Part 14(d)—Submersible Pump Design, Part 4
By Ed Butts, PE, CPI
In the last installment of this series we began a discussion on submersible pump motors and their selection and starting methods. We began with a series of steps intended to provide the designer with a methodical approach to design and application of a submersible motor.
We introduced the initial steps 1 and 2. In this final installment, we’ll conclude by starting with step 3: Motor’s NEC and full load currents, along with an expanded discussion on drop cable sizing and selection, motor cooling and lubrication, efficiency and power factor, and post installation procedures and testing.
Remember in many cases a single motor may be a part of an electrical installation with many diverse and separate loads. This installment will be limited to considerations for a single submersible motor. A future column will outline the entire electrical installation and design.
Step 3: Determine motor’s full load and NEC design amperage
This follows the first two steps discussed in Part 3 and is used to determine the NEC amperage and the actual motor amperage for the selected horsepower. Typically, by code, the NEC current values from NEC Table 430.248 (Single Phase Motors) and 430.250 (Three Phase Motors) are used to design the service and conductor loads and sizes, while the actual motor amperage is used for sizing the short circuit and overload protection.
Much of the tabular values in this column are reprinted from the 2017 edition of the National Electrical Code
(NFPA 70). Caution: The values in Table 1 represent “typical” and in many cases approximate current values for various motor manufacturers and horsepower (HP). All submersible motors, particularly those above 100 HP, should be verified for the precise full-load current as the motor current between manufacturers, motor diameter, and HP can and does vary. Whenever available, always use the NEC and manufacturer’s actual current values:
Step 4: Drop cable and offset wire sizing and selection
There are three distinct criteria I use in selecting submersible pump cable:
- Selecting an insulation class complying with the underwater (wet) environment, maximum rated voltage, and the temperature rating for the needed ampacity and for the terminals in the electrical system.
- Selecting a wire size and material complying with the National Electrical Code (NEC) Table 310.16 and Section 430.22 requiring the wire size (branch conductor) for a single motor to have a minimum ampacity of not less than 125% (× 1.25) of the motor’s full load current.
- Selecting a wire size limiting the entire voltage drop sum to less than or equal to 5% of the motor’s rated voltage at the motor’s applicable full load current (NEC Informational Note).
The type of insulation that must be used for water well applications generally must be 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-volt) applications, a Type RHW designates a rubber (R) outer covering with a wet (W) resistant rating. “T” is used to designate an insulation with a thermoplastic covering.
Other common low-voltage insulation classes for submerged applications include types THW, THWN, XHHW, EPR, XLPE, and PVC. Medium voltage (600-5000V) submersible cables include EPR, XLPE, and polypropylene insulation types.
Submersible pump cable is available in single conductor, twisted (3- and 4-wire), flat jacketed (3- and 4-wire), round (multiple conductors), and special configurations. Conversely, an insulation class for offset conditions where the wire is directly buried must be approved for this service and carries a “U” for underground in their designation (Type UL or USE). When the offset wire is contained within a conduit, then other types of thermoplastic insulation, such as THW, THWN, or THHN, 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.
Over my career, consistent sizing of drop cable for submersible pump motors has been one of my most frustrating and confusing tasks. In the 1970s, sizing of drop cable was relatively easy—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 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.
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 a 90°C service (see Table 2).
Although not necessarily a common issue with inspectors, I have known situations where a pump installer attempted to use a 90°C rated submersible cable in order to use a smaller size of conductor. The installation was rejected because the contractor tried to connect the 90°C rated cable to a 75°C rated terminal or was 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).
This is a violation of the NEC since the higher operating temperature of the 90°C submersible cable would impose this same higher temperature on the lower rated terminal. To avoid this, I recommend limiting the design temperature of all submersible pump cable to a 60°C or 75°C service temperature as the maximum since most of the currently designed industrial electrical equipment available today 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. 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 only using individual conductors in free air and with electrical equipment, terminals, and connections also rated for 90°C service.
Always remember 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 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 potential for fire or failure of a component from overheating.
One final point regarding the wire insulation is warranted, although it 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 when a larger size of flat or twisted #8 or #6 submersible pump cable must be run through a ¾-inch opening on a well seal or a tight opening on a pitless well cap, for example.
