The right cable for a water pump: from motor current to dry and submerged runs

Why isn't horsepower enough by itself?

A motor's mechanical power does not capture all the information about the supply circuit. To choose a cable, the voltage, nameplate current, number of phases, starting method and operating conditions matter. Two pumps with a similar rated power may have very different electrical specifications and route lengths.

In your inquiry, state the exact model of the pump and motor. Also say how many meters the route is from the panel to the equipment and what environments the cable passes through. For a submersible pump, the length of the vertical section and of the ground or overhead parts of the supply must also be included in the review of the whole circuit.

Not treating surface drainage and submersible pumps as the same

A black sheath, the title "pump cable" or even a general mention of water resistance does not settle all submersion conditions. For a fluid other than water, special temperature conditions or a drinking-water application, approval for that same application is needed.

The data needed for sizing from the manufacturer's viewpoint

In its guide to choosing a submersible pump cable, Grundfos raises factors such as the cable length, voltage, the motor's rated current, whether it is single-phase or three-phase, the starting method and the ambient temperature. The choice of cable type also depends on the fluid and the arrangement of the motor cable.

This explanation shows how to gather the information; a table for one specific pump model should not be used for all pumps. The permitted voltage drop and the assumptions of each calculation tool must be consistent with the model and the project's regulations. For a real circuit, the result must be checked with the technical lead.

Reference for this section: Grundfos: choosing a submersible pump cable and the data needed

Check voltage drop and motor protection separately

A long route can reduce the voltage at the motor under load. The starting condition may also differ from steady operation. In a three-phase motor, imbalance or a fault in the supply must be checked alongside the cable itself. Simply enlarging the fuse or removing the protection is not a way to fix low voltage.

If the motor trips repeatedly, the cause may be in the mechanical load, the cooling, the supply or the protection circuit. Choosing the right cable is one part of the system; the motor protection and the pump's operating conditions must also be set according to the manufacturer's documents.

Reference for this section: Grundfos: motor protection and voltage drop

The underwater connection: a point not to take lightly

The motor cable and the extension cable may be joined at one point. Water ingress or an invalid connection can lead to a system failure. In the Grundfos guide as well, a faulty or unapproved connection is presented as a major cause of motor failure.

Ordinary taping or using a connection suited to a dry environment does not replace a kit and method approved for submersion. Making the connection, de-energizing, testing and protecting the route should be left to a qualified person. When ordering, send the motor cable type and the conditions at the connection point along with the pump information.

Reference for this section: Grundfos: the importance of an approved connection in a submersible pump cable

From nameplate power to approximate current

If the current is written on the pump's nameplate, that is the basis to work from. If only the power is written, the following relation can be used for a preliminary estimate for a three-phase motor:

A buying checklist for submersible and drainage pumps

• Write the motor's nameplate current or power and the type of supply.
• Work out the total route length: the well depth plus the horizontal run to the panel.
• For the part inside the water, ask for a cable specified for submersible use.
• Seal the connection or joint with a method and material the manufacturer has approved.
• Check the motor protection and the direction of rotation at start-up.

Frequently asked questions