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 and size of pump cable to use.
There are two classifications of insulated multi-conductor cable used in submersible applications - submersible pump cable and underground feed cable, known commonly 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.
Submersible Pump Cable Makeup - The components of submersible pump cable: the conductors, which are the copper wires that conduct the electricity; the insulation, which is the plastic or rubber material covering the copper conductors that keep the conductors from shorting between themselves or to ground; and, in one type of pump cable, a jacket, which is a rubber or plastic material covering the insulated conductors.
The three types of pump cable commonly used in the United States today are twisted, flat-parallel, and heavy-duty flat-jacketed (see Figures 1, 2 and 3). The latter 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.
Although flat-jacketed cable provides the most mechanical protection, it 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, mainly 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.
Conductor Sizes - Submersible pump cable comes in a multitude of sizes. The smaller cable is identified in AWG sizes from #14 on the smallest end to #4/0 (pronounced four OTT) on the largest end of the scale. Beyond the AWG sizes are the MCM sizes, which start at 250 and go up to 2,000. About the largest pump cable you will ever see in a water well application is 500 MCM. We are talking heavy cable - more than 5 pounds per foot for three conductors. If you are doing residential work, most of the pump cable you use will be in the #6 AWG to #14 AWG range.
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, consider choosing 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 also has a lot to do with the flexibility of the finished product. 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 flexible, approaching that of rubber.
Grounding the Pump - In 1989, the NEC started requiring all submersible pump motors to be grounded to the service entrance. This means running a green ground wire from the motor to the service entrance. You no longer can use steel drop pipe as a grounding link, according to code. For your own sake, don't even think of installing a submersible pump without grounding it according to the code. If something were to happen to one of your customers that could remotely be traced to your doing something not up to code, you would stand to loose everything. It's not worth the risk.
Sizing Tables - The Franklin Electric Submersible Motor Application Installation & Maintenance (AIM) Manual contains cable-sizing tables. A copy of one of the tables is shown above. 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 5-percent voltage drop. From Table 1, 14-gauge, 75-degree C pump cable is suitable for use with a 1-HP 230-volt motor, which is up to 250 feet from the service entrance.
Franklin's manual has tables for both 60-degree C and 75-degree C-rated pump cable, both of which are available in the marketplace. 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. If you do not have a copy of this motor manual, you can view or order it online at www.franklin-electric.com/Manual/contents.html or call Franklin's hotline at 800-348-2420 and ask for a free copy.
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 series on downhole components with a look submersible pump cable splicing. 'Til then….