Voltage Drop Calculator
Volts and percent dropped over any run — copper or aluminum, 1φ or 3φ.
Calculator
How this works
Voltage drop is plain physics: push current through a conductor and some voltage is lost as heat along the way. This calculator uses the standard resistance formulas:
Three-phase: VD = (1.732 × K × I × L) ÷ CM
K is the conductor's effective resistivity in ohm·circular-mil per foot — 12.9 for copper, 21.2 for aluminum, reflecting commonly used 75°C design values. I is the load in amps, L is the one-way run length in feet, and CM is the conductor's cross-section in circular mils (6,530 for 12 AWG, for example).
The 2× in the single-phase formula covers the round trip — current flows out on the hot and back on the neutral or the other hot, so it fights resistance in both directions. On balanced three-phase circuits the return currents partially cancel, which is where the 1.732 (√3) factor comes from. That's why three-phase runs drop less than the same single-phase run.
Percent drop is just the volts lost divided by system voltage. The calculator flags anything above 3% (the common branch-circuit guideline) in amber and anything above 5% (the common overall guideline) in red.
How far can you run copper wire at 3% drop?
| Circuit | Copper wire | Max one-way run |
|---|---|---|
| 15 A at 120 V | 14 AWG | 38 ft |
| 20 A at 120 V | 12 AWG | 45 ft |
| 30 A at 240 V | 10 AWG | 96 ft |
| 40 A at 240 V | 8 AWG | 115 ft |
| 50 A at 240 V | 6 AWG | 146 ft |
| 60 A at 240 V | 4 AWG | 194 ft |
Computed with the single-phase formula above at full load (75°C K value, one-way distance). Longer runs need a larger conductor — run your exact numbers in the calculator.
Worked example: shed subpanel at the back of the lot
Say you're feeding a small workshop shed 150 feet from the house panel. The load works out to 30 A at 240 V single-phase, and there's a coil of 10 AWG copper on the truck. Good enough? Run the numbers before you trench.
Plug in the formula: VD = (2 × 12.9 × 30 A × 150 ft) ÷ 10,380 CM. The numerator is 116,100; divide by the circular mils of 10 AWG and you get 11.2 volts dropped. Against 240 V that's 4.66% — past the 3% branch guideline. The shed sees about 229 V at full load. Lights will run fine, but a table saw or compressor starting under that kind of sag will grumble, and motors that chronically run at low voltage draw more current and run hotter than they should.
Step up to 8 AWG (16,510 CM) and the drop falls to 7.0 V, or 2.9% — inside the guideline with nothing to spare. Go to 6 AWG and you're at 1.8%, with headroom if the shed loads ever grow. For a buried run you'll dig exactly once, the smart money is on the bigger conductor: the copper costs less than a second trench.
That's the whole discipline of voltage drop: it rarely matters at 30 feet, and it almost always matters at 150.
Practical tips and common mistakes
- Measure the real run, not the site plan. Wire goes up walls, across ceilings and around obstacles. A 60-foot straight-line distance is often an 85-foot pull. Measure the path the conductor actually takes.
- Don't double the length yourself. The formula's 2× factor already handles the round trip. Entering total conductor length instead of one-way distance is the most common way to get a scary, wrong answer.
- Size for the running load, check the starting load. Motors and compressors draw several times their rated current at startup. A run that's fine at 3% running drop can sag badly on start — if the load is a motor at the end of a long run, be more conservative.
- Remember drop stacks up. A feeder at 2% plus a branch at 3% is 5% total at the outlet. If this run is fed by a long feeder, budget the two together.
- Voltage drop is a design guideline, not a pass/fail inspection item in most cases. But the callbacks are real: dim lights, nuisance tripping, dead tools at the end of long runs. Fix it on paper, not after the drywall.
Frequently asked questions
What is an acceptable voltage drop?
A common design guideline is to keep branch-circuit voltage drop at or below 3%, and the total drop from service to the farthest outlet at or below 5%. These are widely used recommendations rather than hard rules — some sensitive equipment needs tighter limits, so check the equipment specs and your locally adopted code.
Do I enter one-way length or total wire length?
Enter one-way length — the distance from the panel to the load. The calculator already accounts for the round trip: the 2× factor in the single-phase formula (1.732× for three-phase) covers current flowing out and back through both conductors.
What size wire do I need for 100 feet?
It depends on the load and voltage, not just distance. As an example, a 20 A load at 120 V over 100 feet drops about 7.9 V (6.6%) on 12 AWG copper — too much for the 3% guideline. Use our wire size calculator to get the smallest conductor that meets your target drop.
Why is voltage drop worse at 120 V than 240 V?
The volts lost are the same for a given wire, length and current — but as a percentage of a smaller starting voltage, the impact doubles. A 6 V drop is 5% of 120 V but only 2.5% of 240 V. That is why long runs are much more forgiving on 240 V circuits.
Does temperature affect voltage drop?
Yes, conductor resistance rises with temperature. This calculator uses K values of 12.9 for copper and 21.2 for aluminum (ohm·circular-mil per foot), which reflect commonly used 75°C design assumptions. Cooler conductors will drop slightly less; hotter ones slightly more.
Is aluminum wire worse for voltage drop?
Aluminum has about 64% higher resistance than copper for the same size, so it drops more voltage in the same run. In practice you typically go up about two sizes when switching from copper to aluminum to get an equivalent drop, which is why aluminum is mostly used in larger feeder sizes where it stays cost-effective.