Selecting and Sizing Submersible Pump Cable
Find Out Which Type, Size of Pump Cable to Use
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 pump 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 or disconnect, and can be buried in the ground, although some local codes require underground high-voltage wiring, even UF, to be run in conduit. Submersible pump cable can be used between the wellhead and the control panel or disconnect so long as it is run in conduit.
Submersible Pump Cable
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 to keep the conductors from shorting between themselves or to ground.
- In one type of pump cable, a jacket, 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. The latter offers extra abrasion protection to help the pump cable stand up to the beating it takes during the installation process, during normal operation from vibration, and when the torque-induced motion of the pump’s 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 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.
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 water well applications is 500 MCM. We are talking heavy cable — more than five pounds per foot for three conductors. If you are doing residential work, most of the pump cable you use will be in the #6 to #14 AWG range.
“Stranding” 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.
The dielectric material that is 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 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 lose everything. It’s not worth the risk.
The Franklin Electric Submersible Motor Application Installation and Maintenance (AIM) Manual contains cable-sizing tables. 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. For example, 14-gauge, 75-degree-C pump cable is suitable for use with a 1-HP, 230-volt motor that is up to 250 feet from the service entrance.
One those tables, there are five squares that show lengths in bold. These lengths only meet the NEC ampacity requirements for individual conductors (not jacketed) in free air or water, not in conduit. The lengths not in bold meet the requirements for individual conductors or jacketed cable in free air, in water and in conduit. Flat jacketed and flat parallel both are considered jacketed.
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 its motor manual, call Franklin’s hotline at 800-348-2420 and ask for a free copy.
Two or More Cable Sizes
Depending on the installation, any number of combinations of cable may be used. For example, consider a typical replacement/upgrade installation. The well already has 160 feet of buried #10 cables between the service entrance and the wellhead. A new 3-HP, 23-volt, single-phase motor is being installed to replace a smaller motor. The question: Since there already is 160 feet of #10 AWG installed, what size cable is required in the well with a 3-HP, 230-volt, single-phase motor setting at 310 feet?
In this case, a 3-HP, 230-volt motor can be used with up to 300 feet of #10 AWG cable. In this example, the application has 160 feet of #10 AWG copper wire already existing. The formula for solving this problem:
Using the formula, 160 feet (existing) ÷ 300 feet (max. allowed) = 0.533. This means 53.3 percent of the allowable voltage drop or loss, which is allowed between the service entrance and the motor, occurs in this wire. This leaves 46.7 percent of some other wire size to use in the remaining 310 feet downhole wire run.
The tables in the motor manual show #8 AWG copper wire is good for 470 feet. Use the formula again: 310 feet (used) ÷ 470 feet (allowed) = 0.660. Adding this to the 0.533 determined earlier equals 1.193. Since this combination is greater than 1, the voltage drop will not meet the U.S. NEC recommendations.
The tables show #6 AWG copper wire is good for 750 feet. Using the formula, 310 ÷ 750 = 0.413, and using these numbers, 0.533 + 0.413 = 0.0946, which is less than 1, demonstrates that the wire will met the NEC-recommended voltage drop.
This works for two, three and more combinations of wire, and it does not matter which size wire comes first in the installation.
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 this series on submersible pump system accessories and components with a look at submersible pump cable splicing. ’Til then...