Well Pump Wire Sizing: Why a Long Run Needs a Bigger Wire Than Ampacity Alone
Size a well pump circuit from the motor current, then check voltage drop over the real run length. A 1.5 HP pump 200 ft away is the classic case where the wire passes ampacity but fails voltage drop.
Step-by-Step Procedure
Start with the motor full-load current, not the horsepower alone
For a single-phase motor, use the standard full-load current table value for that horsepower and voltage. A 1.5 HP, 230 V single-phase motor plans at about 10 A. Do not use a running current guessed from a clamp meter on a healthy pump, because starting and loaded current are different questions.
Size the conductor for 125% of full-load current
Multiply full-load current by 1.25 for the conductor: 10 A x 1.25 = 12.5 A minimum ampacity. On that check alone, 14 AWG copper passes, because its 75 C ampacity is 20 A and its small-conductor limit is 15 A. That is exactly why this step alone is not enough for a long well run.
Check voltage drop with the round-trip length
A well 200 ft from the panel is 400 ft of conductor path. Use the resistance from the site wire table: 14 AWG copper is 2.525 ohms per 1,000 ft, 12 AWG is 1.588 ohms per 1,000 ft. Voltage drop is current x resistance x round-trip length / 1,000.
Upsize for voltage drop, then re-check the breaker and ground
For a 25 A motor branch-circuit breaker, the equipment grounding conductor table value is 10 AWG copper. If the phase conductors are upsized for voltage drop, the grounding conductor must also be proportionally upsized. Do not just upsize the breaker to cure a pump that will not start; check voltage at the pump, starting current and the motor overload first.
Confirm the pump, control box and local rules
Check whether the pump uses a control box, a pressure switch and any required disconnect, and check the locally adopted NEC edition. This guide is a planning estimate only; well and pump installation should be verified by a licensed electrician.
Worked Practical Example
1.5 HP well pump, 230 V, 200 ft one-way
A 1.5 HP, 230 V single-phase well pump, 200 ft from the panel, copper wire.
- •Motor full-load current: about 10 A for 1.5 HP at 230 V single-phase
- •Conductor ampacity check: 10 A x 1.25 = 12.5 A, so 14 AWG passes ampacity alone
- •Voltage drop, 14 AWG: 10 A x 2.525 ohms per 1,000 ft x 400 ft / 1,000 = 10.10 V, which is 4.39% of 230 V and fails a 3% branch-circuit guide
- •Voltage drop, 12 AWG: 10 A x 1.588 ohms per 1,000 ft x 400 ft / 1,000 = 6.35 V, which is 2.76% of 230 V and passes
- •Breaker planning value: 10 A x 2.50 = 25 A maximum inverse-time breaker under the motor branch-circuit rule, subject to the locally adopted code and the pump manual
Common Mistakes to Avoid
- ✕Sizing the wire from ampacity only and never checking voltage drop on a long well run.
- ✕Using the one-way length instead of the round-trip length in the voltage-drop calculation.
- ✕Upsizing the breaker to fix a pump that will not start, when the real problem is voltage drop at the pump.
- ✕Forgetting to upsize the grounding conductor when the phase conductors are upsized for voltage drop.
Frequently Asked Questions
What size wire do I need for a 1.5 HP well pump 200 ft away?
As a planning example, 12 AWG copper passes both ampacity and a 3% voltage-drop guide at 230 V and 200 ft, while 14 AWG passes ampacity but fails voltage drop at 4.39%. Always check your actual pump current, distance and locally adopted code.
Why does my well pump hum but not start?
Low voltage at the pump from a long, undersized run is one common cause, but it is not the only one. Check voltage at the pump under load, the control box, capacitor and pressure switch, and get a licensed electrician involved before changing breaker sizes.