Voltage Drop CalculatorWell pump wire size calculator
Wire size for a submersible well pump. Enter the cable run from the panel to the motor, with the pump depth included, and the motor’s maximum nameplate amps.
Smallest wire for 5%
Copper| Wire size | 10 AWG |
| Metric, same or larger | 6 mm² |
| Area | 5.26 mm² |
| Voltage drop | 7.31 V |
| Percent drop | 3.18 % |
| At the load | 222.69 V |
Within 5% target
At other limits
- 8 AWG at 3%
- 10 AWG at 5%
- 14 AWG at 10%
Worked out with
- Copper at 75 °C (K = 12.9)
- 490 ft one way; the return wire is added
- 6 A, the current the load draws
- Single-phase, 230 V
10 AWG. Drop 7.31 volts, 3.18 percent. Within 5% target.
Result from https://voltagedropcalculator.app/well-pump-wire-size-calculator?phase=single&voltage=230¤t=6&length=490&material=copper&target=5&mode=find. This checks voltage drop only, not code compliance.
- The breaker doesn't change. Voltage drop sets the wire size, not the protection. Keep the breaker the circuit needs.
- Check the wire fits. Look up the largest wire the breaker and device terminals take. If 10 AWG won't fit, ask an electrician how to connect it where you live.
- Cheaper ways to the same result: the same power at 460 V (half the current) needs 14 AWG; in aluminum, 8 AWG (larger, but cheaper per foot). A sub-panel nearer the loads also shortens the run.
- A larger ground wire may be needed too when you upsize. Check your local code.
How this is worked out
V = 2 × K × I × L ÷ CM
2 × 12.9 × 6 A × 490 ft ÷ 10,383 CM = 7.31 V
7.31 V ÷ 230 V × 100 = 3.18 %. At the load: 230 V − 7.31 V = 222.69 V.
- L is the one-way length; the 2 counts the return conductor.
- K is 12.9 for copper and 21.2 for aluminum, in ohm·CM per foot on a 75 °C basis.
- Not checked here: temperature derating, conduit fill, power factor and code compliance.
Measure to the motor, not the well head
- The length is the whole cable from the service entrance to the motor: the run to the well plus the depth the pump is set at. Pump makers’ tables are measured this way.
- Example: 150 ft of buried cable to the well and a pump set at 340 ft is 490 ft. Enter 490 ft one way; the return wire is counted for you.
- The control box doesn’t change it. A single-phase control box can sit anywhere along the run; you still measure to the motor.
Which current and which drop
- Use the motor’s maximum nameplate amps (service factor amps), not full-load amps. Pump makers size their cable at that current, and it differs between brands, so take it from your motor’s label or its maker.
- Makers allow a 5% drop at maximum nameplate amps, so the motor gets at least 95% of the service-entrance voltage. That is why this page starts at 5%. Choose 3% for a tighter design.
- Keep the voltage at the motor within 10% of its rating. Too little voltage makes the motor draw more current and run hot, and undersized cable is a common cause.
Worked example: a 1/2 hp pump, 490 ft of cable
A 1/2 hp 2-wire pump draws about 6 A at 230 V and 12 A at 115 V at maximum load (Franklin Electric’s figures for its motors). Over 490 ft of copper cable:
| Supply | At 5% | At 3% |
|---|---|---|
| 230 V, 6 A | 10 AWG 3.18 % | 8 AWG 2.00 % |
| 115 V, 12 A | 4 AWG 3.16 % | 3 AWG 2.51 % |
At 230 V, 12 AWG would come to 5.05 %, just over 5%, so the answer is 10 AWG. The same pump at 115 V draws twice the current and needs 4 AWG: one reason pumps above 1/2 hp are 230 V. The calculator above starts with the 230 V case.
This calculator and the pump maker’s table
This calculator uses copper resistance at 75 °C (K = 12.9; see the method page), a warm-wire figure. In spot checks its allowed lengths come out roughly 5–30% shorter than pump makers’ tables, so treat its answer as a cautious estimate. Where the motor maker’s chart and this page differ, follow the maker’s chart and the electrical code; CentriPro notes that the NEC takes precedence over any maker’s chart.
Cable, splices and grounding
- Use cable made and listed for submersible pumps down the well. It comes twisted, flat parallel or flat with a heavy jacket.
- Copper is the norm. Franklin says aluminum must be two sizes larger than copper, with oxidation inhibitor on the connections; CentriPro calls for copper only.
- Splices in the well must be watertight.
- Ground the motor and control box with a ground wire no smaller than the motor’s power wires. With plastic drop pipe or casing, run a dedicated grounding conductor from the panel to the motor.
- A brief dip when the pump starts is normal. The makers’ tables are set by the running current; a large or lasting drop points to undersized cable or a loose connection.
Frequently asked questions
Do I include the well depth in the wire length?
Yes. Pump makers measure the cable from the service entrance to the motor, so the length is the run to the well plus the depth the pump is set at. A 150 ft trench to a pump set at 340 ft is 490 ft of cable. Enter that one way: the calculator counts the return wire.
Which amps do I use for a well pump?
The motor’s maximum nameplate amps, also called service factor amps, from the motor label or the maker’s data. Pump makers size cable at that current, not at full-load amps, and amps differ between brands, so use the figure for your motor.
What voltage drop do pump makers allow?
Franklin Electric, Grundfos and CentriPro size their cable tables for a 5% drop at maximum nameplate amps: the motor gets at least 95% of the voltage at the service entrance. Franklin gives a 3% option too. Supply voltage at the motor should stay within 10% of its rating.
Does the control box change the length?
No. A single-phase control box without a line contactor can sit anywhere along the run; you still measure the whole cable to the motor. With a line contactor, Franklin and Grundfos limit the cable before the box to a quarter of the allowed length.
Why is a 115 V pump so much harder to wire?
The same horsepower draws about twice the current at 115 V as at 230 V, so the drop doubles on the same wire. Franklin makes 115 V submersible motors only up to 1/2 hp; larger single-phase pumps are 230 V.
Can I use two wire sizes on one run?
Yes. Pump makers add up the share of the allowed length each section uses, and the total must stay at or under 100%. An existing buried run uses part of the budget, so the cable down the well must fit what is left. To check a run like that here, work out each section with “Is my wire OK?” and add the two percentages.
Related: voltage drop calculator, voltage drop chart, ampacity vs voltage drop, wire gauge chart, how the calculator works.