Guidelines for electrical installations are contained in the National Electrical Code (NEC). The code contains general requirements regarding the type of cable to use when installing pumps, but it gives the installer some latitude as to the specific type of cable. This article helps pump installers 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, commonly known as UF. Submersible pump cable runs between the wellhead and the pump in submersible applications, and is approved by UL (formerly Underwriters Laboratories) 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. Installers can often bury it in the ground, although some local codes require underground high-voltage wiring, even UF, to 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.
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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. Insulation keeps the conductors from shorting between themselves or to ground.
- In one type of pump cable, a jacket. This rubber or plastic material covers the insulated conductors.
The three types of pump cable in common use in the United States include twisted, flat parallel and heavy-duty flat jacketed. The last one offers extra abrasion protection to help the pump cable stand up to the beating it takes during installation, from vibration during normal operation and when the torque-induced motion of the pump’s starting and stopping causes the pump cable to rub against the well casing.
The flat jacketed cable provides the most mechanical protection, but costs the most. The ruggedness of flat jacketed cable offers better performance for applications involving steel well casing. It also helps for hard rock holes with high potential for abrasion of the pump cable between the drop pipe and the inner wall of the well. On the other hand, less demanding PVC well casing applications can work with with the twisted or flat parallel cable. In conditions that do not lend themselves to excess abrasion, contractors will find these types cheaper than flat jacketed (and the two have about the same price).
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 ought") 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. For residential water well work, most pump cable sits in the #6 to #14 AWG range.
Stranding
“Stranding” describes the individual copper strands that make up the conductors that, in turn, 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 you find flexibility important — for instance, if you live in a cold climate where temperatures can makes cable difficult to work with — choose a product with finer stranding.
Insulation
The dielectric material used to insulate 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 has the most flexibility, but also highest price tag. 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 code. If something happens to a customer that could remotely traced back to your installation, you stand to lose everything. It’s not worth the risk.
Sizing Tables
The Franklin Electric "AIM Manual" features 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% voltage drop. For example, 14-gauge, 75-degree-C pump cable is suitable for use with a 1-horsepower, 230-volt motor up to 250 feet from the service entrance.
On 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 buy. If you do not have a copy of this resource, click here.
Two or More Cable Sizes
Depending on the installation, installers may use any number of cable combinations. 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-horsepower, 230-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-horesepower, 230-volt, single-phase motor setting at 310 feet?
The motor in question can be used with up to 300 feet of #10 AWG cable. In this example, the application has an existing 160 feet of #10 AWG copper wire. The formula for solving this problem:
160 feet (existing) ÷ 300 feet (max. allowed) = 0.533
This means 53.3% 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% 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 does not meet the U.S. NEC recommendations.
We go back to the tables, which show #6 AWG copper wire as good for 750 feet. Using the formula (310 ÷ 750 = 0.413) and combining that result with what we got earlier (0.413 + 0.533) gets us 0.0946, which is less than 1. The #6 AWG copper 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.
