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 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 ConstructionThe components of submersible pump cable are 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, the 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, 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.
Conductor SizesSubmersible 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 ever will see in water well application is 500 MCM. We are talking heavy cable – more than 5 pounds per foot for 3 conductors. If you are doing residential work, most of the pump cable you use will be in the # 6 to # 14 AWG range.
Stranding“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.
InsulationThe dielectric material used to insulate the individual conductors, and in the jacket of heavy-duty flat-jacketed pump cable, it has a lot to do with the flexibility of the finished product as well. Rubber is the most flexible, but it is also 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 PumpIn 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 TablesThe Franklin Electric Submersible Motor Application Installation & Maintenance (AIM) Manual contains cable-sizing tables. A copy of one of the tables is shown on p. 18. 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.
Notice the five squares that show lengths in bold. These lengths only meet the National Electrical Code 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 the company’s 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.
Pump Cable SizingLet’s explore sizing pump cable when two or more cable sizes are involved. Depending on the installation, any number of combinations of cable may be used. The following is from Franklin Electric’s Submersible Motor Manual.
Figure 4 shows a 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 are 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?
From Table 1, 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 is shown at the lower left.
Using the formula, 160 feet (existing) ÷ 300 feet (max. allowed) is equal to 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 table shows #8 AWG copper wire is good for 470 feet. Using 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 1.0, the voltage drop will not meet the United States NEC recommendations.
Table 1 shows #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 1.0, show that it will meet 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 ....