The Perfect Splice

There are three key elements that lead to the perfect underwater splice.

By Ed Butts, PE, CPI

One of the most overlooked but critical aspects of a successful submersible pump and motor installation is the electrical connection provided between the motor leads and submersible pump (drop) cable. Inadequate connections result in physically weak and high-resistance connections, which can cause damage or failure to an entire pumping system.

This connection in a water well pumping installation is commonly referred to as the splice. Believe it or not, long before the advent of heat shrink tubing, one of the first tasks I learned also happened to be one of the principal ways pumpmen often distinguished themselves—the way we made our splices.

In most cases, we not only had to make our splices fail-proof, but as fast as possible to get the pump installed rapidly. This month’s edition of Engineering Your Business will outline the many facets involved in making the perfect splice and some precautions to avoid a bad splice.

In my opinion, there are three distinct elements that go into the so-called perfect underwater splice:

  • Mechanical integrity and strength
  • Electrical integrity and low resistance
  • Physical dimensions and compactness.

While most would agree with the first two items, many of you may be confused by and question the third. However, drop cable now includes a fourth wire used as an equipment grounding conductor and many people are working to get more out of smaller wells by using larger pumps, which increases the overall size of the drop cable bundle. This means compactness has become even more important to a successful installation. So, we will examine each one separately.

The Early Days

But before moving into our discussion, let’s look at the various splicing and drop cable methods used since the inception of submersible pump motors.

Figure 1. Two types of cable used for low voltage splicing.

Even from the early days of submersible pump and motor installation during the 1950s, there has always been an obvious disconnect between the factory boxed and packaged pump and motor unit and the drop cable that conveys power to the motor.

Although the unit was typically supplied with a pre-installed motor lead, it was generally impractical and burdensome to supply the pumping unit with the pre-spliced drop cable to the proper length, which could be several hundred feet. Therefore, the installer was usually forced to purchase the drop cable separately and attach—or “splice”—the cable onto the motor-lead This procedure was generally performed in the field right before the installation started and could take hours to complete.

Figure 2. Scotchcast resin/splice kit.

Splicing the drop cable during my early days of pump installation generally revolved around the size of motor along with the size and type of drop cable we were using. Three-wire cable was the norm at the time and the only variation of choice was whether the wire was flat, round, or twisted single conductors in shape, and neoprene rubber or thermoplastic in insulation.

Regardless of which splice method was used, the one constant through the years has been the use of pre-formed butt connectors to connect the conductors together. The principal advantage in using a butt connector for underwater splices lies in its universally round shape, which helps to prevent protrusions through overlying tape wraps and the strength of the connection.

Figure 3. Poured splice using motor lead.

Generally, butt splices are made using special pliers—often referred to as Sta-Kon pliers—or for larger connections, a specialized anvil or compression tool.

Beginning in the 1950s, there were two common methods of attaching the motor lead to the drop cable. The first involved the use of a taped splice, which we will discuss at greater length later, and the second method involved the use of an epoxy-poured splice.

Although I only made around four or five epoxy-poured splices, I can relate that performing an epoxy-poured splice required great patience and fairly  decent weather to do it right. This type of splice involved the pre-connection of the three pairs of wires usually through the use of heavily crimped butt
connectors once the leads and drop cable had been pulled through separate ends into a clear plastic sleeve acting as the outer shield and encasement for the splice.

Once the connections had been made and the splice joint pulled back to the approximate center of the clear sleeve (the sleeve was generally clear to enable the installer to know when the splice joint was situated in the middle), a heated or catalytic solution of an epoxy resin was generated and poured from the top to the bottom of the sleeve. The pour was always using care to ensure the splice was poured rapidly to prevent premature setting of the resin, the sleeve was full of the resin when done, and all air bubbles were dislodged.

This type of splice was often referred to as a Scotchcast splice. Although not in common use today for water system installations due to the high cost, time to complete, and specialized training and equipment needed, they are still commercially available and commonly used for underwater splice connections for power and communication cables used in ocean or undersea transmission settings (See Figures 1 and 2).

