How to Size a Power Cable for a Deep Well Submersible Pump

A surface motor is normally located close to its control panel and remains accessible for inspection. A borehole pump motor may operate tens or hundreds of metres below ground.

The electrical route can include:

• Transformer-to-panel cable

• Panel-to-wellhead cable

• Cable down the borehole

• Factory motor leads

• Underwater splice

• Terminal and connection resistance

Every section contributes to the total voltage drop.

The submerged cable must also operate in a restrictive environment. It may be exposed to:

• Continuous immersion

• Hydrostatic pressure

• Water temperature

• Water chemistry

• Abrasion against the casing

• Cable-clamp pressure

• Pump vibration

• Mechanical loads during installation

• Long-term insulation ageing

A cable that is suitable for a dry industrial building is not automatically suitable for a submerged borehole installation.

Begin with the exact motor rather than the pump series name.

Record:

• Rated motor power

• Rated voltage

• Frequency

• Single-phase or three-phase supply

• Rated current

• Full-load or maximum operating current

• Power factor

• Efficiency

• Starting method

• Number of motor leads

• Insulation class

• Permitted voltage tolerance

• Manufacturer’s maximum cable-length data

SLAPK submersible pumps can be configured for different voltages and frequencies, including project-specific 50 Hz and 60 Hz requirements. Available configurations depend on the selected motor and pump model.

Do not assume that two motors with the same kilowatt rating have the same current. Rated current changes with voltage, phase, efficiency, power factor and motor design.

Use the confirmed motor data sheet for the exact order.

Do not use well depth alone as cable length.

The complete electrical route may include:

• Distance from transformer to control panel

• Distance from control panel to wellhead

• Additional cable inside the panel

• Depth from wellhead to the motor connection

• Service loop at the wellhead

• Cable required for termination

• Routing around structures

• Planned maintenance allowance

If the well is 120 m deep but the control panel is 45 m from the wellhead, the electrical route is already substantially longer than the downhole section alone.

Use the actual one-way route length when consulting the applicable motor cable table or performing the voltage-drop calculation. Apply the correct formula for the phase arrangement; do not manually double or multiply lengths unless the selected calculation method requires it.

The factory motor lead must also be included if the manufacturer’s sizing method does not already account for it.

The cable’s current-carrying requirement is normally based on the motor’s confirmed current and the applicable electrical rules.

Do not calculate cable size from kilowatts alone when a rated current is available.

The design review should consider:

• Normal operating current

• Motor service factor, if applicable

• Starting current

• Starting duration

• Frequency of starts

• Ambient temperature

• Cable grouping

• Installation method

• Harmonic content from a VFD

• Required protective-device coordination

Starting current does not usually mean that the cable must carry locked-rotor current continuously. However, the cable must maintain enough voltage during starting for the motor to develop adequate torque.

A long, undersized cable can create a large voltage drop during startup even when its continuous ampacity appears acceptable.

Ampacity is the continuous current a cable can carry under specified conditions without exceeding its allowable conductor or insulation temperature.

The published ampacity depends on:

• Conductor material

• Conductor cross-sectional area

• Insulation material

• Number of loaded conductors

• Cable construction

• Ambient temperature

• Installation in air, conduit, ground or water

• Grouping with other cables

• Local electrical regulations

A cable that passes the voltage-drop calculation must still satisfy ampacity requirements.

Conversely, a cable that can safely carry the current may still be too small because of excessive voltage drop over a long distance. Both checks are necessary.

Voltage drop occurs because every conductor has electrical resistance and impedance.

For a simplified resistive calculation, voltage drop increases with:

• Cable length

• Motor current

• Conductor resistivity

It decreases as conductor cross-sectional area increases.

For a copper conductor, a simplified relationship is:

Voltage drop is proportional to current * cable length ÷ conductor area

A complete AC calculation may also account for:

• Power factor

• Conductor reactance

• Cable configuration

• Operating temperature

• Three-phase or single-phase supply

For a balanced three-phase motor, use a three-phase voltage-drop formula or an approved manufacturer cable table. For a single-phase motor, use the correct single-phase method.

Do not mix the two calculations.

The calculated voltage at the motor terminals should remain within the motor manufacturer’s permitted range under both running and starting conditions.

The same voltage loss represents different percentages at different system voltages.

