Part 2. Submersible pump cable.
By Ed Butts, PE, CPI
We kicked off a three-part series last month on defining the basics of electrical cable and conductor selection.
In part two this month, we will continue the discussion with an emphasis on submersible pump cable. We will conclude the topic in the January 2024 issue with defining voltage drop. It is important to note these three parts are intended as companion columns, so some tables and figures referenced in one column will be found in the other two.
Introduction to Submersible Pump Cable
A submersible pump cable is arguably one of the most important elements and a key component for a trouble-free and long-lasting submersible pump installation. The cable must often be able to withstand hundreds of pounds per square inch of applied pressure while sized to effectively convey potentially hundreds of volts and amps continuously to the motor and withstand minor scrapes and cable damage without resulting in excessive leakage to ground.
Although submersible pump cable generally uses copper conductors, it is occasionally available in aluminum along with various insulating materials including PVC, rubber, and hybrids with several construction types such as single conductor and multiple conductors including twisted, flat jacketed, and round jacketed.
The motor lead is defined as the short length of cable that is generally provided with the pump and motor. The drop cable is typically physically connected (spliced) to the motor lead and extends from the motor to the surface, connecting the motor with the power supply.
The proper selection of a drop cable for a specific well pump must include several salient factors including allowable operating voltage, ampacity, operating temperature, voltage drop, environment, and cost.
The cost often becomes one of the most important and contentious factors, particularly in competitive bid situations. Clients should be forewarned that saving a few dollars on the front end by using an undersized wire or inadequate insulation type may come back to haunt them in the end, particularly with large horsepower units.
All the losses experienced in installing a submersible pump will be evident in the installation’s operational and life-cycle costs. Thus it is important to evaluate the scope of these losses combined with the chosen cable size as well as the projected life.
Submersible Drop Cable Selection and Sizing
As drop cable choices for a new installation are generally a one-time investment, it’s important the cable be sized and selected based on the optimal design criteria. If the cable is undersized, the performance of the pump may suffer as the motor may be affected by exposing it to an inadequate operating voltage, reducing its life.
There are six distinct basic criteria I use in selecting submersible pump cable:
- Selecting an insulation class that complies with the underwater environment, maximum rated voltage, and the temperature rating for the needed ampacity and terminals in the electrical system.
- Selecting a wire size and material that complies with the National Electrical Code (NEC) Table 310.16 and Section 430.22 that requires the wire size (branch conductor) for a single motor to possess a minimum ampacity of not less than 125% (×1.25) over the motor’s full load current or operation at no more than 80% of the rated ampacity of the conductor. For our purposes, the term “size” refers to the cross-sectional area or circular mils of the individual power conductors in the drop cable.
- Selecting a wire size that limits the entire voltage drop to less than or equal to 5% of the motor’s rated voltage at the motor’s applicable full load current or amperage (FLA) (NEC Informational Note).
- Selecting a wire size to ensure that no less than 65% of the motor’s rated voltage is available during starting conditions (locked rotor). This is generally provided if the third criteria listed above is observed. However, specific installations and reduced voltage starters may require this additional consideration.
- As the current NEC requires a ground wire to all submersible motors, all cables and design criteria assume the minimum use of four conductor cables: three for power to the motor and a single equipment ground. Certain motors, such as wye-delta, may require six power conductors to the motor.
- Apply any derating or adjustments to the selected conductors in accordance with NEC requirements.
Even though this is not yet required in all states or jurisdictions, many states are now requiring all components in a potable water supply well be NSF-61 approved. Although this typically applies to the pump and motor, it’s not too much of a stretch to see the day that submersible cable may be included. I suggest all water system designers verify if this applies to their locality and respond accordingly.
Generally, the type of insulation used for water well applications must be listed and approved for use in a wet location. This is often indicated by a “W” within the insulation class designation. For example, for low voltage (less than 600 volts) applications, a Type RHW designates a rubber (R) heat-resistant (H) outer covering with a wet (W) location rating.
Other common low voltage insulation classes for submerged applications include thermoplastic types such as THW, THWN, and XHHW. Medium voltage (more than 1000 volts) submersible cables include EPR, XLPE, and polypropylene insulation types.
Submersible pump cable is available in single conductor, twisted (three and four wire), flat jacketed (three and four wire), round (multiple conductors), and other special configurations.
Conversely, an insulation class for offset conditions, where the wire is directly buried, must be specifically approved for this environment, and carry a “U” (for Underground) in their designation, such as Type UL or USE. When the offset wire is contained within a buried conduit subject to moisture, other types of wet rated thermoplastic insulation, such as THW or THWN, are generally acceptable.
In addition to the insulation class, the voltage rating of the cable is critical. Low voltage ratings include 300V and 600V and medium voltage ratings include 1000V, 2000V, and 5000V. In most jurisdictions, approval of the assembled pump cable by an authorized testing lab, such as UL or CSA, is also required.
