Voltage Drop on 10 AWG at 100 Feet
Voltage drop
4.14% on the run
120 volts, single phase, 20 amps, 100 feet one way, 10 AWG copper.
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.
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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 size10 AWG
- Conductor materialCopper
Voltage drop4.14% 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 | 4.97 V |
|---|---|
| Voltage at the load | 115 Vfrom 120 V at the panel |
| Smallest conductor under 3% | 8 AWG |
| Longest run at 10 AWG under 3% | 72 ftone way |
| Conductors to pull | 3including the equipment ground |
The material list this produces
| 10 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 | 2.07% on the run | 10 AWG |
| 100 ft | 4.14% on the run | 8 AWG |
| 150 ft | 6.21% on the run | 6 AWG |
| 200 ft | 8.29% on the run | 4 AWG |
| 300 ft | 12.43% on the run | 3 AWG |
What else matters here
Going from 12 to 10 cuts the drop by more than a third, from 6.58 percent to 4.14, because the circular mil area goes from 6,530 to 10,380. That is the general shape of it: one size up is roughly a 37 percent reduction in drop.
Four percent is a common landing place and it is a judgment call. For a receptacle circuit feeding hand tools it is usually accepted. For a circuit running a motor continuously, or LED drivers that flicker below a certain voltage, take it to 8 AWG and be done with it.
The practical difficulty with 10 AWG in a receptacle circuit is the devices. Most 15 and 20 amp receptacles are not rated for a 10 AWG conductor in the back, so the usual approach is to run 10 to a junction box near the load and pigtail down to 12 for the last few feet, where the length no longer matters.
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
Is 4 percent voltage drop acceptable?
It is inside the 5 percent total figure the NEC notes suggest for feeder plus branch circuit, and it is not a code violation in itself. Whether it is acceptable depends on the load: a drill does not care, a well pump motor does.
Can I put 10 AWG wire on a 20 amp breaker?
Yes. The breaker protects the smallest conductor in the circuit, so larger wire on a smaller breaker is always allowed. The reverse is not.
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.
The voltage drop calculator takes any dimensions you like and hands back the same list.