These situations happen 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 not only can cause the installer grief and added work, but can be injurious to the water system since the insulation can easily rub and scrape 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 by 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.
The second consideration, providing an ampacity rating that is at least 125% (× 1.25) of the motor’s full load current (FLC) (motor FLC < 80% of the conductor’s maximum ampacity) is a simple matter of referring to the current revision of NEC Table 310.15 (B)(16): “Allowable Ampacities for Not More Than Three Insulated Current-Carrying Conductors in Raceway, Cable, or Earth (Direct Buried), Rated 0-2000 Volts Based on Ambient Temperature of 30°C (86°F)”.
I recommend always using the values in Table 3a to be safe—although many inspectors may allow the use of ampacity from Table 3b for individual drop conductors in the well.
The current values in Table 3a are based on no more than three current-carrying conductors in a single raceway. Although a grounding conductor counts towards the raceway (conduit) fill, it is not considered as a current-carrying conductor. Any more than three current-carrying conductors require a current derate as shown in Table 3b.
Please note the allowable number of current and non-current-carrying conductors in a single raceway is a function of the conductor size (AWG-kcmil) and type (copper or aluminum), the insulation type and thickness, raceway type (rigid, PVC, tubing), and the raceway size. There are too many variables to list all the potential combinations of allowable raceway fill. Refer to the NEC for these maximum values.
For those installations using single insulated conductors in free air, you can use the table: “Allowable Ampacities for Single Insulated Current-Carrying Conductors, Rated Up To and Including 2000 Volts in Free Air Based on Ambient Temperature of 30°C (86°F).”
Note many inspectors feel the values in Table 4 apply to single conductors installed in an open well environment. However, always check and verify this with the local authority before implementing or using these values.
The tabular values in Tables 3a and 4 are based on an ambient air temperature of 30°C (86°F). A higher air temperature than what is shown will require an appropriate adjustment factor to derate the current based on the actual air temperature according to Table 5. [Source: NFPA 70-National Electrical Code 2017, Table 310.15(B)(2)(a)]
Important note: In many circumstances, the use of both Adjustment Factors Nos.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 determine both factors and any necessary corrections
For our example installation, we selected a 50 HP, 460 VAC, 3-phase, 6-inch-diameter submersible motor. From the manufacturer’s data, the full load amperage (FLA) is 68 amps and this value must be used to size the submersible cable and overload device. A total of 68 full load amperes requires a copper conductor for 68 FLA × 1.25 = 85 amps. From Table 310.15(B)(16), either a #4 AWG 75°C rated or #3 AWG 60°C rated copper conductor cable would comply with the NEC.
The next criteria, No. 3, is to check the selected cable size for voltage drop. It is not as simple as Nos. 1 and 2. Although the NEC does not specifically address a maximum or allowable voltage drop, it does include an informational note recommending a maximum voltage drop limit of 5% of the supply voltage for maximum circuit efficiency and performance.
The third cable selection criteria, although recommended to not exceed a total of 5% voltage drop over the entire distance, including the drop cable in the well plus any offset cable between the power supply and wellhead, is often a judgement factor based on the experience and knowledge of the designer.
There have been numerous instances where I designed the drop or offset cable to a voltage drop of less than 2% and others where I have allowed as much as a 10% voltage drop. From the above selection criteria, we have concluded a #4, 75°C or 90°C or a #3, 60°C rated conductor will work for our example installation.
The proposed pump setting in our example is 150 feet with a 30-foot offset to the VFD. The voltage drop can be determined in two ways: (1) Use one of the many charts available from pump and motor manufacturers which indicate the voltage drop based on conductor temperature, motor amperage, and distance, or (2) Use the following formula that provides for any variance of design. In actuality, the proper sizing of drop cable should include various factors such as cable impedance and motor power factor. However for simplicity, the following formula will work well for most submersible and non-submersible motors.
