Voltage Drop Calculator
Calculate wire voltage drop instantly and find the correct AWG size for any circuit run.
Ensure your electrical circuits operate safely by determining the exact voltage lost over wire distances and preventing equipment damage.
A multimeter can tell you what a wire is doing right now. It can’t tell you what a 100-foot run out to the garage, or a 12V line out to a solar array, is going to do before you’ve even bought the cable — that’s the gap a voltage drop calculator fills. Feed it the current, the one-way distance, and the wire gauge, and it works out whether the load on the other end gets full power or arrives a few volts short.
The formula behind the number
For a single-phase circuit, voltage drop comes from:
VD = (2 × K × I × L) ÷ CM
I is the current in amps, L is the one-way distance in feet, CM is the wire’s cross-sectional area in circular mils, and K is a resistivity constant — roughly 12.9 for copper, 21.2 for aluminum. The factor of 2 accounts for the round trip through both the outgoing and return conductor. Three-phase circuits swap that 2 for 1.732 (√3), since the three conductors share the load more evenly and lose proportionally less voltage over the same distance.
Once you have the volt figure, the percentage that actually matters for code compliance is simple: % drop = (VD ÷ source voltage) × 100.
Where the 3% and 5% numbers come from
NEC 210.19(A), Informational Note No. 4, suggests keeping voltage drop on a branch circuit to around 3%, and the combined feeder-plus-branch drop to around 5%. It’s worth being precise about what that note actually is: a recommendation for efficient operation, not a mandatory code violation if you cross it. Nothing fails an inspection over voltage drop alone — but push past those numbers and you start seeing real symptoms: motors running hot and inefficient, dimmer switches behaving oddly, electronics browning out under load.
Working through a branch circuit example
Take a 20A, 120V branch circuit running 100 feet on 12 AWG copper (6,530 circular mils):
VD = (2 × 12.9 × 20 × 100) ÷ 6,530 = 7.9V, or 6.6% of 120V — well past the 3% target.
Stepping up to 10 AWG (10,380 CM) brings that to 4.97V, or 4.1%. Still over. Moving to 8 AWG (16,510 CM) gets you to 3.1V, or 2.6% — the first size that actually clears the recommendation. This is the part a lot of people miss when they size wire purely off the NEC ampacity table: 12 AWG is rated to carry 20A all day long without overheating, but ampacity and voltage drop are two separate checks, and a long run can fail the second one while passing the first comfortably.
Wire gauge reference
| AWG (copper) | Circular mils | Resistance (Ω/1,000 ft) |
|---|---|---|
| 14 | 4,110 | 2.525 |
| 12 | 6,530 | 1.588 |
| 10 | 10,380 | 0.999 |
| 8 | 16,510 | 0.628 |
| 6 | 26,240 | 0.395 |
| 4 | 41,740 | 0.249 |
| 2 | 66,360 | 0.156 |
| 1/0 | 105,600 | 0.098 |
Aluminum runs about 61% higher resistance than copper at the same gauge, which is why aluminum feeders are typically sized two gauge steps larger than the copper equivalent to land on similar drop performance.
Why 12V and 24V systems are a different problem entirely
The same formula applies to DC, but low system voltage changes the stakes. A 3% drop on a 120V circuit is 3.6V — plenty of margin. A 3% drop on a 12V battery system is 0.36V, and there’s almost nothing left to spare. This is why a wire size that’s perfectly fine for a household outlet can be badly undersized for a solar array or an RV running the same current at 12V.
Take a 12V solar setup pulling 25A from the panels to the charge controller, 40 feet away. On 10 AWG wire, the math comes out to roughly 2.5V of drop — over 20% of the system voltage, which is unworkable. Getting that down to a 3% target (about 0.36V) actually requires stepping up to something in the 2 AWG range, because the low source voltage amplifies the percentage so dramatically. The same 25A over the same 40 feet on a 120V circuit would lose a small fraction of a percent on far thinner wire.
The same problem shows up at a smaller scale in 12V LED strip installs all the time: a strip drawing 5A, 20 feet from its power supply, on 14 AWG wire loses about 0.63V — a 5.2% drop, enough to visibly dim the far end of the run. Swapping to 10 AWG cuts that to roughly 0.25V, about 2%, and the strip lights evenly end to end.
Typical runs and the wire size they actually need
| Application | Typical load | One-way distance | Wire that holds ≤3% drop |
|---|---|---|---|
| 120V branch circuit (outlets, lighting) | 15–20A | 100 ft | 8–10 AWG |
| 12V LED strip / accent lighting | 3–5A | 20–25 ft | 10–12 AWG |
| 12V solar array to charge controller | 20–25A | 40 ft | 2 AWG or larger |
| 12V car audio amplifier | 40–60A | 10–15 ft | 4–6 AWG |
| 240V feeder (subpanel, shop, well pump) | 30–50A | 150 ft | 6–8 AWG |
These are starting points, not substitutes for running the actual numbers — distance, load, and voltage all shift the result, and a run that’s 20% longer can easily push you up a full wire gauge.
Fixing a circuit that fails the check
- Go up a gauge (or two). The most direct fix — lower resistance per foot means less drop for the same current and distance.
- Shorten the run where possible. Relocating a panel, junction box, or charge controller closer to the load has an outsized effect since drop scales linearly with distance.
- Raise the system voltage. Doubling from 12V to 24V cuts the required current in half for the same load, which cuts voltage drop roughly in half too — a common fix in solar and RV systems with long wire runs.
- Switch to three-phase where the load allows it. The √3 factor instead of 2 means meaningfully less drop over the same distance and gauge.
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The NEC recommends a maximum of 3% voltage drop on branch circuits, which equals 3.6 volts on a 120V system. For the combined total of feeder and branch circuit, the limit is 5%, or 6 volts. Sensitive electronics and data equipment often require tighter limits of 1.5–2%.
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The most direct fix is to upsize the wire gauge — increasing from #12 to #10 AWG, for example, reduces resistance by roughly 40% and cuts voltage drop proportionally. Switching from aluminum to copper conductors also reduces drop significantly. In some cases, increasing the source voltage (e.g., using 240V instead of 120V) is the more practical solution on very long runs.
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Yes — and DC systems are often more sensitive to voltage drop than AC systems because they operate at lower voltages. A 0.5V drop on a 12V battery system represents over 4% loss, which can prevent devices from functioning correctly. Always calculate voltage drop on any DC run longer than 10–15 feet.
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Standard #12 AWG copper is rated for 20 amps on ampacity, but a 100-foot run at 20 amps produces approximately 5.9V drop on a 120V circuit — nearly 5%, which exceeds the NEC recommendation. For a 100-foot run, #10 AWG copper is the correct choice, bringing the drop down to approximately 3.7V (3.1%).