For electricians & homeowners
Voltage Drop Calculator
Over a long run, voltage leaks away as heat and the far end ends up starved. This tool tells you exactly how much you lose, whether it passes the NEC 3% limit, and the wire size that fixes it.
A voltage drop calculator built for electricians — clear enough for anyone.
Full power leaves the source — but the long run bleeds voltage away as heat, so the house at the end is starved and the lights dim. That's the drop this tool measures.
NEC-based
Built to the current code
Transparent
Every tool shows its formula
Free & private
No signup, runs in your browser
Voltage Drop Calculator
The voltage lost over your wire run — and whether it passes the NEC 3% limit.
Enter values to see the verdict.
Max run at 3%: — ft
Ampacity — can the wire carry the current?
Equipment grounding conductor — NEC Table 250.122
Recommended — smallest size passing both checks
↑ Recommended wire— Open Wire Size Calculator →Compare sizes — tap to select (drop % · amps)
Method — the working, on NEC 2026
R, X from NEC Ch. 9 Table 9 (PVC conduit). Ampacity from Table 310.16 with 310.15 ambient & bundling adjustments; terminals capped per 110.14(C). Breaker per 240.4(D)/240.6. EGC per Table 250.122 and 250.122(B). Built on NEC 2026.
Step by step
How to use the voltage drop calculator
Four inputs, one clear verdict. Fill each field to match your circuit and the result updates as you type — no signup, no reset needed.
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01
Enter voltage & load
Your system voltage (120, 240, 208…) and the current in amps. Working from watts or horsepower? Open Advanced and switch the unit — it converts for you.
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02
Set the one-way length
Distance from source to load, one direction only — in feet. The tool doubles it for the return leg automatically, so you never count both runs yourself.
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03
Pick wire size & material
Choose the AWG or kcmil you plan to run and whether it's copper or aluminum. Set phase to single-phase, three-phase, or DC to match the circuit.
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04
Read the verdict
The gauge shows your percent drop with a pass, borderline, or fail call against your target, plus the voltage at the load and the farthest you can run that wire.
The concept
What is voltage drop?
Voltage drop is the loss of voltage as current travels along a conductor, caused by the wire's own resistance. It follows Ohm's law — the longer or thinner the wire, or the more current it carries, the more voltage is used up before it reaches the load.
Every conductor has resistance, so some drop is unavoidable — the goal is to keep it small. When it grows too large, the far end is starved: lights dim and flicker, motors lose torque and overheat, and electronics misbehave. Every volt lost in the wire is also energy wasted as heat, which is why long runs get a voltage-drop check, not just an ampacity check.
Each step is voltage handed to the wire's resistance instead of the load.
The math
Voltage drop formula.
The voltage drop formula is VD = (2 × K × I × L) ÷ CM for single-phase, where K is the conductor's resistivity (12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way length in feet, and CM is the wire's area in circular mils. Percent drop = (VD ÷ V) × 100.
Single-phase & DC
Current flows out and back, so the length that matters is twice the one-way run.
Three-phase
That's √3. Phases share the return path, so drop is lower for the same wire.
Our calculator goes further
For AC it uses the NEC Chapter 9 impedance method — adding reactance and power factor — so it can differ by a few tenths. The example below reconciles both.
Worked example
One size up fixes it — here's the proof
Same circuit, two wires: 120 V · single-phase · 20 A · 100 ft one-way · copper. Only the conductor changes. The drop crosses back under the NEC 3% line — the exact move the calculator recommends, worked by hand and confirmed by the tool.
Upsizing one size — 10 AWG to 8 AWG — moves the run left across the NEC 3% line.
10,380 circular mils
VD = (2 × 12.9 × 20 × 100) ÷ 10,380 = 4.97 V
4.97 ÷ 120 = 4.14% · tool 4.0%
16,510 circular mils
VD = (2 × 12.9 × 20 × 100) ÷ 16,510 = 3.13 V
3.13 ÷ 120 = 2.60% · tool 2.6%
Same numerator, bigger conductor — the drop falls because the circular-mil area rose. The tool reads a hair under the textbook K-value because it uses the conductor's true AC impedance; both agree the wire goes up a size. Verify it above: enter 120 V, 20 A, 100 ft, copper.