For example, a fixed voltage loss is more significant in a lower-voltage system than in a higher-voltage system.

Excessive voltage drop can cause:

• Reduced starting torque

• Extended acceleration time

• Higher motor current

• Contactor chatter

• Repeated overload trips

• Motor overheating

• Unstable VFD operation

• Failure to start under hydraulic load

• Reduced service life

SLAPK troubleshooting information identifies low voltage, an excessively thin wire and an overly long cable as possible reasons a submersible motor may fail to start.

Cable selection must therefore be coordinated with the actual voltage available at the site, not only the nominal system voltage.

A motor draws substantially more current during direct-on-line starting than during normal operation.

Because voltage drop is related to current, the temporary drop during startup can be much larger than the running drop.

The starting review should consider:

• Available transformer capacity

• Generator capacity

• Supply-system impedance

• Cable length and conductor size

• Motor locked-rotor current

• Pump starting torque

• Wellhead and panel connections

• Starting method

• Acceleration time

A motor may operate normally after reaching speed but still fail to start because the terminal voltage collapses during acceleration.

Possible starting methods include:

• Direct-on-line starting

• Star-delta starting

• Autotransformer starting

• Soft starter

• Variable-frequency drive

The selected method must be compatible with the motor leads, voltage, control panel and project requirements.

Do not assume that installing a soft starter or VFD automatically permits a smaller cable. The complete manufacturer instructions and electrical design still apply.

Copper is commonly used for submersible pump cables because of its conductivity, connection reliability and availability.

If aluminium conductors are considered for a surface section, the designer must account for:

• Larger required conductor area

• Termination compatibility

• Oxidation control

• Mechanical strength

• Thermal expansion

• Connector ratings

• Transition to copper motor leads

Do not make an underwater copper-to-aluminium connection unless the complete splice system is specifically designed and approved for that purpose.

The conductor material used in the voltage-drop calculation must match the actual cable.

The downhole portion should be a cable specifically rated for continuous immersion and the expected water conditions.

Selection factors include:

• Fresh water or saline water

• Maximum water temperature

• Hydrostatic pressure

• Chemical exposure

• Oil or hydrocarbon exposure

• Flat or round construction

• Cable outside diameter

• Flexibility

• Abrasion resistance

• Voltage rating

• Insulation and jacket material

• Drinking-water approval, where required

A standard flexible cable may appear physically similar but may not be designed for continuous underwater service.

Confirm both the insulation and the outer jacket. A waterproof jacket does not compensate for conductor insulation that is unsuitable for the voltage, temperature or immersion period.

Submersible pump cables are available in flat and round constructions.

Flat cable may be useful where the clearance between the pump, rising main and casing is limited. It can sit more closely against the pipe or cable guard.

Round cable may be preferred for certain glands, seals, cable clamps and mechanical protection systems.

The correct choice depends on:

• Motor connection

• Well casing clearance

• Cable gland design

• Cable-clamping method

• Splice kit

• Installation equipment

• Manufacturer approval

Cable shape does not determine conductor capacity. Confirm the conductor area, insulation, voltage rating and dimensional drawing.

Cable selection also affects whether the pump assembly can pass through the borehole.

Check the combined envelope of:

• Pump and motor

• Rising main

• Cable

• Cable guard

• Cable clamps

• Underwater splice

• Check valves

• Pipe couplings

• Centralizers

The splice may be thicker than the cable itself and can become the controlling installation dimension.

Verify the minimum casing internal diameter along the complete installation path. Allow practical clearance for casing deviation, joints, deposits and retrieval.

For related dimensional guidance, see 6-Inch vs 8-Inch Borehole Pumps: How to Choose.

A deep-well pump commonly requires a connection between the factory motor leads and the extended drop cable.

This connection must provide:

• Reliable electrical continuity

• Low connection resistance

• Mechanical strength

• Electrical insulation

• A permanent water barrier

• Compatibility with the cable insulation

• A smooth outside profile

• Resistance to installation stress

The SLAPK cable-connection instructions show preparation of the conductor ends, mechanical joining, insulation wrapping and multiple sealing layers. The approved method and materials should be followed for the actual cable supplied.

A splice that is electrically insulated but not water-tight can fail after immersion.

Water entering a cable or joint can lead to:

• Low insulation resistance

• Ground faults

• Phase-to-phase faults

• Corrosion of conductors

• Protection trips

• Motor winding damage

Use a manufacturer-approved submersible splice kit or documented connection procedure.

