What Size Wire for a Well Pump 300 Feet Away?
Voltage drop
4.94% on the run
240 volts, single phase, 10 amps running, 300 feet one way, 12 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 voltage240 V
- PhaseSingle phase
- Load current10 A
- One way run length300 ft
- Conductor size12 AWG
- Conductor materialCopper
Voltage drop4.94% 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 | 11.85 V |
|---|---|
| Voltage at the load | 228.1 Vfrom 240 V at the panel |
| Smallest conductor under 3% | 8 AWG |
| Longest run at 12 AWG under 3% | 182 ftone way |
| Conductors to pull | 3including the equipment ground |
The material list this produces
| 12 AWG copper conductor, ft | 990 |
|---|---|
| Conduit or cable, ft | 330 |
| Breaker, 10 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 | 0.82% on the run | 14 AWG |
| 100 ft | 1.65% on the run | 14 AWG |
| 150 ft | 2.47% on the run | 12 AWG |
| 200 ft | 3.29% on the run | 10 AWG |
| 300 ft | 4.94% on the run | 8 AWG |
What else matters here
Well pumps are the textbook voltage drop failure, and this is why. Ten amps is a small load, 12 AWG looks like plenty of wire for it, and at 300 feet the pump sees 228 volts instead of 240.
A motor at low voltage does not run slower, it draws more current to make the same torque, and the extra current turns into heat in the windings. That shortens the motor's life quietly, over years, and then it fails and gets replaced with an identical motor on the same undersized wire.
The running current is also not the number that hurts. Starting current on a submersible is several times the running figure, and the momentary drop at start is correspondingly larger, which is what stops some pumps starting at all on a long thin run. Size for 3 percent at running load and the starting case takes care of itself. Here that means 8 AWG.
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
What size wire for a well pump 300 feet away?
8 AWG copper for a 10 amp pump at 240 volts, to hold 3 percent. 12 AWG carries the current fine but drops nearly 5 percent over that distance.
Why do well pumps fail from voltage drop?
A motor on low voltage compensates by drawing more current, which heats the windings. It does not fail immediately, it fails early, and the replacement usually goes onto the same wire.
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.