Voltage Drop on a 20 Amp Circuit at 150 Feet
Voltage drop
6.21% on the run
120 volts, single phase, 20 amps, 150 feet one way, 10 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 length150 ft
- Conductor size10 AWG
- Conductor materialCopper
Voltage drop6.21% 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 | 7.46 V |
|---|---|
| Voltage at the load | 112.5 Vfrom 120 V at the panel |
| Smallest conductor under 3% | 6 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 | 495 |
|---|---|
| Conduit or cable, ft | 165 |
| 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
A hundred and fifty feet at 120 volts is where a 20 amp branch circuit stops being practical in ordinary wire. Even 10 AWG lands at 6.21 percent, and getting under 3 percent needs 6 AWG, which will not terminate on a receptacle at all.
The fix is almost never a bigger branch circuit. It is a sub panel: run a 240 volt feeder to a small panel near the loads and take short branch circuits off it. At 240 volts the percentage drop on the same copper is half, and the branch circuits at the far end are ten feet long instead of a hundred and fifty.
That is the general rule for anything past about 100 feet at 120 volts. Move the panel, do not grow the wire.
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 a 20 amp circuit?
At 120 volts and a full load, about 46 feet in 12 AWG and 72 feet in 10 AWG before you pass 3 percent. Past roughly 100 feet the sensible answer is a 240 volt feeder to a sub panel rather than a larger branch circuit.
Why is 240 volts better for a long run?
The same power draws half the current at twice the voltage, and drop is proportional to current. Halve the current and halve the drop, then divide by a voltage that is twice as large, and the percentage drop is a quarter of what it was.
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.