Cutting or Damaging Conductor Strands

Removing insulation carelessly can reduce the effective conductor area and weaken the connection.

Joining Different Conductor Sizes Improperly

The connector and sealing system must accommodate both conductors and remain mechanically secure.

Leaving Sharp Projections

Sharp conductor ends or connector edges can penetrate the insulation during wrapping or installation.

Using Ordinary Electrical Tape Alone

A few layers of general-purpose tape do not necessarily form a reliable deep-water seal.

Trapping Moisture Inside the Joint

Cable ends and connection materials must be clean and dry before sealing.

Making the Splice Too Large

An oversized joint can catch on casing joints or become damaged during lowering.

Placing the Joint Under Repeated Mechanical Stress

The splice should not carry the suspended cable load or remain sharply bent against a coupling.

The power cable should be supported along the rising main so that it does not hang freely, rub against the casing or move excessively during operation.

Cable supports should:

• Hold the cable without crushing it

• Resist the water and installation environment

• Avoid sharp edges

• Accommodate thermal and mechanical movement

• Remain secure during pump starting

• Permit safe removal of the assembly

Cable guards should be used where required around the pump and coupling sections.

Do not use metallic fastening materials that can cut into the cable jacket or create an unsuitable galvanic condition.

The fastening interval and material should follow the pump, cable and installer requirements.

Cable selection cannot be separated from the motor protection system.

The control panel may include:

• Short-circuit protection

• Motor overload protection

• Phase-loss protection

• Phase-sequence protection

• Under-voltage and over-voltage protection

• Ground-fault protection

• Dry-running protection

• Temperature monitoring

• Level control

• Soft starter

• Variable-frequency drive

Protection settings should be based on the confirmed motor and manufacturer instructions.

An overload relay should not be set higher merely to prevent nuisance trips caused by low voltage or a damaged cable. Find and correct the electrical or hydraulic cause.

A VFD changes the electrical waveform supplied to the motor.

Long motor cables can increase reflected-wave effects, voltage stress, leakage current and electromagnetic interference. The allowable cable length may depend on:

• Drive model

• Carrier frequency

• Output voltage

• Motor insulation

• Cable construction

• Grounding

• Output reactor

• dV/dt filter

• Sine-wave filter

Follow the VFD and motor manufacturer’s maximum cable-length instructions.

The cable must also be suitable for the drive output and installed with the required grounding and shielding arrangement.

Do not apply a standard direct-on-line cable table to a VFD system without checking the drive documentation.

The pump installation must include the grounding or protective conductor required by the applicable electrical code and equipment design.

Grounding helps protective devices respond to insulation failure and reduces shock risk.

Do not use the rising main, safety cable or well casing as an unverified substitute for the required protective conductor.

The grounding arrangement should include:

• Motor grounding connection

• Control-panel grounding

• Cable protective conductor

• Bonding of required metallic components

• Correct termination

• Continuity testing

Local electrical regulations and site conditions determine the final design.

Before lowering the pump, inspect and test the cable and motor assembly.

Recommended checks may include:

• Visual inspection of the cable jacket

• Verification of conductor continuity

• Phase-to-phase resistance comparison

• Insulation-resistance testing

• Ground-conductor continuity

• Splice inspection

• Motor winding-resistance comparison

• Confirmation of phase identification

• Verification of cable length

Use the test voltage and procedure specified by the motor and cable manufacturer.

Disconnect sensitive electronic equipment, including a VFD, before performing insulation-resistance tests when required by its instructions.

Record the results before installation. These baseline values help diagnose future problems.

Repeat the required electrical checks after lowering the pump but before normal operation.

This can reveal damage caused by:

• Pulling the cable

• Contact with casing edges

• Crushed insulation

• A damaged splice

• Incorrect termination

• Water entering the connection

During commissioning, record:

• Supply voltage

• Motor terminal or panel voltage

• Phase-to-phase voltage balance

• Running current on each phase

• Starting behavior

• Insulation resistance

• Protection settings

• Pump flow and discharge pressure

Current imbalance may indicate voltage imbalance, a connection problem, cable damage or a motor fault.

Assume a three-phase borehole pump will be installed 150 m below the wellhead, while the control panel is 35 m from the well.

