Voltage Drop Calculation Table by Wire Size
Enter one circuit scenario, then compare every listed conductor size at once. The highlighted row is the first size that meets the selected voltage-drop target—not a final ampacity approval.
Default reference · 120 V · 15 A · 50 ft one-way · copper · single-phase · 3% target
Adjust calculation120 V · 15 A · 50 ft · copper · 1-phase · 3%
Use design or operating current and one-way distance. Switching ft/m converts the active length value. DC and single-phase use a factor of 2; balanced three-phase uses √3. Treat 3% and 5% as common design references, not universal legal limits.
No conductor rows match the current search or status filter.
The table can scroll horizontally on smaller screens. The wire-size column stays available as the row identifier.
| Target status | |||||||
|---|---|---|---|---|---|---|---|
| 18 AWG | 9.95% | 11.944 V | 108.06 V | 15.1 ft | > 3% target | 179.2 W | 1,620 / 0.82 |
| 16 AWG | 6.25% | 7.500 V | 112.50 V | 24.0 ft | > 3% target | 112.5 W | 2,580 / 1.31 |
| 14 AWG | 3.92% | 4.708 V | 115.29 V | 38.2 ft | > 3% target | 70.6 W | 4,110 / 2.08 |
| 12 AWG | 2.47% | 2.963 V | 117.04 V | 60.7 ft | Minimum by drop | 44.4 W | 6,530 / 3.31 |
| 10 AWG | 1.55% | 1.864 V | 118.14 V | 96.6 ft | ≤ 3% target | 28.0 W | 10,380 / 5.26 |
| 8 AWG | 0.98% | 1.172 V | 118.83 V | 153.6 ft | ≤ 3% target | 17.6 W | 16,510 / 8.37 |
| 6 AWG | 0.61% | 0.737 V | 119.26 V | 244.1 ft | ≤ 3% target | 11.1 W | 26,240 / 13.30 |
| 4 AWG | 0.39% | 0.464 V | 119.54 V | 388.3 ft | ≤ 3% target | 7.0 W | 41,740 / 21.15 |
| 3 AWG | 0.31% | 0.368 V | 119.63 V | 489.5 ft | ≤ 3% target | 5.5 W | 52,620 / 26.66 |
| 2 AWG | 0.24% | 0.292 V | 119.71 V | 617.3 ft | ≤ 3% target | 4.4 W | 66,360 / 33.63 |
| 1 AWG | 0.19% | 0.231 V | 119.77 V | 778.5 ft | ≤ 3% target | 3.5 W | 83,690 / 42.41 |
| 1/0 AWG | 0.15% | 0.183 V | 119.82 V | 982.3 ft | ≤ 3% target | 2.7 W | 105,600 / 53.51 |
| 2/0 AWG | 0.12% | 0.145 V | 119.85 V | 1238.1 ft | ≤ 3% target | 2.2 W | 133,100 / 67.44 |
| 3/0 AWG | 0.10% | 0.115 V | 119.88 V | 1560.9 ft | ≤ 3% target | 1.7 W | 167,800 / 85.03 |
| 4/0 AWG | 0.08% | 0.091 V | 119.91 V | 1968.4 ft | ≤ 3% target | 1.4 W | 211,600 / 107.22 |
| 250 kcmil | 0.06% | 0.077 V | 119.92 V | 2325.6 ft | ≤ 3% target | 1.2 W | 250,000 / 126.68 |
| 300 kcmil | 0.05% | 0.065 V | 119.94 V | 2790.7 ft | ≤ 3% target | 1.0 W | 300,000 / 152.01 |
| 350 kcmil | 0.05% | 0.055 V | 119.94 V | 3255.8 ft | ≤ 3% target | 0.8 W | 350,000 / 177.35 |
| 400 kcmil | 0.04% | 0.048 V | 119.95 V | 3720.9 ft | ≤ 3% target | 0.7 W | 400,000 / 202.68 |
| 500 kcmil | 0.03% | 0.039 V | 119.96 V | 4651.2 ft | ≤ 3% target | 0.6 W | 500,000 / 253.35 |
| 600 kcmil | 0.03% | 0.032 V | 119.97 V | 5581.4 ft | ≤ 3% target | 0.5 W | 600,000 / 304.02 |
| 750 kcmil | 0.02% | 0.026 V | 119.97 V | 6976.7 ft | ≤ 3% target | 0.4 W | 750,000 / 380.03 |
| 1000 kcmil | 0.02% | 0.019 V | 119.98 V | 9302.3 ft | ≤ 3% target | 0.3 W | 1,000,000 / 506.71 |
Calculated values use the simplified circular-mil K-factor method for preliminary comparison. Actual conductor resistance and impedance vary with temperature, conductor construction, installation method, AC effects, and source data. Ampacity is a separate design check.
Scenario: 120 V, 15 A, 50 ft one-way, copper, single-phase AC, 3.00% maximum drop. Voltage-drop screening only; verify ampacity separately.
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How to Read the Voltage Drop Table
The highlighted row answers one specific question: which listed conductor first meets the selected voltage-drop percentage under the entered assumptions. It does not determine the final conductor size by itself.
Voltage drop is not an ampacity check
A row can pass the selected voltage-drop target and still be unacceptable for installation. Verify conductor ampacity, overcurrent protection, insulation and terminal temperature ratings, correction factors, raceway fill, grounding, equipment instructions, and the locally adopted electrical code separately.
Enter One-Way Distance, Not Round-Trip Length
The length input is the source-to-load distance once. The calculation applies the required circuit multiplier internally, so do not double the entered distance.
Single-phase / two-wire DC: the formula uses a factor of 2. Balanced three-phase: the simplified formula uses √3.