Another version of this type of splice, shown in Figure 3, involves the same type of poured splice to connect the drop cable to a power cable receptacle. The connection between the motor and the power cable receptacle is then performed using a two-ended jam nut assembly that plugs into the motor as well as into the end of the power cable receptacle.

Either of these methods can take up to an hour to complete and great care must be taken to ensure the butt connectors do not touch in any way, as a shorted connection could occur otherwise. Yet many of these splices, originally made in the 1950s and 1960s, can still be found powering well pump motors in water well and oilfield applications.

Figure 4. Mechanical splice.

Another method of splicing the motor lead to the drop cable used a mechanical splice connection, which is shown in Figure 4. This method was fairly easy to make on new drop cable with good insulation and involved centering the butt connector in the middle of a plastic tube with a rubber sealing ferrule and threaded cap used to compress the ferrule against the wire insulation.

This type of splice kit was commonly shipped with new pump and motor units for several years and was popular with pump installers as no heat or specialized methods were needed to create an effective splice.

However, this type of splice also had limitations as they were not effectively interchangeable between wire sizes and overtightening the cap could cause leakage of water into the connection.

Figure 5. Taped splice.

The next type of splice known to almost all old-timers is the taped splice, shown in Figure 5. For many of us, performing a good taped splice was akin to a baptism into the pump business. In my case, I was forced to watch a more senior pumpman demonstrate several of them before I was allowed to make my first one.

Our standard procedure used a butt connector connection between the motor lead and drop cable. This was followed by a coating of Scotchkote solution, three to four wraps of 3M #70 or #2242 rubber tape, and then finished with several wraps of 3M #33 vinyl plastic tape.

I readily admit I was proud of my taped splices, and to my knowledge never had one fail. In fact, in a few cases we actually used them to fish pumps from wells (more on that next month).

Figure 6. Heat shrink kit.

Finally, the innovation and widespread use of heat shrink splice kits (Figure 6) has become almost universal in the pump industry and has essentially replaced all of the prior methods, including fully taped splices and even on larger conductors up to #500 MCM. It sometimes seems as if the splicing part of our work has come full circle as we are back to using heat to make splices just like they did in the 1950s.

Most submersible motors are factory shipped with either a removable or epoxy set motor lead already attached to the motor. This facilitates factory testing of the motor and provides the installer with a pre-assembled and ready means to connect to the submersible pump cable.

To provide the best possible and most reliable working installation, the appropriate methods must be used to physically and electrically connect the drop cable to the motor leads. There are two primary considerations to mechanical strength: the physical strength of the connection and the integrity of the splice developed through adequate resistance to bending of the splice.

Mechanical Integrity and Strength

Physical strength lies in the method used to connect or join the two separate conductors together into a single assembly, which if improperly done can literally cause failure to the entire pump installation.

Although many unique methods are used to perform this task, connecting the conductors together is performed generally using butt connectors—often referred to as ferrules, pins, or sleeves—or even by the trade name of Sta-Kon connectors or clamps.

Although the term implies that the two conductors are actually connected, in reality, the butt connector usually provides a physical joint with the needed electrical continuity provided by and across the connector itself.

It is imperative that common electrical devices such as wire nuts, split bolts, or bolted/pressure lugs never be used to create an underwater splice since the sharp and irregular edges and threads common to these devices can wear through the insulating layer over time, exposing the conductor to water and resulting in a partial or dead short to ground.

In addition, the threads on these devices can loosen with thermal expansion and contraction due to operating temperature variations, which along with insufficient mechanical stress often developed during assembly can result in an inconsistent or total loss of electrical continuity.

Non-insulated butt connectors or splices are cylindrical, factory made electrical sleeves made of high-conductivity copper with a corrosion-resistant plating. They usually include marked indicators to match the applicable wire size.