The motor nameplate and supplier data provide the rated voltage, current, starting method and permitted cable length.

The engineer should:

1. Add the downhole cable, surface route, service allowance and factory motor leads as required by the selected calculation method.

2. Identify the rated and starting current.

3. Select a submersible-rated copper cable with suitable insulation.

4. Check its ampacity under the actual installation conditions.

5. Calculate running voltage drop.

6. Check the available motor voltage during startup.

7. Compare the result with the manufacturer’s permitted range.

8. Increase the conductor size if either voltage-drop or ampacity requirements are not met.

9. Check the cable and splice outside dimensions against the casing clearance.

10. Confirm the cable with the control-panel, starter or VFD supplier.

11. Document the final conductor area, length, splice method and protection settings.

The final cable cannot be selected from installation depth alone.

• Selecting conductor size from motor kilowatts alone

• Ignoring the surface cable between the panel and wellhead

• Using nominal well depth as total cable length

• Checking ampacity but not voltage drop

• Checking running voltage but not starting conditions

• Using single-phase calculations for a three-phase motor

• Using ordinary cable for continuous immersion

• Ignoring water temperature and chemistry

• Installing an unapproved underwater splice

• Allowing the cable to rub against the casing

• Failing to include the splice in the clearance check

• Ignoring VFD maximum motor-cable length

• Increasing overload settings instead of correcting low voltage

• Failing to test insulation before and after installation

Send the following information to the motor, cable and control-panel supplier:

• Complete pump and motor model

• Rated motor power

• Rated voltage and frequency

• Single-phase or three-phase supply

• Rated current

• Starting method

• Transformer or generator capacity

• Distance from supply to control panel

• Distance from panel to wellhead

• Pump installation depth

• Factory motor-lead length and size

• Proposed cable material and cross-sectional area

• Flat or round cable requirement

• Water temperature and chemistry

• Minimum casing internal diameter

• VFD model and operating frequency range

• Required local electrical standard

• Grounding arrangement

• Expected starts per hour

• Continuous or intermittent operating duty

What size cable do I need for a deep-well pump?

The answer depends on motor current, voltage, phase, complete cable length, voltage-drop limit, starting method, installation conditions and local regulations. Motor power and well depth alone are not sufficient.

Can I use a larger cable than the manufacturer’s minimum size?

Often yes, provided it is compatible with the terminals, splice, control equipment and well clearance. A larger conductor reduces voltage drop but increases cost, weight and outside diameter.

Does the cable length include the distance to the control panel?

Yes. All electrical sections that contribute to voltage drop must be included according to the chosen calculation method.

Can I use a normal flexible cable underwater?

Only if it is specifically rated for continuous immersion, the required voltage, temperature, pressure and water conditions. General flexible cable should not be assumed suitable.

Does a VFD allow me to use a smaller cable?

Not automatically. The cable must still meet ampacity, voltage-drop, insulation, grounding and drive-manufacturer requirements. Long VFD output cables may require additional filters or reactors.

Why does my pump fail to start even though the supply voltage looks correct?

The voltage may fall substantially during starting because of cable resistance, weak supply capacity or high starting current. Measure and evaluate the voltage under starting conditions.

Should the underwater splice be above the water level?

This is often impractical in a deep-well installation. When the splice is submerged, it must use an approved permanent underwater sealing method.

How can I confirm the cable was not damaged during installation?

Perform the specified continuity, winding-resistance and insulation-resistance tests before and after lowering the pump. Compare the results with the recorded baseline.

A deep-well pump cable must be selected through both an electrical and an installation review.

The final selection should:

• Carry the required current safely.

• Limit running and starting voltage drop.

• Use the complete electrical route length.

• Match the voltage, phase and starting method.

• Withstand continuous submersion.

• Use a reliable underwater splice.

• Fit inside the casing with the complete assembly.

• Coordinate with the control panel and protection system.

• Meet applicable electrical regulations.

Do not order a cable from motor power or well depth alone. Confirm the exact motor current, total cable length, voltage-drop calculation, conductor area, insulation system and installation environment before production.

Send SLAPK your required flow and head, well depth, pump setting depth, motor power, voltage, frequency, phase, total cable route, starting method, water temperature and casing internal diameter.

Our engineers can recommend a suitable QJ or SP borehole pump and provide the motor data, rated current, factory lead information and cable-length guidance required for your project.