Circular mils =
Conductor resistivity (Copper) = 12.45 ohms at 60°C—12.9 ohms at 75°C—13.3 ohms at 90°C
(Aluminum) = 20.5 ohms at 60°C—21.2 ohms at 75°C—21.9 ohms at 90°C
1Multiplier: Use 1.732 for 3-phase loads, 2 for single phase loads
For our example: 50 HP, 460 VAC, 3-phase submersible, FLA = 68 A with 75°C rated copper cable
Required circular mils =
11,375 cm = #8 conductor. This is considerably less than the NEC minimum conductor size of a #4. Therefore, the previously determined size using a #4 copper conductor with a 75°C rated insulation is selected (68 FLA × 1.25 = 85 amps < 85 amps maximum with no adjustments required [Re: NEC Table 310.15(B)(16]).
Step 5: Motor cooling and lubrication
Although an important factor for all electric motors, maintaining proper cooling and lubrication is a critical design and application factor for a submersible motor. If the requirements for providing the appropriate flow rate and velocity past the motor exist as an initial design consideration, most of any last-minute changes made in the field can be avoided.
Cooling of the motor is a simple case of forcing the entire (or partial) flow rate to go past the motor first before entering the pump suction. In most cases this flow rate will be adequate to provide the minimum water velocity to pass the motor that is needed to remove and carry away the heat generated from the motor during operation.
Even though this value may vary between manufacturers, as a rule of thumb, a minimum velocity of .25 feet per second (fps) is required for a 4-inch diameter motor, .50 fps for a 6- and 8-inch motor, and up to .80 fps for 10-inch and larger diameter motors.
These values can be met in several ways. The simplest is to divert and force the pumped water to travel by the motor on its journey to the pump inlet in a bottom fed application or by using a shroud (Figure 1) for a top-feed installation or where the differential between the motor and well casing sizes is too great to provide the minimum velocity. Table 7 can be used as a source of the annular area between various motor diameters and well casing or shroud sizes as well as the minimum flow rate required for each differential.
In many cases, advance examination of the submersible motor nameplate data is not feasible. However, all of the needed information required to properly design a submersible motor installation including HP and voltage, full load and locked rotor amps, fusing or circuit breaker sizing, and KVA code is available from either the motor manufacturer or pump supplier. Whenever possible, these sources should be consulted and the actual motor values used for design of the drop cable, motor starter, overload, and short circuit devices.
Step 7: Power factor and efficiency considerations
Due to its unique construction and design, the inherent power factor and efficiency of a submersible motor is generally less than that for a comparable above-ground type of motor. For example, a typical 50 HP, 3600 RPM standard vertical hollowshaft type of motor may exhibit a full load efficiency of 90% and power factor of 85% while a comparable HP and speed submersible motor will demonstrate a lower full load efficiency and power factor of 83% and 84%, respectively.
This difference in motor characteristics directly impacts factors such as starting (locked rotor) and running amperage, power draw, and circuit breaker/fusing sizing and must be considered whenever sizing a new or retrofitting a submersible motor into an installation that had formerly supported a conventional motor, even those that are being exchanged HP for HP.
Step 8: Post installation testing
Most of the post installation procedures and tests closely resemble those for any other type of motor. However, there is one critical post installation test that should be performed immediately upon installation. This test is to verify the insulation resistance of the motor and drop cable.
Since a submersible motor is required to operate under several hundred feet of water at times, it is essential the motor windings are safely protected from the intrusion of water. This is generally performed during manufacture by embedding or potting the windings within a protective covering using a non-conductive substance, such as epoxy. This allows cooling and lubricating water to travel around the interior of the motor in close proximity to electricity without short-circuiting. The insulation resistance test is conducted in the same fashion for all submersible motors—independent of horsepower, phase, or voltage.
The test is best done with a megohmmeter capable of generating 500 or 1000VDC, but even a conventional ohmmeter will work as long as it has a scale of at least 100,000 ohms or higher (setting: R × 100K). The test is performed to verify the installation and its relative resistance to leakage, including the motor, motor lead, and drop cable.
The following values represent my opinion and may or may not coincide with those recommended by various motor manufacturers. However, I have found them to be reliable and closely approximate other values over the years:
As shown in Figure 2, the example installation is now complete for a submersible type of pump.
This concludes this installment of The Water Works. In the next column, we will examine using vertical turbine and submersible pumps as booster pumps.
Until then, keep them pumping!
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.