Complete guide to wiring a well pump

Sizing the circuit and conductors

Start with the motor nameplate. Submersible pumps list full load amps (FLA) and service factor amps (SFA). For branch circuit conductor sizing, NEC 430.22 requires 125% of the motor FLA. For overcurrent protection, NEC 430.52 allows up to 250% FLA for an inverse time breaker on a single motor, higher for time delay fuses.

Most residential 1/2 to 1 HP submersibles run on 240V single phase, 2-wire or 3-wire. A 1 HP pump pulls roughly 9 to 10 amps FLA at 240V. A 20A 2-pole breaker with 12 AWG copper covers it for short runs. Once the well is more than about 100 feet from the panel, voltage drop drives the conductor size, not ampacity.

Common pitfalls to check before pulling wire:

• Voltage drop calculated at SFA, not FLA, kept under 3% per NEC Chapter 9 informational notes.
• Pump manufacturer wire chart compared against your NEC calc, then take the larger.
• Equipment grounding conductor sized per NEC 250.122 based on the OCPD.
• Submersible cable rated for potable water if it contacts the supply, typically marked PE or PVC jacket suitable for well service.

Disconnect, controller, and overload

NEC 430.102(B) requires a disconnecting means within sight of the motor or capable of being locked in the open position. For a buried well, the disconnect goes at the wellhead or at the pressure tank inside, with lockout. A fused disconnect or a 2-pole breaker in a weatherproof enclosure both qualify if rated as motor circuit disconnects per 430.109.

3-wire submersibles need a control box with the start capacitor, run capacitor, and start relay. Mount it indoors, vertical, away from heat, with at least the clearance the manufacturer specifies. 2-wire pumps have the starting components downhole and need no control box, just the disconnect and overload protection.

Field tip: label the disconnect with the pump HP, voltage, and depth on a phenolic tag. The next tech in 15 years will thank you when troubleshooting at 2 AM.

GFCI, bonding, and grounding

NEC 210.8(F) requires GFCI protection for outdoor outlets on dwelling units, and 422.5 lists specific appliances. Submersible well pumps are not on the mandatory GFCI list at the branch circuit level in most code cycles, but check local amendments. Many AHJs in flood-prone or agricultural areas now require GFCI on the pump circuit.

Bonding is non-negotiable. NEC 250.112(M) requires the metal well casing to be bonded to the equipment grounding system. Run a copper EGC, sized per 250.122, from the pump motor frame back to the service. Bond the casing at the wellhead with a listed clamp suitable for direct burial.

Steel casings have been used as grounding electrodes under NEC 250.52(A)(8) when they meet the requirements, but verify with your inspector before relying on it. Most jobs treat the casing as bonded equipment and use a separate ground rod or the existing GES.

Wire methods from panel to wellhead

Direct burial submersible cable, listed as type UF or pump cable rated for direct burial, is the workhorse. From the panel, run THWN-2 in PVC conduit to a weatherproof junction box near the well, then transition to submersible cable down the casing.

Burial depth comes from NEC 300.5. For a 240V residential branch circuit:

1. UF cable direct buried: 24 inches minimum.
2. Rigid metal or IMC: 6 inches minimum.
3. PVC conduit: 18 inches minimum.
4. Under a 4 inch concrete slab with conduit: 18 inches.
5. GFCI protected residential branch, not over 120V, 20A: 12 inches, but most pumps are 240V so this does not apply.

Splices in the well between the drop cable and motor leads use heat-shrink butt connectors with adhesive lining or kits listed for submersion. Tape splices fail. Always use a torque pad or tie wraps to secure the cable to the drop pipe every 10 feet so the splice does not carry the weight of the column.

Pressure switch, low water, and controls

The pressure switch is the on/off control for the pump. Standard residential cut-in/cut-out is 30/50 or 40/60 PSI. Wire it on the load side of the disconnect, ahead of the motor or control box. Use the correct horsepower rated switch, typically 1 or 2 HP at 230V for residential.

Low water cutoff protection is cheap insurance against burning a motor when the well runs dry. Pumptec, Cycle Sensor, and similar electronic devices monitor amperage and shut the pump down on dry run, rapid cycling, or overload. They wire between the pressure switch and the motor leads. On 3-wire setups, follow the controller manufacturer's diagram exactly because the start winding feed matters.

Field tip: when a pump trips on overload but tests fine cold, the wire from the pressure switch is usually the suspect. Loose terminals heat up under load and read normal at rest.

Commissioning and documentation

Before energizing, megger the drop cable and motor leads. NEMA standards put a healthy submersible motor at 20 megohms or higher cable to ground when new, dropping over time. Anything under 1 megohm is a fault. Record the reading on the job ticket and inside the control box.

Check rotation on 3-phase pumps by reading the amperage on all three legs and the discharge pressure. Reverse rotation produces low pressure and lower than nameplate amps. Swap any two leads at the disconnect to correct.

Leave the homeowner with a single sheet listing pump HP, voltage, FLA, depth set, breaker size, wire size, and the megger reading at startup. That sheet becomes the baseline every future service call measures against.