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Single-Phase & Three-Phase Voltage Drop Calculator

A conductor sized correctly for ampacity can still arrive at the load several volts low, and the 3 and 5 percent figures everyone quotes are recommendations in informational notes, not enforceable requirements. This tool calculates drop for single-phase and three-phase runs, names the smallest conductor that meets your target, and shows every intermediate value it used.
Code basis
NEC 2023, 210.19 / 215.2
Editions
2017, 2020, 2023
Modes
Single-phase, three-phase
Conductor sizes
24
Materials
Copper, aluminum
Methods
Table 8 DC resistance
Targets
3% branch, 5% combined

Circuit

Voltage options change with the mode. A two-pole 208 V circuit from a three-phase panel is a single-phase load.

Mode

Current actually flowing, not the breaker rating.

Panel to load. The return path is already accounted for.

What the verdict is judged against.

Conductor

Uses launch-safe Table 8 resistance data. Table 9 impedance is held back until verification closes.

Parallel sets divide the resistance. Permitted from 1/0 upward.

Match the edition your jurisdiction enforces.

Voltage drop

Over target

3.89%

8 AWG copper, 150 ft one-way at 40 A. Target is 3 percent.

Volts dropped
9.34 V
Voltage at load
230.7 Vfrom 240 V nominal
Smallest size within target
6 AWGDrop only. Check ampacity separately.
Resistance used
0.778 ohm/kft
  • Drop is 3.9 percent, above the 3 percent target. 6 AWG is the smallest tabulated size that meets it over 150 ft.
  • The 3 and 5 percent figures are recommendations in informational notes, not enforceable requirements. Some jurisdictions and some other standards do enforce a limit.
  • Drop is calculated from direct-current resistance at 75 degrees C. The unverified Table 9 impedance path is not shipped in the public calculator.
Size this conductor for ampacity

Derivation

StepBasisEffectValue
DC resistance, 8 AWG copper stranded at 75 CChapter 9, Table 8—0.778 ohm/kft
Circuit resistance over 150 ft, doubled for the return conductor—× 20.2334 ohm
Drop at 40 A——9.34 V
Share of the 240 V nominal system voltage210.19(A), Informational Note—3.89 %
Derivation of the voltage drop over this run, from tabulated resistance to percentage

Drop by size

SizeOhm/kftDropPercentVerdict
12 AWG1.9823.769.90%Over target
10 AWG1.2414.886.20%Over target
8 AWG0.7789.343.89%Over target
6 AWG0.4915.892.46%Within target
4 AWG0.3083.701.54%Within target
3 AWG0.2452.941.23%Within target
2 AWG0.1942.330.97%Within target
Voltage drop at the entered current and length across nearby conductor sizes.

Every row assumes 40 A over 150 ft at 240 V, single-phase. Ampacity is not checked here.

Calculated against the NEC 2023 edition using 210.19(A) informational note, 215.2(A) informational note, Chapter 9, Table 8. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Table 9 impedance mode is not publicly selectable until the data verification gate closes.
  • Conductor operating temperature other than 75 degrees C. Resistance falls as a conductor runs cooler, so a lightly loaded circuit drops slightly less than shown.
  • Coated copper and solid conductors, both of which are tabulated separately and run a few percent higher.
  • Harmonic current on the neutral of a three-phase, four-wire system supplying nonlinear load.
  • Motor starting inrush, where momentary drop is judged against the equipment manufacturer's tolerance rather than these percentages.
  • Ampacity, overcurrent protection and raceway fill, which are covered by the other calculators.

Data last verified

Calculation Method

Four steps, matching a hand calculation from the conductor properties table.

  1. Step 1

    Resistance lookup

    Reads ohms per 1000 ft from the stranded uncoated resistance data at 75 degrees C. Table 9 impedance mode stays hidden until the R/X dataset is verified for launch.

  2. Step 2

    Circuit length

    Single-phase doubles the one-way length, because current returns on a second conductor. Three-phase multiplies by the square root of three, which accounts for the 120-degree displacement between line currents.