NEC limits
What's an acceptable voltage drop?
The NEC recommends keeping voltage drop at or below 3% on a branch circuit or a feeder, and 5% for the two combined, measured to the farthest outlet. These figures are recommendations in Informational Notes — not enforceable rules — but most electricians and inspectors treat 3% as the design target.
- Branch circuit≤ 3%
- Feeder≤ 3%
- Feeder + branch, combined≤ 5%
One case is mandatory
Sensitive electronic equipment under NEC 647.4(D) has an enforceable limit — 1.5% on a branch and 2.5% combined.
Ampacity sizes a wire for heat, not distance — a long run can pass ampacity and still fail 3%. That's why voltage drop is a separate check.
What 3% equals on common systems
Factors & fixes
What affects voltage drop — and how to reduce it.
Voltage drop rises with longer runs, higher current, thinner wire, and aluminum over copper — the four levers in the formula. To cut it, the usual fix is to upsize the conductor; you can also shorten the run, raise the voltage, or split the load across circuits.
What increases it
Length
Drop rises linearly — double the run, double the drop.
Current
More amps through the wire, proportionally more drop.
Thinner wire
Smaller AWG = more resistance. Going bigger is the main lever down.
Aluminum
Higher resistance than copper for the same size.
How to reduce it
Upsize the conductor usual fix
More copper, less resistance, less drop — the move the calculator recommends.
Shorten the run
Relocate a subpanel closer to the load where the layout allows.
Raise the voltage
Wire a 240 V load at 240 V, not 120 V — half the current, far less drop.
Split or parallel
Spread the load across circuits, or run parallel conductors on large feeders.
Upsized for voltage drop? Upsize the ground too.
Per NEC 250.122(B), when you enlarge the ungrounded conductors for voltage drop, the equipment grounding conductor must grow by the same proportion. The calculator handles this automatically — the Ground wire tab flags when it's been increased.
Reference chart
Voltage drop chart.
Maximum one-way run for copper wire on a 120 V single-phase circuit before it exceeds the NEC 3% limit. Pick your wire size and load current for the answer. For 240 V, double the distance; for aluminum, multiply by about 0.6; for three-phase, by about 0.87.
Voltage-drop limit only. These distances assume the wire can already carry the current — they don't check ampacity. Always confirm the conductor's ampacity for your load (the calculator's Ampacity tab does this). Need a different voltage, target, or aluminum? The calculator gives an exact answer for your circuit.
| Wire size | 15 A | 20 A | 30 A | 40 A | 50 A | 60 A | 100 A | 150 A | 200 A |
|---|---|---|---|---|---|---|---|---|---|
| 14 AWG | 39 ft | 29 ft | 19 ft | 15 ft | 12 ft | 10 ft | 6 ft | 4 ft | 3 ft |
| 12 AWG | 60 ft | 45 ft | 30 ft | 23 ft | 18 ft | 15 ft | 9 ft | 6 ft | 5 ft |
| 10 AWG | 100 ft | 75 ft | 50 ft | 38 ft | 30 ft | 25 ft | 15 ft | 10 ft | 8 ft |
| 8 AWG | 154 ft | 115 ft | 77 ft | 58 ft | 46 ft | 38 ft | 23 ft | 15 ft | 12 ft |
| 6 AWG | 245 ft | 184 ft | 122 ft | 92 ft | 73 ft | 61 ft | 37 ft | 24 ft | 18 ft |
| 4 AWG | 387 ft | 290 ft | 194 ft | 145 ft | 116 ft | 97 ft | 58 ft | 39 ft | 29 ft |
| 2 AWG | 632 ft | 474 ft | 316 ft | 237 ft | 189 ft | 158 ft | 95 ft | 63 ft | 47 ft |
| 1/0 AWG | 1,000 ft | 750 ft | 500 ft | 375 ft | 300 ft | 250 ft | 150 ft | 100 ft | 75 ft |
| 2/0 AWG | 1,200 ft | 900 ft | 600 ft | 450 ft | 360 ft | 300 ft | 180 ft | 120 ft | 90 ft |
| 4/0 AWG | 1,935 ft | 1,452 ft | 968 ft | 726 ft | 581 ft | 484 ft | 290 ft | 194 ft | 145 ft |
| 250 kcmil | 2,308 ft | 1,731 ft | 1,154 ft | 865 ft | 692 ft | 577 ft | 346 ft | 231 ft | 173 ft |
| 350 kcmil | 3,158 ft | 2,368 ft | 1,579 ft | 1,184 ft | 947 ft | 789 ft | 474 ft | 316 ft | 237 ft |
| 500 kcmil | 4,444 ft | 3,333 ft | 2,222 ft | 1,667 ft | 1,333 ft | 1,111 ft | 667 ft | 444 ft | 333 ft |
What is an acceptable voltage drop?
