The Voltage Drop Calculator estimates how much voltage is lost in the conductors between a source and a load. Enter wire gauge, one-way length, load current, system voltage and circuit type (DC, single-phase AC or three-phase AC); the tool returns drop in volts and as a percent of source voltage, then flags common design guidance thresholds.
Calculate voltage drop over a wire run
Current through a wire's resistance produces an IR loss that never arrives at the load terminals. Longer runs, smaller conductors and higher current all increase that loss. The Voltage Drop Calculator uses published ohm-per-length values for copper or aluminium AWG sizes rather than asking for resistivity by hand on every estimate.
For a simple DC or single-phase estimate with a round-trip conductor path:
V_drop ≈ I × R_one_way × 2
% drop = (V_drop / V_source) × 100R_one_way comes from the wire table for the chosen gauge and metal, scaled to the one-way length. Enter one-way length (source to load), not the sum of hot and neutral, when the calculator already applies the return-path factor for the selected circuit type. Double-counting length is a frequent cause of inflated results that look like a wire problem when the real issue is data entry.
Loads that draw continuous current feel soft voltage as heat, dimming or undervoltage trips. Brief inrush may tolerate more drop than a continuous heater on the same run.
Choose single phase, three phase or DC
Circuit type changes the multiplier that turns one-way resistance into the voltage seen between supply and load. Direct current and ordinary single-phase AC both use a two-wire out-and-back path in the simple resistive model. Three-phase estimates use a different geometric factor. Pick the mode that matches the installed system before comparing percent drop to a target.
| Type | Typical factor on I × R_one_way |
|---|---|
| DC (two-wire) | ×2 (out and back) |
| Single-phase AC | ×2 (similar two-wire path) |
| Three-phase AC | ×√3 ≈ ×1.732 for line drop estimates |
Exact field formulas vary with power factor and whether drop is reported line-to-line or line-to-neutral. This calculator uses the standard teaching factors above for copper/aluminium AWG estimates. For motor starting or high-reactance feeders, a full impedance calculation belongs in design software, not a resistive AWG table alone. Select copper or aluminium to match the conductors installed; aluminium needs a larger gauge for the same drop target because its resistivity is higher.
Read the AWG wire table
American Wire Gauge labels diameter: larger gauge numbers mean thinner wire and higher resistance per foot. The table stores approximate DC resistance in ohms per 1,000 feet for copper (and a parallel aluminium column) at a reference temperature. Scaling to the run length gives R_one_way for the drop equation.
| AWG | Approx. copper Ω / 1,000 ft (typical table) | Notes |
|---|---|---|
| 14 | ~2.525 | Common branch lighting |
| 12 | ~1.588 | 20 A copper branch circuits |
| 10 | ~0.999 | Heavier loads / longer runs |
| 8 | ~0.628 | Subpanels and long feeders |
| 6 | ~0.395 | Larger feeders |
Values differ slightly between NEC Chapter 9 tables, manufacturer data and temperature assumptions. The calculator states which table temperature it uses beside the result. Hot conductors raise resistance; a sunny attic run drops more than a basement run of the same gauge. Stranded vs solid also shifts resistance a little; use the table that matches the cable purchased when the estimate is close to a hard limit.
Calculate drop for 12 AWG over 100 feet at 15 A
The teaching fixture uses copper 12 AWG, one-way length 100 ft, load 15 A, source 120 V and a single-phase two-wire path. Those inputs land just above a common 3% branch target and show why length matters even when ampacity looks fine on a short chart.
1. Resistance for 100 ft one-way. Using ~1.588 Ω per 1,000 ft for 12 AWG copper: `` R_one_way = 1.588 × (100 / 1000) = 0.1588 Ω ``
2. Round-trip drop. `` V_drop = 15 A × 0.1588 Ω × 2 = 4.764 V ≈ 4.76 V ``
3. Percent of 120 V. `` % drop = (4.76 / 120) × 100 ≈ 3.97% ``
Result: about 4.76 V, or 3.97%. Voltage at the load is near 115.2 V before other losses. That percent sits above the common 3% branch-circuit guidance discussed next.
If the same load ran on 10 AWG (~0.999 Ω/1,000 ft): `` R_one_way = 0.0999 Ω V_drop = 15 × 0.0999 × 2 ≈ 3.00 V ≈ 2.5% ``
Upsizing gauge is the usual fix when length and current are fixed. The Voltage Drop Calculator should reproduce the 4.76 V / 3.97% fixture when those inputs are entered with the same resistance table.
