Selecting the Right Pump Cable
Guidelines for electrical installations are contained in the National Electric Code (NEC). The code contains general requirements regarding the type of cable to be used when installing pumps, but the installer is given some latitude as to the specific type of cable. The purpose of this article is to help the pump installer decide which type of pump cable will be best for his applications.
There are two classifications of insulated multi-conductor cable used in submersible applications - submersible pump cable and underground feed cable commonly known as UF. Submersible pump cable is used between the wellhead and the pump in submersible applications and is approved by Underwriters Laboratories (UL) for underwater duty. It is designed to safely carry electrical loads inside water wells so long as it is sized properly. UF, on the other hand, is used between the wellhead and the pump control panel and can be buried in the ground, local codes permitting. Some local codes require underground high-voltage wiring, even UF, to be run in conduit.
The three types of pump cable commonly used in the United States today are twisted, flat parallel (also known as flat yellow) and heavy-duty flat-jacketed (see Figures 1, 2 and 3). The last offers extra abrasion protection to help the pump cable stand up to the beating it takes during the installation process and during normal operation from vibration and when the torque-induced motion of the pump starting and stopping causes the pump cable to rub against the well casing. The flat-jacketed cable provides the most mechanical protection as we have said, but it also is the most expensive. Use flat-jacketed for sure with steel well casing, or if you are working in a hard rock hole where abrasion of the pump cable between the drop pipe and the inner wall of the well is likely to occur. On the other hand, if you are using PVC well casing, the twisted or flat parallel would be a reasonable choice because of the lower cost - the two are about the same price - and the fact that the conditions in the well do not lend themselves to excessive abrasion.
Stranding - This is the term used to describe the individual copper strands that make up the conductors that make up the pump cable; the finer the stranding, the more flexible the cable. You can buy size 10 pump cable with as few as 19 strands from some manufacturers and as many as 49 strands from others. If flexibility is important to you - for instance, if you live in a cold climate where the cold temperature makes the cable difficult to work with - choose a product with finer stranding.
Insulation - The dielectric material used to insulate the individual conductors and in the jacket of heavy-duty flat-jacketed pump cable has a lot to do with the flexibility of the finished product as well. Rubber is the most flexible but it also is the most expensive. The compounding of PVC insulation and jacket material can be adjusted to make the final product very pliable, approaching that of rubber.
Sizing tables - The Franklin Electric submersible motor manual contains cable-sizing tables. Since Franklin makes most of the motors we are installing, its tables are as good as any. These tables list the maximum number of feet of each size cable you can run from the service entrance to the motor for each horsepower motor, based on a maximum five-percent voltage drop. If you do not have a copy of the motor manual, call Franklin's hotline at 800-348-2420 and ask for a free copy.
Franklin's manual has tables for both 60-degree C- and 75-degree C-rated pump cables, both of which are available in the market place. The higher temperature rating allows for smaller cable sizes in certain situations. Make sure you use the table appropriate to the cable you are buying.
As with most major components in a water system, you have a lot of options available in your choice of pump cable. With a little research and some forethought, you will be able to choose the right product for your particular situation.
Next month, we will continue the section on submersible pump cable with a look at cable splicing. 'Til then .... ND