Consistent sizing of drop cable for submersible pump motors and other electrical applications has been one of my most frustrating and confusing tasks over my career. In the early days of the 1970s, sizing of drop cable was relatively easy as everything was sized on one type of insulation class for the maximum operating temperature, generally 60°C (140°F).
Today, in an effort to squeeze more out of less, electrical insulation classes or ratings for many types of electrical equipment, including most submersible pump cable, is rated for any one or multiples of three single operating temperatures: 60°C (140°F), 75°C (167°F), or 90°C (194°F), a dual temperature rating of 60°/75° or 75°/90°, or even a dual rating of 75° for wet environments and 90° for dry environments.
Most submersible pump cables made today possess a temperature rating of no less than 75°C or a dual rating of 75°C/90°C with many larger and medium voltage cable rated for as high as 90°C service. See Table 1.
Although not necessarily a common issue with inspectors, there are situations where I have known a pump installer to attempt to use a 90°C rated submersible cable in order to use a smaller size of conductor in the well, only to have the installation rejected because they tried to connect the 90°C rated cable to a 60°C or 75°C rated terminal or were exceeding the tabular amperage value for the conductor size and temperature rating (i.e., using a #4 AWG 60°C 70 amp rated conductor with a 75°C conductor load rating of 85 full load amps or 90°C conductor load rating of 95 amps).
Both are violations of the NEC since the higher operating temperature of the 75°C and 90°C submersible cables would impose this same higher temperature on the conductor and lower rated terminal.
To avoid this situation, I generally recommend limiting the design temperature of all submersible pump cable to a 60°C or 75°C maximum service temperature since most of the currently designed and existing industrial electrical equipment available in today’s marketplace is rated for at least a 60°/75°C or 75°C service and many are actually rated for either 75°C/90°C or 90°C service.
However, there is no guarantee of this, particularly when reusing older electrical equipment with a 60°C sole rating. Also, most inspectors I have encountered will require you to apply the lower rated operating temperature of 60°C or 75°C when using a device with a dual rating of either 60°/75°C or 75°/90°C.
In my opinion, using a cable with and applied at a 90°C temperature rating should be reserved for new installations and with electrical equipment, terminals, and connections that are also rated for 90°C service. Always remember that heat is the greatest killer of electrical equipment and devices, and just because you can operate a conductor at 90°C (194°F) doesn’t mean you necessarily should!
Using the lower temperature rating will also provide a few other benefits since the operating temperature of the cable, terminals, and inside of the enclosure will be lower, resulting in less fire potential or component failure from overheating.
One final point regarding the wire insulation is warranted, although this may seem like a trivial concern. I have observed numerous occasions where the wire, especially those with rubber insulation, could not easily be routed through the factory-provided well seal opening. This is particularly true, for example, when a larger size of flat or twisted #8 or #6 submersible pump cable must be run through a ¾-inch opening on a standard 6-inch well seal or a tight opening on a pitless well cap.
This is happening more frequently today as homeowners are asking for more water and wells have become deeper, requiring a higher pump and motor horsepower, even from 6-inch wells. These situations can not only cause the installer grief and added work but can be injurious to the water system since the insulation can easily rub and abrade on the tight opening or threads.
Besides the obvious possible solution of drilling a larger opening in the well seal or cap, using a wire with a thermoplastic insulation or running the wires through the opening as individual conductors to a junction box can often provide just enough of a smaller size to make this type of installation work.
Finally, to comply with criteria No. 6 earlier, one or both of the adjustment factors (derate) may need to be applied (Table 2a and Table 2b).
It is important to note that in many circumstances the use of both Adjustment Factors 1 and 2 may be required for a single application. The designer is cautioned to examine the allowable maximum raceway fill plus the ambient air temperature to ascertain both factors and any necessary corrections.
Insulation Types and Ratings of Submersible Pump Cable
The majority of conductors used for water well pump service are encased inside of a thermoplastic (PVC) insulation or thermoset (rubber) insulation. Drop cable should be UL-listed for wet applications.
The selection of a submersible cable must not only consider the ampacity and allowable temperature of the conductor or cable but the physical size as well, as many heavy-duty twisted and jacketed round cables will not fit into a tight well or one with a liner or large drop pipe. In these situations, the designer or installer must carefully examine the physical dimensions of the proposed drop cable, and if necessary, choose a flat cable or smaller, higher-rated temperature cable with transformed boost voltage at the surface if permitted by the AHJ.
This is never a preferred or attractive option, as the smaller cable will operate at a higher temperature and voltage drop, but in some cases, if the conductor complies with NEC amp values, it is the only viable alternative.
Another option I have used with single-phase submersible motors, especially 3 HP and above, is derating the size of the starting or red conductor to two AWG sizes smaller. As the starting phase of a motor is an instantaneous and short-term inrush event, downsizing of the red lead is a permissible way to reduce the overall bundle size.
