditions of use. It sets the minimum legal wire size before you consider voltage drop.
• Resistance (ohms per 1,000 ft): how much a conductor opposes current per thousand feet, the R in the 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 buildings and uses the √3 factor.

Tips to keep voltage drop in check

• Size up on long runs. If you are near the 3% line, the next larger size buys margin for little extra cost.
• Use the higher voltage where you can. At the same wattage a 240 V circuit draws half the current of a 120 V circuit, so its percent drop is a quarter as large on the same wire.
• Measure the real route. Include vertical runs, bends and detours; a straight-line measurement underestimates the length.
• Buy extra wire. Slack at both ends and routing around obstacles add length beyond the measured run.
• 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

• It uses Table 8 direct-current resistance at 75°C for stranded conductors; hotter conductors drop more.
• It does not model reactance, power factor or harmonics, which matter on large AC feeders and inductive loads (NEC Chapter 9, Table 9 covers AC impedance).
• It does not check ampacity, derating, conduit fill or termination ratings, all of which can require a larger wire than voltage drop alone.
• It assumes a balanced load on three phase and a steady current, not motor inrush or varying demand.
• Final sizing must follow the NEC edition your jurisdiction has adopted and any local amendments, verified by a licensed electrician for permitted work.

Related electrical calculators

This page answers "how much voltage will I lose on this run, and is the wire big enough for the drop?" For the neighboring questions:

• To pick a wire size from ampacity and length together, use the Wire Size Calculator.
• To check that the conductors fit the raceway, use the Conduit Fill Calculator.
• To turn a nameplate in watts or kVA into amps, use the Watts to Amps Calculator or the kVA to Amps Calculator.
• To work out volts, amps, ohms or watts on a circuit, use the Ohm's Law Calculator.

Voltage drop and Ohm's law

Voltage drop is 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 uses up is the current times the round-trip wire resistance. That is exactly what the formula computes: the × 2 (or √3) and the ÷ 1,000 convert one-way feet and ohms per 1,000 ft into the total resistance in the circuit. To explore the V = I × R relationship on its own, or to solve for power and resistance, the Ohm's Law Calculator handles the general case.

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

Low-voltage DC systems (RV and marine wiring, solar arrays, landscape lighting and battery banks) are where voltage drop bites hardest, because the percentage is measured against a small number. A 0.5 V drop is trivial on a 240 V circuit (0.21%) but a painful 4.17% on a 12 V system. A 12 V, 10 A load just 20 feet away drops 6.60% on 12 AWG copper and needs 8 AWG (2.59%) to stay within 3%. The single-phase formula with its × 2 factor applies directly to two-wire DC circuits; enter the nominal DC voltage, the load current and the one-way run. Table 8 values are DC resistances, so they apply exactly.

Sources

• National Fire Protection Association: NFPA 70, National Electrical Code (NEC), 2023 edition (adopted in most states; local adoption differs, so check which edition your jurisdiction enforces): Chapter 9, Table 8 (conductor properties, DC resistance at 75°C) and the voltage-drop informational notes to 210.19(A) and 215.2(A).
• The formulas are Ohm's law applied to the conductor loop (2 × L for single phase and DC, √3 × L for balanced three phase) and need no external data beyond Table 8.