3% and 5% Voltage Drop Guidance
Three percent is a common design target for an individual branch circuit, while five percent is often used as a combined feeder-plus-branch design target. These values should not be treated as one universal legal limit for every circuit or piece of equipment.
| Source voltage | 1% | 2% | 3% | 5% |
|---|---|---|---|---|
| 12 V | 0.12 V | 0.24 V | 0.36 V | 0.60 V |
| 24 V | 0.24 V | 0.48 V | 0.72 V | 1.20 V |
| 48 V | 0.48 V | 0.96 V | 1.44 V | 2.40 V |
| 120 V | 1.20 V | 2.40 V | 3.60 V | 6.00 V |
| 208 V | 2.08 V | 4.16 V | 6.24 V | 10.40 V |
| 240 V | 2.40 V | 4.80 V | 7.20 V | 12.00 V |
| 277 V | 2.77 V | 5.54 V | 8.31 V | 13.85 V |
| 480 V | 4.80 V | 9.60 V | 14.40 V | 24.00 V |
Voltage Drop Equations Used by This Table
For a two-wire DC or single-phase circuit, the preliminary circular-mil relationship is:
The entered length is one-way. The factor of 2 represents the outgoing and return conductor path.
For a balanced three-phase circuit:
The result is an approximate line-to-line voltage drop for balanced loading.
Variables
- Vdvoltage drop, volts
- Kmaterial resistivity constant in ohm-cmil/ft
- Iload current, amperes
- Lone-way length, feet
- CMconductor area, circular mils
Percentage drop is calculated as 100 × Vd ÷ source voltage. The solver keeps full internal precision and rounds only the displayed values.
Voltage Drop Calculation Examples
120 V, 15 A, 50 ft copper
12 AWG: Vd = 2 × 12.9 × 15 × 50 ÷ 6,530 ≈ 2.96 V, or 2.47%.
At a 3% target, 14 AWG is above the target and 12 AWG is the smallest listed size below it by voltage drop alone.
240 V, 30 A, 150 ft copper
8 AWG: Vd = 2 × 12.9 × 30 × 150 ÷ 16,510 ≈ 7.03 V, or 2.93%.
10 AWG is above a 3% target in this simplified comparison, while 8 AWG is below it.
480 V, 50 A, 250 ft three-phase copper
6 AWG: Vd = √3 × 12.9 × 50 × 250 ÷ 26,240 ≈ 10.64 V, or 2.22%.
8 AWG is above a 3% target in this simplified balanced three-phase comparison; 6 AWG is below it.
Common Voltage Drop Calculation Mistakes
Entering round-trip length
This tool expects one-way source-to-load distance and applies the circuit multiplier internally.
Using breaker rating automatically
Use the design or operating current appropriate to the engineering check. Breaker rating and actual load current are not always interchangeable.
Ignoring feeder drop
A branch-circuit target does not erase voltage drop already consumed upstream in a feeder.
Rounding too early
Target status is evaluated from the unrounded result, even when the displayed percentage appears equal to the target.
Assuming copper and aluminum are interchangeable
Different material constants change the calculated drop and may change the minimum listed size by voltage drop.
Treating the result as approval
The table is a preliminary design aid, not a substitute for project-specific engineering or code review.
Engineering Basis and Source Transparency
Original calculated output; no complete NEC or IEEE conductor table is reproduced.
This table uses the simplified circular-mil K-factor method for preliminary voltage-drop comparison. Detailed AC design can require verified conductor resistance, reactance, temperature, installation configuration, power factor, harmonics, and equipment-specific limits.
- NFPA 70 (2026), National Electrical CodeOfficial code source for electrical requirements and voltage-drop context. The edition adopted by the authority having jurisdiction may differ.
- IAEI — Voltage Drop CalculationsPractical discussion of NEC voltage-drop references and the distinction between general design guidance and application-specific requirements.
- Turn2Engineering Voltage Drop CalculatorCompanion single-scenario calculator and internal method reference.
Dataset and Review Details
- Publisher
- Turn2Engineering
- Calculation basis
- 2KIL/CM and √3KIL/CM
- Default K values
- Copper 12.9; aluminum 21.2 ohm-cmil/ft, used as simplified design constants
- Length convention
- One-way input
- Conductor range
- 18 AWG through 1000 kcmil; metric area shown as an approximate conversion
- Rights status
- Original calculated comparison; restricted standards tables excluded
- Known exclusions
- Temperature-adjusted AC impedance, harmonics, parallel runs, motor starting, nonlinear loads
Voltage Drop Calculation Table FAQs
Enter the one-way distance from source to load. The table applies a factor of 2 for a two-wire or single-phase path and √3 for a balanced three-phase calculation.
No. Three percent is widely used as design guidance for many branch circuits, but the controlling requirement depends on the application, equipment, locally adopted code edition, and authority having jurisdiction.
A conductor may carry the current thermally yet lose too much voltage over a long run. The final size must satisfy both voltage-drop performance and all applicable ampacity and installation rules.
For a balanced three-phase line-to-line calculation, the phase relationships produce the √3 multiplier. A two-wire or single-phase loop uses the outgoing-and-return factor of 2.
Detailed AC calculations can include conductor temperature, AC resistance, reactance, power factor, raceway configuration, harmonics, and other effects that the simplified K-factor method does not model.
There is no single wire size for every 100-foot run. The required size by voltage drop depends on source voltage, load current, conductor material, circuit type, and the selected voltage-drop target. Enter 100 ft as the one-way distance and use the highlighted row as a voltage-drop-only screening result, then verify ampacity separately.
For the same conductor, current, material, and distance, the calculated voltage loss in volts is the same under this simplified single-phase model, but that loss is a smaller percentage of a higher source voltage. For example, a 3 V drop is 2.5% of 120 V but 1.25% of 240 V.