Free Submersible Well Pump Wire Sizing Calculator
Calculate required submersible copper cable gauge (AWG), terminal voltage drop, and backup generator wattage for deep well pump motors.
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How to Size Wire for Submersible Well Pumps
Submersible well pumps operate under harsh electrical conditions: they are submerged hundreds of feet underwater at the end of very long copper cable runs. Undersized wire causes excessive voltage drop during motor starting, triggering the thermal overload switch and permanently burning out the motor windings.
1. Voltage Drop Limits (Franklin Electric Standards)
National Electrical Code (NEC) permits up to a 5% total voltage drop from service entrance to pump terminals. However, leading pump manufacturers (Franklin Electric, Goulds) recommend keeping running voltage drop under 3.0% to ensure sufficient starting torque when water pressure switches close.
2. Voltage Drop Calculation
- #14 AWG Copper: 3.07 Ω / 1,000 ft
- #12 AWG Copper: 1.93 Ω / 1,000 ft
- #10 AWG Copper: 1.21 Ω / 1,000 ft
- #8 AWG Copper: 0.764 Ω / 1,000 ft
- #6 AWG Copper: 0.491 Ω / 1,000 ft
- #4 AWG Copper: 0.308 Ω / 1,000 ft
Frequently Asked Questions
Why can't I use standard Romex NM-B wire inside a well casing?
Standard indoor NM-B wire insulation degrades rapidly when submerged in cold well water, leading to ground fault short circuits. You must use specialized submersible pump cable (heavy dual-layer PVC/nylon rated for continuous submersion) with waterproof heat-shrink splice kits.
What generator size is needed to run a 1 HP well pump during a power outage?
A 1 HP 230V well pump draws ~11.5 amps running (2,600 Watts), but draws 35 to 45 amps for a fraction of a second during locked-rotor starting (LRA). To overcome this starting inertia without stalling, your generator must have at least 5,000 to 6,000 peak starting Watts.
How do I test if my well pump motor has a ground short?
Turn off the circuit breaker and disconnect the pump leads in the control box. Use a digital multimeter or megohmmeter (megger) set to high resistance (ohms). Measure between each motor lead and the metal well casing / ground wire. Any reading below 20 MΩ indicates insulation breakdown; under 0.5 MΩ indicates a dead short.
What is torque arrestor and why is it clamped onto the drop pipe?
Every time a submersible motor starts, electromagnetic reaction torque causes the pump to twist violently in the casing. A rubber torque arrestor clamped just above the pump expands against the casing walls to absorb this twist, preventing the wire insulation from rubbing against jagged steel casing edges.