The NEC recommends 3% or less on a branch circuit or feeder, and 5% or less for the two combined, measured to the farthest outlet. Three percent is the target most electricians design to; sensitive electronic equipment under NEC 647.4(D) is held to a stricter, mandatory 1.5%.
How do you calculate voltage drop?
For single-phase, VD = (2 × K × I × L) ÷ CM — where K is 12.9 (copper) or 21.2 (aluminum), I is the current in amps, L is the one-way length in feet, and CM is the wire's circular-mil area. Divide by the source voltage for percent. The calculator above does this and refines it with AC impedance.
How can I reduce voltage drop?
Upsize the conductor (the usual fix), shorten the run, raise the system voltage (240 V instead of 120 V), or split the load across circuits. Copper also drops less than aluminum. When you upsize for voltage drop, upsize the ground wire too, per NEC 250.122(B).
Is voltage drop a code requirement?
Mostly no. The 3% and 5% figures live in NEC Informational Notes, which are recommendations, not enforceable rules. The exception is sensitive electronic equipment under 647.4(D), where the limit is mandatory. Even so, staying within 3% is standard practice and often expected by inspectors.
Do I enter one-way or round-trip length?
Enter the one-way distance — source to load, one direction. The formula's ×2 already accounts for the return leg, and this calculator doubles it automatically. Entering the round-trip length would double the drop by mistake.
Copper or aluminum — which drops more?
Aluminum drops more. For the same size and length it has higher resistance, so a copper run reaches about 1.65× as far before hitting the same limit. Aluminum is still common on larger feeders because it's lighter and cheaper — you just size up to compensate.
What's the difference for single-phase, three-phase, and DC?
The method is the same; only the multiplier changes. Single-phase and DC use ×2 for the out-and-back path; three-phase uses ×1.732 (√3). For the same wire, current, and length, a three-phase run drops roughly 13% less than single-phase.
What happens if voltage drop is too high?
The far end is starved: lights dim and flicker, heaters run cool, and motors lose torque and overheat — drawing extra current and shortening their life. Electronics can reset or misbehave, and the lost voltage is wasted as heat in the wire.
Keep going
Related electrical calculators
Voltage drop is one step in sizing a run. These follow the same job — pick the wire, protect it, and fit it in the raceway — and use the same NEC-based engine.
How we keep this accurate
These calculators follow the National Electrical Code (NEC 2026): conductor properties from Chapter 9, Table 8/Table 9, and the 3% branch-circuit / 5% total voltage-drop recommendation (NEC 210.19 & 215.2 informational notes). Results are for planning and estimating. Code adoption, local amendments, temperature, and install conditions vary — always confirm against the edition your jurisdiction enforces and have work verified by a licensed electrician.
Last updated July 2026 · Verified against the National Electrical Code (NEC 2026).