Voltage Drop Calculator
Voltage lost over a wire run — current, distance, gauge, with the 3%/5% verdict.
Updated
You need
7.72 Vdropped
6.4% of 120 V over 100 ft
- Percent drop
- 6.4%
- Voltage at the load
- 112.3 V
✗ Too high — over 5%. Upsize the wire.
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In short
How much voltage drop is acceptable on a circuit?
Voltage drop = 2 × one-way distance × current × ohms per foot. At 20 A over 100 ft of 12 AWG copper, rated 1.93 ohms per 1,000 ft, that is 7.72 V lost: 6.4 percent of a 120 V supply, leaving 112.28 V at the load. NEC Informational Notes recommend 3 percent on a branch circuit, so this run is too thin.
Those percentages appear in Informational Notes, which NEC 90.5 states are not enforceable as requirements, though energy codes and local amendments often make them binding anyway.
How to use the voltage drop calculator
Enter the current the circuit carries in amps, the one-way distance from the panel to the load in feet, the copper conductor size, and the supply voltage, and the tool returns the volts lost along the run, that loss as a percentage of the supply, the voltage actually arriving at the load, and a verdict on whether the run is sized sensibly.
The defaults model a very common case: 20 A over 100 ft of 12 AWG copper on a 120 V circuit. Copper is assumed throughout, which is what most branch wiring uses. Enter the current the load genuinely draws rather than the breaker rating, since drop scales directly with amps and sizing to a breaker you never approach will send you to needlessly heavy cable.
Do not know the current in amps?
This tool asks for the current the circuit actually carries. If your load is quoted in watts, the watts to amps calculator converts it at your supply voltage.
Open the watts to amps calculator →7.72 V
Lost along the run
20 A over 100 ft of 12 AWG copper
6.4%
Of a 120 V supply
past both recommended targets
112.28 V
Arriving at the load
why tools at the far end run weak
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This is the tool for the question that starts why does my shed, garage or detached workshop run tools weakly. A circuit can be entirely safe and still deliver soft voltage at the far end when the run is long and the wire is thin, because the loss happens along the cable rather than at the source, and a meter at the panel reads perfectly normal.
Motors run hot and lose torque on low voltage, incandescent lighting dims noticeably, and some electronics behave strangely. Two things drive the loss and both sit in the formula: length and current. Double the distance and the drop doubles; double the amps and it doubles again. A long feed to an outbuilding is the classic problem case precisely because it maximises both at once.
Read the percentage against the recognised targets. NEC 210.19(A) carries an Informational Note recommending that branch-circuit voltage drop at the farthest outlet not exceed 3 percent, with the combined feeder and branch-circuit drop kept to 5 percent, and NEC 215.2(A) carries a parallel note for feeders.
Both are recommendations rather than code requirements, because NEC 90.5 states plainly that Informational Notes are informational only and not enforceable as requirements. In practice energy codes and local amendments often make the 3 percent figure binding anyway, and it is defensible engineering regardless. Use this to plan a run before you buy cable or to diagnose one already misbehaving, and have a licensed electrician size and install anything permanent.
Do
- Enter the current the circuit actually carries, not the breaker rating.
- Measure the run along the route the cable takes, including vertical drops.
- Compare the percentage against the 3 percent branch and 5 percent overall figures.
- Try the next gauge up and watch the loss fall before buying cable.
- Have a licensed electrician size and install anything permanent.
Don't
- Use these figures for aluminium, since every resistance value here is copper.
- Diagnose a soft outlet at the panel, where the reading looks perfectly normal.
- Treat voltage drop as the only constraint on conductor size.
- Read the Informational Note percentages as enforceable code requirements.
Every copper size from 14 AWG to 4/0, run at the same 20 A over the same 100 ft on 120 V, plus the longest run each size can manage while staying inside 3 percent. The last column is the one to read when you are planning rather than diagnosing, because it answers the question people actually have: how far can I go on the wire I already own?
| Copper conductor | Ohms per 1,000 ft at 75 C | Drop at 20 A over a 100 ft run | Percent of a 120 V supply | Longest 120 V, 20 A run inside 3 percent |
|---|---|---|---|---|
| 14 AWG (limited to a 15 A device by NEC 240.4(D)) | 3.07 | 12.28 V | 10.2 percent | 29 ft |
| 12 AWG | 1.93 | 7.72 V | 6.4 percent | 46 ft |
| 10 AWG | 1.21 | 4.84 V | 4.0 percent | 74 ft |
| 8 AWG | 0.764 | 3.06 V | 2.5 percent | 117 ft |
| 6 AWG | 0.491 | 1.96 V | 1.6 percent | 183 ft |
| 4 AWG | 0.308 | 1.23 V | 1.0 percent | 292 ft |
| 3 AWG | 0.245 | 0.98 V | 0.8 percent | 367 ft |
| 2 AWG | 0.194 | 0.78 V | 0.6 percent | 463 ft |
| 1 AWG | 0.154 | 0.62 V | 0.5 percent | 584 ft |
| 1/0 AWG | 0.122 | 0.49 V | 0.4 percent | 737 ft |
| 2/0 AWG | 0.0967 | 0.39 V | 0.3 percent | 930 ft |
| 3/0 AWG | 0.0766 | 0.31 V | 0.3 percent | 1,174 ft |
| 4/0 AWG | 0.0608 | 0.24 V | 0.2 percent | 1,480 ft |
What each step up in gauge buys you
Wire is not a perfect conductor. It has resistance, and every foot of it converts a little of your supply voltage into heat instead of delivering it to the load. Thicker wire has lower resistance per foot, which makes the gauge selector the main lever you have.