  3. Step 3

    Drop at the operating current

    Multiplied by the current actually flowing. Use the running load, not the breaker rating: a 50 A breaker on a 32 A load produces a 32 A drop.

  4. Step 4

    Percentage of nominal

    Volts dropped divided by nominal system voltage, compared against the selected target. Exceeding the target is reported as a caution, never as a failure.

Drop is calculated at the operating current, not at the design current used for conductor sizing. The 125 percent continuous-load multiplier is a sizing rule for conductors and overcurrent devices; it is not a statement about how much current flows.

Recommended Limits and Their Actual Status

The single most misrepresented point in this calculation. Only the last row is enforceable.

SegmentRecommended maxStatusBasis
Branch circuit, panelboard to outlet3%Recommendation210.19(A), Informational Note
Feeder, service equipment to panelboard3%Recommendation215.2(A), Informational Note
Feeder and branch circuit combined5%RecommendationBoth notes read together
Sensitive equipment with a stated tolerancePer nameplateManufacturer requirementEquipment listing and instructions
Recommended maximum voltage drop by circuit segment, with the enforceable status of each figure.

The two percentages are not additive in the direction most people assume. A feeder held to 3 percent leaves 2 percent for every branch circuit behind it, not 3 more. Where a long feeder is unavoidable, holding it to 1 percent buys back headroom downstream.

Maximum One-Way Run at 3 Percent

Copper at 240 V single-phase. Use it as a sanity check before opening the calculator.

Size20 A40 A60 A100 A
14 AWG————
12 AWG90 ft———
10 AWG145 ft———
8 AWG230 ft115 ft——
6 AWG365 ft180 ft120 ft—
4 AWG580 ft290 ft190 ft—
2 AWG925 ft460 ft305 ft185 ft
1/0 AWG1475 ft735 ft490 ft295 ft
2/0 AWG1860 ft930 ft620 ft370 ft
4/0 AWG2960 ft1480 ft985 ft590 ft
Longest one-way run in feet that stays within 3 percent drop, copper conductors at 240 V single-phase, rounded down to the nearest 5 ft. Computed from the direct-current resistance values in the NEC conductor properties table. A dash means the conductor may not carry that current, whether limited by its tabulated ampacity or by the small conductor overcurrent rule.

Halve these lengths for a 120 V circuit at the same current: drop percentage is inversely proportional to system voltage. Aluminum of the same size reaches roughly 60 percent of the distance.

When Drop Governs the Size

Drop rarely decides a short circuit and frequently decides a long one. These are the cases that come up.

InstallationTypical outcomeAction
20 A receptacle circuit to a detached garage, 180 ftAmpacity allows 12 AWG; drop needs 8 AWGUpsize the ungrounded conductors and increase the grounding conductor proportionally.
100 A subpanel feeder, 240 V, 200 ftDrop lands between 3 and 5 percentAcceptable as a feeder alone. Budget the remainder for the branch circuits downstream.
480 V three-phase feeder, 400 ftUsually passes at the ampacity-driven sizeHigher voltage carries the same power at lower current, so drop is rarely the constraint.
Well pump on 240 V, 350 ft of 10 AWGWell over 3 percent under running loadUpsize the conductors. Starting current makes the momentary drop several times larger.
EVSE at 48 A on a 240 V circuit, 120 ftMarginal on 6 AWG copperCheck 4 AWG. Charging is a continuous load, so the drop is sustained rather than intermittent.
120 V lighting branch at 12 A, 90 ftPasses on 12 AWGNo change needed. At 120 V a short run still leaves headroom for the feeder ahead of it.
Typical voltage drop outcomes by installation and the action each one calls for.

Choosing How to Fix an Excessive Drop

Six options, ordered from the most common to the most situational. Upsizing is not always the cheapest.

  • Increase the conductor size

    The direct fix and usually the cheapest. Resistance roughly halves for every three AWG sizes, so two sizes up cuts drop by about a third. Confirm the terminations and raceway accept the larger conductor before pricing it.

