Voltage Drop Calculator
Voltage drop is the loss of voltage between the start of a wire run and the load at the end of it.
- Copper & aluminum materials
- DC · 1-phase · 3-phase circuit types
- NEC Ch. 9 Tables 8 & 9 source data
It's the reason a long extension cord dims your lights or a winch feels weak at the end of a 50-foot cable. This tool works as a voltage loss calculator too — same measurement, different name people search. Use it to find your voltage drop for copper or aluminum wire, AC or DC current, and single-phase or three-phase circuits — enter your wire size, run length, and load current, and it does the math instantly.
Explore More Calculators & Guides
This calculator covers the general case. If your situation is more specific, one of these goes deeper.
How to Read Your Result (Is Your Voltage Drop OK?)
Run this as an NEC voltage drop calculator and here's how to read the percentage it gives you: under 3% is generally fine for a branch circuit. Above 5% total — branch circuit plus feeder combined — and you're into the range where equipment runs hot, motors struggle to start, and lights visibly dim.
These numbers come from informational notes in the National Electrical Code: 210.19(A), Informational Note No. 4, for branch circuits and 215.2(A), Informational Note No. 2, for feeders. Worth being precise about what that means — they're not hard code violations the way an undersized breaker is. An inspector generally won't fail you on voltage drop alone. But the NEC includes them because ignoring voltage drop causes real problems: wasted energy as heat, motors that draw more current to compensate and burn out faster, and electronics that behave erratically under low voltage. Treat 3%/5% as the number a careful electrician targets, not a rule enforced at your final inspection.
| Voltage drop | What it means |
|---|---|
| Under 3% | Fine for a branch circuit — no action needed |
| 3–5% | Acceptable if it's the feeder portion and total stays under 5%, but check |
| Over 5% | Upsize your wire — you're losing enough voltage to affect performance |
How the Voltage Drop Calculator Works (The Formula)
Knowing how to calculate voltage drop comes down to one relationship: the farther current has to travel through a wire, and the more current it's carrying, the more voltage the wire itself eats up before the current reaches your load. For a single-phase or DC circuit, that's:
VD = 2 × K × I × L / CM
- VD
- voltage drop, in volts
- K
- a constant for the wire material (roughly 12.9 for copper, 21.2 for aluminum, ohms-circular-mil per foot)
- I
- current in amps
- L
- one-way length of the run, in feet
- CM
- the wire's cross-sectional area, in circular mils
The "2" accounts for the fact that current travels out and back — down the hot wire and back through the neutral or ground. Three-phase circuits use √3 (about 1.732) in place of the 2, since the phase geometry cancels out some of the loss a single-phase circuit doesn't get to cancel.
A Worked Example
Say you're running 12 AWG copper wire 50 feet to power a 20-amp load. 12 AWG has a cross-sectional area of about 6,530 circular mils, and copper's K is 12.9.
- VD = 2 × 12.9 × 20 × 50 / 6,530
- VD = 25,800 / 6,530
- VD ≈ 3.95 volts
On a standard 120-volt circuit, that's 3.95 / 120 ≈ 3.3% — just over the 3% branch-circuit guideline. Bump up to 10 AWG (10,380 circular mils) and the same run drops to about 2.5 volts, or roughly 2.1% — back under the limit. That's the calculator doing this same arithmetic for whatever numbers you enter.
Copper vs. Aluminum Wire: Why It Changes Your Result
Aluminum has meaningfully higher resistance than copper — its K value of about 21.2 versus copper's 12.9 means roughly 60% more voltage drop for the same wire size, length, and load. That's why aluminum wiring, common in older homes and in larger feeder cable for cost reasons, usually needs to be sized up one or two gauges compared to copper to hit the same voltage drop target.
| Wire size (AWG) | Copper resistance (Ω/1,000 ft) | Aluminum resistance (Ω/1,000 ft) |
|---|---|---|
| 14 | 3.07 | 5.06 |
| 12 | 1.93 | 3.18 |
| 10 | 1.21 | 2.00 |
| 8 | 0.764 | 1.26 |
| 6 | 0.491 | 0.808 |
The calculator's copper/aluminum toggle applies this difference automatically, but it's worth knowing why the number moves when you switch it — it's not a rounding difference, it's a real property of the metal.
AC vs. DC, and Single-Phase vs. Three-Phase — Which Applies to You?
