GGSTech ToolsHow the borewell cable calculation works
The calculator first estimates motor input current from kW, voltage, power factor and efficiency. It then estimates the one-way cable route from the panel to the wellhead and down to the installed motor. A routing allowance is added for bends, termination, slack and practical installation.
Each standard cable size is checked against a conservative preliminary current limit and a simplified resistive running-voltage-drop calculation. The smallest size satisfying both checks is displayed.
Do not use total bore depth blindly
A 200 m borewell does not necessarily mean a 200 m motor cable. If the pump is installed at 130 m, the vertical cable requirement is approximately that motor setting depth, plus the horizontal route and installation allowance. Conversely, unusual routing can make the actual cable longer than the simple vertical depth.
Why depth changes cable selection
Electrical current is mainly set by motor load, voltage, efficiency and power factor. Increasing depth does not directly appear in the motor-current formula, but a deeper motor usually means a longer cable. Longer cable increases resistance and therefore voltage drop and losses. That is why a cable that is adequate by ampacity may still be too small for a deep borewell.
Running voltage drop
For preliminary screening, this calculator uses conductor resistivity. Actual AC voltage drop also depends on cable reactance, conductor temperature, power factor and cable construction.
For final design, use the manufacturer's resistance/reactance data at the relevant operating temperature and the applicable project or statutory voltage-drop limits.
Starting voltage drop can be more severe
A motor may draw several times full-load current during starting. With a long borewell cable, this temporary current can produce a large terminal-voltage dip. The result can be slow acceleration, contactor chatter, relay operation or failure to start. DOL, star-delta, auto-transformer starter and VFD have different starting behavior.
Typical borewell panel functional chain
Protection checks for a borewell motor
Protection must match the motor and system, not just the cable size. Typical functions may include short-circuit protection, thermal overload, phase failure/reversal, voltage monitoring, earth-fault protection and dry-run or level protection. Settings must be coordinated with motor nameplate current, starting time and manufacturer recommendations.
Submersible cable and joints
Use cable and jointing systems intended for continuous wet/submersible service when the installation requires it. Cable insulation, sheath, conductor material, mechanical support, bend radius, water sealing and joint workmanship are critical. A mathematically correct conductor area does not compensate for an unsuitable cable construction or poor underwater joint.
Earthing and safety
The motor body, panel enclosure and exposed conductive parts must be earthed according to applicable standards and manufacturer instructions. Do not use a calculator result as a substitute for protective-conductor sizing, earth-fault-loop verification or insulation testing.
Worked example
Consider a 15 kW, 415 V, 3-phase borewell motor installed 120 m below ground, with the panel 15 m from the wellhead and 10% routing allowance. The cable route is about 148.5 m one way. The calculator estimates full-load current from the selected PF and efficiency, then checks standard cable sizes for both preliminary ampacity and the selected maximum running voltage drop.
Field troubleshooting: pump does not start
Check all phase voltages, phase sequence where relevant, and panel control supply.
Identify overload, phase-failure, dry-run, level or VFD fault indications before resetting.
With the circuit isolated and proven dead, follow approved procedures for continuity/insulation tests and manufacturer limits.
Excessive cable drop or weak supply can prevent acceleration even when no-load voltage looks normal.
Frequently asked questions
Should I enter borewell depth or pump depth?
Enter the actual motor/pump setting depth for cable routing. Add the panel-to-well surface route separately.
Can I select cable only from current?
No. Check ampacity, voltage drop, starting performance, derating, short-circuit withstand, installation method and manufacturer data.
Does a VFD always allow a smaller cable?
No. VFD starting can reduce inrush, but cable ampacity and VFD output-cable requirements still apply. Long motor leads may require dv/dt or sine-filter consideration depending on the VFD and motor system.
Is this calculator a final design?
No. It is a learning and preliminary selection tool. Final engineering requires actual motor, cable, supply, installation and protection data.