  • Run parallel sets

    Two sets halve the resistance. Permitted from 1/0 upward, and every set must match in size, material, insulation, length and termination method. Practical where a single conductor would exceed what the equipment lugs accept.

  • Raise the system voltage

    The same power at 480 V draws less than half the current it does at 208 V, and drop scales with current. Where a 277/480 V system is available, feeding at the higher voltage and transforming locally often costs less than upsizing copper.

  • Move the distribution point

    Drop scales linearly with length. A subpanel located near the load turns one long run at full load current into a short run plus several short branch circuits, which is frequently the lowest-cost answer on a long site.

  • Reallocate the budget between segments

    The 5 percent figure covers the feeder and the branch circuit together. A feeder held to 1 percent leaves 4 percent for the branch circuits behind it, which can be the difference between upsizing a 300 ft feeder and upsizing nothing.

  • Switch material deliberately, not by default

    Aluminum of the same size has roughly 1.6 times the resistance of copper, so matching a copper run generally takes two sizes larger. On a long feeder that is still often cheaper per foot. Confirm the terminations are listed for aluminum.

FAQs

Is exceeding 3 percent voltage drop a code violation?
No. The 3 percent branch-circuit and feeder figures and the 5 percent combined figure appear in informational notes, which are explanatory material and are not enforceable as requirements. That is why this calculator reports a caution rather than a failure. Treat it as design practice with two exceptions: some jurisdictions adopt a mandatory limit by local amendment, and other standards that may apply to a project, including energy codes and specific equipment listings, do impose limits. Confirm with your AHJ.
Should I enter the breaker rating or the actual load current?
The current that actually flows. Drop is proportional to current, so entering a 50 A breaker rating for a 32 A load overstates the drop by more than half. Size the conductor for ampacity at the design current, which does include the 125 percent continuous-load multiplier, then check drop at the operating current.
Why does the single-phase calculation multiply by two?
Current leaves on the ungrounded conductor and returns on the grounded conductor or on a second ungrounded conductor, so it traverses twice the one-way length. Three-phase uses the square root of three instead, because the line currents are displaced by 120 degrees and the vector sum of the three line-to-line drops is smaller than a straight doubling. Enter the one-way length in both modes; the multiplier is applied for you.
Does this calculator use Table 8 or Table 9?
The public calculator uses the direct-current resistance of stranded uncoated conductors at 75 degrees C. Table 9 impedance mode stays hidden until the encoded R/X values are verified cell by cell against the adopted edition.
Which conductor size does the tool recommend?
The smallest size in the resistance table whose drop falls within the selected target at the entered current and length. It is a drop answer only. It does not check that the conductor can carry the current, that the breaker may protect it, or that it fits the raceway, all of which can require a larger size than drop alone. Run the wire size calculator for the governing constraint across all of them.
Do parallel conductors really halve the drop?
Two sets in parallel halve the effective resistance and therefore halve the drop, provided the sets are electrically identical. Parallel conductors are permitted from 1/0 upward and every set must match in size, material, insulation type, length and termination method, because unequal impedance makes the sets share current unequally and overloads the shorter one.
Does voltage drop change the equipment grounding conductor?
Yes, when the ungrounded conductors are upsized for drop. The grounding conductor is normally sized from the overcurrent device rating, but where the ungrounded conductors are increased in size the grounding conductor must be increased proportionally to their increase in circular mil area. The breaker rating does not change, so the table lookup alone will understate it.
Why is my measured drop different from the calculated figure?
Calculated drop assumes the tabulated resistance of a clean conductor at 75 degrees C carrying the current you entered. In the field, a conductor running cooler has lower resistance, loose or corroded terminations add resistance the calculation does not model, the actual current fluctuates, and the utility supply voltage itself moves. A measurement taken under load at the far end of the run is the authoritative figure; the calculation is for sizing before anything is installed.