If you're not sure which mode to pick, the load tells you. Household circuits, most automotive and marine 12V/24V systems, and solar battery banks are DC or single-phase AC — pick single-phase in the calculator and, for AC loads with a power factor below 1 (motors, some electronics), enter that power factor if you know it. Run it as a DC voltage drop calculator and power factor drops out of the math entirely, since there's no phase angle to account for.
Three-phase shows up almost exclusively in commercial and industrial settings — large motors, HVAC equipment, and feeders in commercial buildings. If your panel or equipment nameplate says three-phase, switch the calculator to 3 phase voltage drop calculator mode; it applies the √3 multiplier instead of the single-phase 2×, and it's genuinely a different calculation, not a scaled version of the single-phase one. The three-phase voltage drop formula is Vd = √3 × I × L × Z ÷ 1000, against 2 × I × L × Z ÷ 1000 single-phase — about 13 percent less drop for the same current, conductor and distance. For 12V and 24V wiring, set the calculator to DC and watch the budget shrink with the voltage: 3 percent of 12 volts is 0.36 volts, so a run that is unremarkable at 240 volts can want two or three sizes more conductor at 12.
Wire Gauge (AWG) and Voltage Drop
Wire gauge and voltage drop are directly connected: a lower AWG number means a physically thicker wire, less resistance per foot, and less voltage lost over the same distance. This is separate from ampacity — the maximum current a wire can safely carry without overheating. A wire can be rated for more amps than your load needs and still have too much voltage drop if the run is long enough. That's the case the standard ampacity tables don't cover on their own.
If this voltage drop calculator wire size result is too high, the fix is almost always to go up one or two wire sizes rather than switching material or shortening the run, which usually isn't practical. Switching this calculator to Find wire size does that search directly: it returns the smallest conductor that holds the drop inside your limit at your actual length, and checks ampacity at the same time, so the answer is legal as well as adequate. Voltage drop reference tables by gauge give the same answer at a glance once the pattern is clear — drop is proportional to length, so doubling the run doubles the drop.
Where Voltage Drop Matters Most
Voltage drop is easy to ignore on short runs and hard to ignore on long ones. It shows up most on landscape lighting circuits stretching a hundred feet or more from the transformer, on solar installations where battery-to-inverter or panel-to-charge-controller cable runs can be long and current is often high, on automotive and marine 12V wiring where every added foot of cable competes with an already-low system voltage, and on any feeder run in a large building or detached garage.
The common thread is low system voltage combined with real distance — the same 3-volt drop that's negligible on a 240-volt circuit is a genuine problem on a 12-volt one, which is why a 12v voltage drop calculator run tends to flag issues that a 120V run on the same wire wouldn't. Solar array wiring is the same calculation with one twist: NEC 690.8 sizes it from the array's short-circuit current with a 125 percent factor applied, not from its rated output. For anything running on 12V or 24V, set the calculator to DC — at those voltages the conductor is decided by voltage drop long before ampacity becomes the constraint.
Frequently Asked Questions
How much voltage drop is acceptable?
Under 3% for a branch circuit and under 5% total (branch plus feeder combined) is the range the NEC's informational notes recommend. Above that, expect visible performance issues like dimming or motor strain.
How do you calculate voltage drop?
Multiply 2 × K (a constant for the wire material) × current in amps × one-way length in feet, then divide by the wire's cross-sectional area in circular mils. Three-phase circuits use √3 instead of 2.
What is voltage drop?
It's the reduction in voltage that occurs as current travels through a wire's resistance, meaning the voltage reaching your load is lower than the voltage at the source.
How is voltage drop calculated in practice?
By plugging your wire's material, gauge, one-way run length, and load current into the formula above — which is exactly what this calculator does for you.
How do you measure voltage drop with a multimeter?
Measure voltage at the source with the load running, then measure again at the load, and subtract. The difference is your real-world voltage drop, which should be close to what the calculator predicts.
What is a power drop?
"Power drop" is sometimes used loosely to mean the same thing as voltage drop, though technically power loss (in watts) and voltage drop (in volts) are related but distinct measurements — voltage drop causes the power loss.
How do you calculate voltage drop in a series circuit?
Add up the voltage drop across each resistive element in the circuit — total voltage drop across a series circuit always equals the source voltage, by Kirchhoff's voltage law.
What is not a factor when calculating voltage drop?
The color of the wire's insulation and its brand have no effect. Only conductor material, cross-sectional area, length, and current — plus power factor for AC — determine the result.
Voltage drop calculations here follow standard NEC-referenced formulas and are provided for planning purposes. For anything affecting code compliance or safety, confirm with a licensed electrician.
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