Check the drop against design guidance
Designers often aim for about 3% drop on a branch circuit and about 5% total including the feeder. Those figures are guidance for efficiency and equipment performance, not a substitute for reading the adopted electrical code in the jurisdiction. Local amendments, continuous-load rules and ambient corrections still apply to every real installation.
The 12 AWG / 100 ft / 15 A / 120 V case at ~3.97% exceeds a 3% branch target. Options: shorten the run, reduce current, raise conductor size, or raise system voltage where the equipment allows (for example 240 V instead of 120 V for the same power at half the current). Motors and LED drivers can misbehave when voltage is soft even if conductors are ampacity-legal. Ampacity answers whether the wire overheats; voltage drop answers whether the load sees enough volts. Both checks matter.
Document the assumed metal, temperature and circuit type beside any percent that drives a purchase order. A later aluminium substitution invalidates a copper-based estimate without any change to length or amps.
Choose a wire size to stay under a target
When a maximum percent (or maximum volts) is entered, the calculator can recommend the smallest AWG in its table that keeps estimated drop under that target for the stated length and current. Recommendations still need an ampacity check: a gauge that passes drop may fail ampacity, and the reverse is common on short, heavy circuits.
Long one-way runs to outbuildings are classic upsize cases. A 12 AWG copper branch that is fine at 40 ft can fail a 3% target at 100 ft under the same 15 A load, as the worked example shows. Parallel conductors and higher voltage systems are further options when a single conductor would become impractical. Mixing aluminium ohm values with a copper label understates drop and can leave a remote load starving.
Ampacity versus voltage drop
Ampacity tables answer thermal limits for insulation. Voltage drop answers usable volts at the far end. A short heavy run may pass drop yet sit near ampacity; a long light run may pass ampacity yet fail a 3% drop target. Run both checks.
Power factor and conductor reactance matter more on long AC feeders and large motors than on short DC control runs. When the resistive table and a field measurement disagree by a wide margin, look for loose connections, undersized terminations or an incorrect one-way length before blaming the AWG chart.
Frequently asked questions
How do you calculate voltage drop in a wire?
Multiply current by the round-trip resistance of the run (or apply the three-phase factor to one-way resistance), then divide by source voltage and multiply by 100 for percent. Use the ohm-per-length value for the installed metal and gauge at the stated temperature.
What is the drop for 12 AWG, 100 ft, 15 A at 120 V?
About 4.76 V, or roughly 3.97%, using typical copper 12 AWG resistance near 1.588 Ω per 1,000 ft and a two-wire return path. Load voltage is then near 115 V before other losses.
Is 3% voltage drop a code requirement?
Treat 3% branch and 5% total as common design guidance. Confirm requirements in the electrical code adopted locally; this page is not a code citation and does not replace inspection rules.
Does the length field mean one-way or round-trip?
Enter one-way length when the calculator applies the return-path factor for the selected circuit type. Check the field hint on the tool. Entering round-trip length on top of a ×2 factor doubles the estimate by mistake.
Copper or aluminium?
Match the metal installed. Aluminium has higher resistance; the same AWG drops more volts than copper. Upsize aluminium relative to a copper estimate when the drop target is fixed.
Does voltage drop replace ampacity sizing?
No. Ampacity prevents overheating. Voltage drop protects equipment performance at the far end. A circuit can pass one check and fail the other; both belong in the worksheet.
Why is three-phase different?
The geometry of line currents changes the factor from ×2 to about ×√3 for typical line-voltage drop estimates. Using the single-phase factor on a three-phase feeder misstates the result.
What temperature is assumed?
Tables quote a reference temperature. Hot conduits increase resistance and drop. Use the calculator's stated reference or a temperature-corrected table for critical runs in high ambient spaces.
Can percent drop be lowered without changing gauge?
Yes: shorten the run, lower the current, or use a higher system voltage so current falls for the same power. Each option changes different constraints (routing, load, equipment ratings).
Summary
The Voltage Drop Calculator turns gauge, length, current and voltage into a volt and percent loss estimate for DC, single-phase and three-phase runs. The 12 AWG, 100 ft, 15 A, 120 V fixture lands near 4.76 V and 3.97%, above typical 3% branch guidance.
Upsize conductors, shorten runs or rethink voltage when the estimate exceeds the project target, and still verify ampacity and local code separately.