This means you can use a #12 conductor for the red lead where two #8 wires are required for the run windings (black and yellow). In these cases, I have often used two runs of the now obsolete two-wire submersible cable, such as #8/2 for the black and yellow conductors and #12/2 for the ground and red lead or single conductor wires for all three conductors plus the ground.
Although this is more hassle, time consuming, and the cables must be routed together, I have used this method several times to make an installation happen when there was no other simple solution.
Obviously, you should verify this is permitted by your local AHJ before commencing. Common types of 600-volt rated submersible pump cables on the market are flat cable (Figure 1a), round cable (Figure 1b), twisted cable (Figure 1c), heavy-duty or armored cable (Figure 1d), and single conductor (Figure 1e).
Flat Submersible Pump Cable: This has a PVC or rubber outer insulation that provides additional cable protection from abrasion and mechanical impact. When it comes to most submersible pumps, the durability and extra conductor protection provided by flat cable is advantageous.
In deep wells, there is a good chance that the cable will rub against the surface, liner pipe, or weld, causing it to abrade, so the jacket often provides the added much-needed protection. A flat submersible pump cable is more expensive than a twisted copper cable but is comparable in cost to round and heavy-duty cables.
Round Submersible Pump Cable: This is a cable with three to six power conductors and a ground wire. Compared to a typical flat cable, it is larger in diameter and not as durable because the conductors in the cable are laid in a circular pattern and not parallel. However, this cable is more durable than an unjacketed twisted cable and is often preferred for a submersible pump in a large well casing that requires a three to six power conductor construction such as a wye-delta motor or motor-sensing equipment.
Twisted Submersible Pump Cable: Commonly available in sizes up to #2 in four-wire construction, it is primarily suitable for typical light and medium duty submersible pumps. This type of cable is recommended when the well is shallow enough so that access to the wire will not be too expensive in case it must be repaired or replaced.
Twisted copper cable is less expensive than other cables in this range because it does not possess extra protection afforded by an outer jacket. To clear out older stock of three-conductor submersible cable, many installers may use three-conductor cable with a separate ground wire. A twisted copper cable is significantly more affordable compared to other different popular types on the market.
Heavy-Duty Flat or Armored Submersible Pump Cable: This is a variety of flat cable styles with an extra rating for heavy-duty use. The cable is double jacketed and often armored for extra protection. It is usually reserved for submersible pump installations with extreme conditions because a regular flat cable performs just as well in most heavy-duty applications. Armored or heavy-duty rated construction is another option for crooked wells or those subject to abrasion.
Typical technical data for 600-volt rated, four-wire, submersible pump cable is shown in Table 3. Other types of cables are sometimes referred to under the umbrella term “submersible pump cable” because they are used for related purposes.
Medium Voltage Submersible Cable: Medium voltage rated cables (MV-90 and MV-105) are available in copper and aluminum conductors in several wire gauges and configurations and used for submersible pump motors that operate between 1000 up to 15,000 volts. The XLPE insulated and sheathed three-core submersible cables are ideal for usage for submersible pump motors in deep wells. Examples of a typical cross-section construction and multiple conductors are illustrated in Figures 2a, 2b, and 2c.
Securing and Supporting Drop Cable
Because of gravity, vibration from the pipe string, and the weight of the cable itself, submersible pump cable—even when firmly attached to the drop pipe—can sag or break the support tape over time. It is therefore critical that adequate support and strength in the cable support method be employed.
Band-it and screw clamps and twisted solid wire should be used with caution as extreme clamping pressure can press into the wire insulation, resulting in premature insulation failure.
When the cable is supported with tape by the drop pipe, a clean and dry surface of at least 6 inches in length on the pipe should be used. Using ¾-inch-wide electricians’ tape should be reserved for #10 and smaller cable while 2-inch-wide 10 mil plumbers’ tape should be used to support larger cable.
For best results, placing the tape wrap above the pipe coupling following joint assembly will allow using the coupling as a protective backstop should the tape slide down the pipe. An initial two wraps of tape around the pipe followed by a second wrap around the cable and then affixing the assembly to the pipe with two additional half-lapped wraps is suggested. This will help to prevent the cable weight pulling downward through the tape.
Drop cable should be affixed to the drop pipe at spacings no greater than 10 feet or at roughly twice per joint. While they are designed for submersible pumps, many of the available cables are also perfect for various industries when a waterproof, durable cable is needed. Most notably, submersible pump wires are used in irrigation systems and motors, food processing plants, food preservation systems, and mining facilities.
This concludes this month’s—and this year’s—edition of Engineering Your Business. Next month, we will conclude this series dedicated to electrical oriented themes and this three-part miniseries on electrical wire types and sizing with an overview on determining and calculating voltage drop.
I wish all of you a safe and happy holiday season. Until next month (and next year), work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.