Voltage Drop Calculator: Wire Size for a Long Run
Enter the one way run, the load, the conductor and whether it is copper or aluminum, and it returns the drop in volts and percent, the voltage actually arriving at the load, and the smallest conductor that keeps the run under three percent. It also tells you how far this wire can go before it fails that test.
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Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- System voltage240 V
- PhaseSingle phase
- Load current30 A
- One way run length120 ft
- Conductor size10 AWG
- Conductor materialCopper
Voltage drop3.73% on the run
Between 3 and 5 percent. Legal, and still worth fixing on a branch circuit that feeds a motor, a heater or a long lighting run. Going up to 8 AWG brings it under 3 percent.
| Drop | 8.95 V |
|---|---|
| Voltage at the load | 231.1 Vfrom 240 V at the panel |
| Smallest conductor under 3% | 8 AWG |
| Longest run at 10 AWG under 3% | 97 ftone way |
| Conductors to pull | 3including the equipment ground |
The material list this produces
| 10 AWG copper conductor, ft | 396 |
|---|---|
| Conduit or cable, ft | 132 |
| Breaker, 30 A, each | 1 |
| Terminations and lugs, each | 6 |
What this calculator assumes
- The formula is the standard one: drop equals 2 times K times amps times the one way length, divided by the circular mil area of the conductor. Three phase uses 1.732 in place of the 2.
- K is 12.9 for copper and 21.2 for aluminum, the usual figures for a conductor running warm in conduit. Circular mil areas come from NEC Chapter 9, Table 8.
- Enter the one way distance, not the loop. The formula doubles it for you, because current has to get back.
- The 3 percent branch circuit and 5 percent total figures in 210.19 and 215 are informational notes rather than requirements. Nobody fails an inspection on voltage drop alone, but motors, LED drivers and long well pump runs care a great deal.
The formula, and the mistake everyone makes with it
Voltage drop on a single phase circuit is two times K times the current times the one way length, divided by the circular mil area of the conductor. K is 12.9 for copper and 21.2 for aluminum. Three phase uses 1.732 instead of the 2.
The two is there because the current has to come back. That is the mistake: people measure the loop, enter it, and the calculator doubles a number that was already doubled. Enter the distance from the panel to the load, one way, and let the formula do the rest.
Circular mil areas come from Chapter 9, Table 8 of the NEC. A 12 AWG copper conductor is 6,530 circular mils, a 10 is 10,380, an 8 is 16,510. The jump from 12 to 10 is a 59 percent increase in area and therefore a 37 percent cut in drop, which is why one size up so often fixes it.
Three percent is a recommendation, not a rule
The three percent branch circuit figure and the five percent total figure live in informational notes to 210.19 and 215 of the NEC. Informational notes are not enforceable. Nobody red tags a job for voltage drop alone.
The equipment does not read the code book though. A motor starting on a long undersized run draws more current at lower voltage, heats up, and trips on overload or dies quietly over a few years. LED drivers flicker. Well pumps at the end of a 400 foot run are the classic case, and so are detached garages and barns.
Some jurisdictions have adopted the drop limits as local amendments and made them enforceable, and some engineers specify them. If the run is long, design to three percent and it will pass anywhere.
Aluminum is not the problem, the connection is
Aluminum has about 61 percent the conductivity of copper, which is why K is 21.2 instead of 12.9. For the same drop you go up roughly two sizes. That trade is often worth taking on a long feeder, where the copper price on 200 feet of 2/0 is a real number.
What goes wrong with aluminum is almost never the wire in the middle. It is the terminations. Aluminum creeps under pressure and oxidizes into a resistive film, so it needs terminals listed AL or CU-AL, an antioxidant compound on the strands, and a torque wrench rather than a feel for it.
Do that properly and an aluminum feeder is fine. Do it by eye on a lug rated for copper only and you have built a heater inside a panel.
Questions people ask
What wire size do I need for a 100 foot run at 20 amps?
On a 120 volt circuit, 10 AWG copper. 12 AWG at that length and load lands around 4.3 percent, which is over the recommended three. On 240 volts the same run in 12 AWG is about 2.2 percent and is fine, because drop as a percentage halves when the voltage doubles.
Is 3 percent voltage drop a code requirement?
No. It sits in informational notes to NEC 210.19 and 215, which are advisory. Some local amendments and many engineered specs make it binding, and motors and electronics care regardless of what the code says.
Do I enter the one way distance or the total wire length?
One way, panel to load. The formula multiplies by two for a single phase circuit because the current makes a round trip. Entering the loop length doubles the answer.
How much bigger does aluminum need to be?
About two sizes for the same voltage drop, because aluminum carries roughly 61 percent of copper's conductivity. Where copper needs 2 AWG, aluminum wants 1/0, and the terminations need to be listed for aluminum.
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Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.