The values behind it come from NEC Chapter 9, Table 8, which lists conductor DC resistance at 75 C: 12 AWG solid uncoated copper is 1.93 ohms per 1,000 ft, 10 AWG is 1.21, and 8 AWG is 0.764. Each step up the gauge scale cuts the resistance by roughly a third, so moving two sizes from 12 to 8 more than halves the loss on an identical run without changing anything else.
Read it: The ladder flattens as it descends, so the biggest wins come from the first steps up in gauge; the reference table above adds the longest run each size can manage inside 3 percent.
Computed from NEC Chapter 9, Table 8 resistance at 75 C, the same values behind the gauge selector.
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Full guide
What Size Wire Do I Need? Voltage Drop Over a Long Run
The current goes out and comes back, so a 100 ft run is 200 ft of copper — and moving to 240 V helps twice over.
Read the full guide →The formula, worked line by line
This is Ohm law applied to the cable rather than the load. Current flowing through resistance produces a voltage across that resistance, and here the resistance belongs to the conductors themselves. The only detail that catches people out is the factor of two: current leaves the panel along one conductor and returns along the other, so the resistance you are fighting covers twice the one-way distance you measured.
Conductor resistance is tabulated per 1,000 ft, so it is divided by 1,000 to give ohms per foot before being multiplied by the round-trip length. The percentage is then that loss compared with the supply voltage you started from, which is why the identical absolute drop is only half as serious on a 240 V circuit as on a 120 V one, and why long feeds to outbuildings are so often run at 240 V.
voltage drop = 2 × distance × current × (ohms per 1000 ft ÷ 1000)
percent drop = voltage drop ÷ supply voltage × 100
voltage at the load = supply voltage − voltage drop
2 × 100 ft × 20 A × 0.00193 = 7.72 V- Round trip
- 2 × 100 ft
- Current
- 20 A
- Resistance, 12 AWG copper
- 1.93 ÷ 1,000 = 0.00193 ohms per foot
- Voltage drop
- 2 × 100 × 20 × 0.00193 = 7.72 V
- At the load
- 120 − 7.72 = 112.28 V, a 6.4 percent loss
That is over the 5 percent figure the NEC notes give for the whole path, so the verdict says to upsize. Step to 8 AWG copper at 0.764 ohms per 1,000 ft and the same run drops just 3.06 V, or 2.5 percent, comfortably inside the 3 percent branch-circuit recommendation.
Change the supply voltage instead of the wire and watch what happens. The same 20 A over the same 100 ft of 12 AWG still loses 7.72 V in absolute terms, because nothing about the conductor changed, but on a 240 V circuit that is only 3.2 percent rather than 6.4. The absolute loss is identical; its significance halves.
That is the real reason 240 V is preferred for long feeds, alongside the fact that a given amount of power at 240 V needs half the current, which halves the absolute drop as well and compounds the advantage.
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Questions people ask
How much voltage drop is acceptable?
The recognised targets are 3 percent on a branch circuit and 5 percent for the combined feeder and branch-circuit path. Those figures come from Informational Notes attached to NEC 210.19(A) and 215.2(A), and NEC 90.5 states that Informational Notes are informational only and not enforceable as requirements, so they are recommendations rather than code. Energy codes and local amendments often make the 3 percent figure binding in practice, and it is sound engineering either way. This tool grades a run as good at or under 3 percent, acceptable to 5, and needing thicker wire above that.
Covered in depth in What Size Wire Do I Need? Voltage Drop Over a Long Run →
What wire size do I need for a 100 ft run at 20 A?
On 120 V, 12 AWG copper over 100 ft at 20 A drops 7.72 V, which is 6.4 percent and past the 5 percent figure, so it is too thin for that run. Stepping to 10 AWG brings it to 4.84 V, or 4.0 percent, which is inside 5 percent but not 3. Going to 8 AWG gives 3.06 V, or 2.5 percent, comfortably inside both targets. If you can run the load at 240 V instead, the same 12 AWG lands at 3.2 percent without changing the cable at all.
Why do long runs need thicker wire?
Because voltage drop grows in direct proportion to both length and current, and a long run to an outbuilding usually maximises both. Double the distance and the loss doubles; double the amps and it doubles again. Resistance accumulates over every foot of conductor, so by the time the current reaches the far end there is measurably less voltage left to work with, even though the panel reads perfectly normal. Thicker wire has lower resistance per foot, which is the only lever that fixes it once the route and the load are fixed.
Does this calculator work for aluminium wire?
No, it assumes copper, which is what most branch wiring uses. Aluminium has substantially higher resistance for the same gauge: NEC Chapter 9, Table 8 lists 12 AWG aluminium at about 3.18 ohms per 1,000 ft against 1.93 for copper, roughly 65 percent more, and the same ratio holds across the common sizes. An aluminium run of a given gauge will therefore drop about 65 percent more voltage than the copper figure shown here. Sizing aluminium conductors is a job for a licensed electrician working from the correct table.
My detached garage outlet reads low — is voltage drop the cause?
Very likely, if the run is long and the conductor is thin. The panel can read a healthy voltage while the far end of a long circuit reads soft, because the loss happens along the wire rather than at the source, so measuring at the panel tells you nothing. That soft voltage is what makes tools run weakly and motors run hot at an outbuilding. Enter the run length, the actual load in amps and the gauge to see exactly how much you are losing, then have an electrician confirm the diagnosis before anything is rewired.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
The conductor resistances the calculator uses, from Chapter 9 Table 8 at 75 C: 1.93 ohms per 1,000 ft for 12 AWG uncoated copper, 1.21 for 10 AWG and 0.764 for 8 AWG.
NFPA 70, National Electrical Code, Chapter 9 Table 8 — NFPA, Free registered access; table numbering is edition-sensitive
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