How to optimise cable sizing in submersible pumps

Sizing and selecting the right pump and motor is crucial in any submersible pump installation. But it’s also important to size your cables correctly.

In this module, we will show you how to optimise your cable-sizing process. You will also learn about the Grundfos cable-sizing tool and how it can benefit you in your work. But first, we’re going to briefly take you through the different cables.

First of all, the voltage provided by the cables are based on local legislations, so it’s important to always ensure that the voltage of your cables is consistent with these legislations.

At Grundfos, there is a distinction between motor cables and drop cables.

The motor cable is the cable that is delivered with the pump. Typically, a submersible motor consists of a motor and a motor cable. A general rule of thumb is that motor cables must always be submerged in liquid – exceptions occur, however, and we’ll get back to that shortly.

The drop cable extends from the motor cable, connecting the pump with the power supply. There are a number of different drop cable types, materials and layouts depending on the application in which the pump is operating.

But how do you decide which type of drop cable type to use? Well, there are four key factors.

First of all, standard motor cables are suited for raw drinking water, and making them suitable for other liquids is something that needs to be evaluated from case to case. This demonstrates the first of the four aforementioned factors: selecting your drop cable type based on the pumped media.

Voltage losses also have a significant influence. During the cable-sizing process, you may find that the amperage of your motor cable is at such a low level that it can be used in the air, allowing you to actually use your motor cable as a drop cable in the air, despite the fact that it is intended for submerged operation.

Next, there is the financial factor. All the losses you experience in your submersible pump installation will also be evident in your installation’s operation costs. Therefore, it is important to evaluate the size of these losses combined with the chosen cable size. That way, you can reduce life cycle costs, and, at the same time, improve the conditions of your submersible motor.

Finally, there’s cable configuration – or, the cable layout. The motor cable has an impact on the drop cable type you select, so if your motor cable is jacketed, for instance, you need to ensure that the drop cable matches it. Jacketed cables offer the most cost-efficient solution.

As drop cable choices are a one-time investment, it’s important that you size and select it based on the optimal criteria. If not, you risk wasting money every day the pump is in operation.

The easiest way to size and select your cables is by using the Grundfos cable-sizing tool.

You can find the cable-sizing tool on Grundfos Product Center. Once here, simply click “Tools”, select “Cable Calculator”, and enter the values of your installation. We’ll take a closer look at these values a bit later.

By “cable size”, we refer to the cable cross-section – of the individual wires in the drop cable. The submersible drop cable is a key component for a trouble-free submersible pump installation. If the cable is undersized, the performance of the pump may be affected, which might expose the installation to a reduced supply voltage.