Voltage Drop on 12 AWG at 100 Feet
Voltage drop
6.58% on the run
120 volts, single phase, 20 amps, 100 feet one way, 12 AWG copper.
Over 5 percent. This run is too long for the conductor on it. Go up in size, shorten the run, or feed it at a higher voltage, because at this drop motors run hot and equipment warranties start to have opinions.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- System voltage120 V
- PhaseSingle phase
- Load current20 A
- One way run length100 ft
- Conductor size12 AWG
- Conductor materialCopper
Voltage drop6.58% on the run
Over 5 percent. This run is too long for the conductor on it. Go up in size, shorten the run, or feed it at a higher voltage, because at this drop motors run hot and equipment warranties start to have opinions.
| Drop | 7.9 V |
|---|---|
| Voltage at the load | 112.1 Vfrom 120 V at the panel |
| Smallest conductor under 3% | 8 AWG |
| Longest run at 12 AWG under 3% | 46 ftone way |
| Conductors to pull | 3including the equipment ground |
The material list this produces
| 12 AWG copper conductor, ft | 330 |
|---|---|
| Conduit or cable, ft | 110 |
| Breaker, 20 A, each | 1 |
| Terminations and lugs, each | 6 |
One way run length compared
Drop is a straight line in length. Double the run and you double the drop, which is why the same wire that is fine in a basement is not fine across a yard.
| One way run length | Voltage drop | Smallest conductor under 3% |
|---|---|---|
| 50 ft | 3.29% on the run | 10 AWG |
| 100 ft | 6.58% on the run | 8 AWG |
| 150 ft | 9.88% on the run | 6 AWG |
| 200 ft | 13.17% on the run | 4 AWG |
| 300 ft | 19.75% on the run | 3 AWG |
What else matters here
Six and a half percent. The load sees 112 volts instead of 120, which is outside what most equipment is designed around and well outside the 3 percent the NEC suggests for a branch circuit.
This is the most common long circuit mistake in residential work: 12 AWG is the right size for 20 amps on ampacity grounds, so nothing about the wire gauge looks wrong, and the problem only shows up as dim lights, a hot cord or a motor that trips its overload in summer.
Twelve AWG holds 3 percent out to 46 feet on this circuit. Past that you are into 10 AWG, which takes it to 4.14 percent at this length, and 8 AWG is what actually gets you under 3 at 100 feet.
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.
Questions people ask
How far can you run 12 gauge wire on a 20 amp circuit?
On ampacity alone, as far as you like. On the 3 percent voltage drop guideline at 120 volts and a full 20 amp load, about 46 feet. At 240 volts the same wire and load reaches roughly twice that, because drop as a percentage halves when the voltage doubles.
Do I enter the one way run or the total length of wire?
One way, panel to load. The formula multiplies by two for a single phase circuit because the current has to come back, so entering the loop length doubles the answer and sends you two sizes larger than you need.
Is 3 percent voltage drop actually required by code?
No. The 3 percent branch circuit figure and the 5 percent total figure live in informational notes to NEC 210.19 and 215, and informational notes are advisory rather than enforceable. Some jurisdictions adopt them as local amendments and many engineered specs require them, and motors and electronics behave badly on a long undersized run whatever the code says.
The voltage drop calculator takes any dimensions you like and hands back the same list.