Most single butt connectors will accommodate a range of wire sizes, such as #12/10 or #8/6 AWG. The appropriate wire size rating and length should always be used, but a short section (1 to 2 inches) of copper tubing of the proper inside diameter can also be used when standard butt connectors are unavailable.

The most important step in performing a physical connection is using the proper connector and applying the appropriate amount of force to compress the sleeve onto the electrical conductor that is necessary to secure the connection with adequate strength to resist the amperes and voltage that travels through the connection.

This is conducted using a crimping tool. For the smallest conductors, a crimping tool consists of a hand-held pair of modified pliers that when squeezed by manual force pushes a pre-formed indention die into the connector, compressing it securely onto the conductor. This is effective for crimping up to roughly #6 AWG connectors.

Larger (#4 AWG up to 1000 MCM) wire sizes require a greater compressive force to secure the higher amperage. This is usually performed by either using a larger crimping tool with greater length and leverage (similar in size to bolt cutters) or an electric, hydraulic, or pneumatic power crimping tool.

Many of the larger hand-held or power-operated crimping tools are provided with adjustable or interchangeable dies to accommodate various wire sizes and have ratchet, preset, or adjustable limits or settings to make sure the proper compressive force (up to 12 tons) is applied to the connection.

In addition to commercially available crimping tools, many pump installation firms have built their own crimping device.

Due to the common differences in wire gauges between motor lead and drop cable conductors, the use of a standard butt connector may not be appropriate. In these instances, the use of a parallel butt connector may be indicated. This is where the two conductors slide over each other and are then crimped together or double the length of the stripped motor lead conductor and fold back in the middle to provide twice the original size.

The other critical mechanical aspect of an effective splice lies in the integrity of the connection. Just as the physical strength of the connection is vital, providing the greatest resistance to fatigue breakage from repeated bending is also vital.

Electrical Integrity and Conductivity

The second facet of a perfect splice is the electrical conductivity and integrity. The lowest practical electrical resistance (conversely, the highest conductivity) across the connection is just as important if not more important.

While a butt connector serves a useful purpose in joining the two separate conductors, in some instances—particularly those for high amperage (more than 100 amps), larger wire size (larger than #2 AWG) installations, and where a great amount of power (voltage and amps) must travel through a relatively small area of conduction—reinforcing the connection through soldering may also be warranted.

Properly soldering the connection will not only generate additional physical strength but enhance the electrical conductivity by lowering the overall circuit resistance. Soldering must not be performed by anyone without the proper degree of training and should be performed only by experienced and competent individuals using appropriate methods and preparation along with flux and rosin core solder.

Take note that the heat or flame used to apply solder must not extend past the connection to prevent possible injury or burning the wire’s insulation.

The following is a brief outline of a proper compression splice:

  1. Strip the insulation on each conductor carefully to avoid nicking or cutting conductor strands and to the proper length so the conductors can be inserted fully into the full length. The wires should be visible in the inspection hole of the connector.
  2. If necessary, strip back the insulation on the smaller wire to double the length of exposed conductor in order to fold back the conductor to fill more space on the connector.
  3. Using the appropriate crimping tool, fully compress the connector on both conductors.
  4. If warranted, conduct soldering of the joint to reinforce the connection and lessen the electrical resistance.
  5. Double-check both ends of the butt connector. Look for smooth and uniform surfaces along with incomplete or inadequate crimps or sharp or extended points or corners where crimping has resulted in spreading, splitting, or fragmentation of the connector. This can result in small but sharp points that can wear through the taped or heat shrink wrapping.
  6. Proceed with soldering of the joint, if required.

Physical Dimensions and Compactness

The final aspect of a perfect splice is likely the factor most overlooked: physical dimensions and compactness. Although the relative size of the splice is generally not considered as an important element of the perfect splice, it is often one of the most important.

There are two reasons for this. The first is the one previously stated regarding the larger drop cable overall size now in use due to the fourth wire used as the equipment ground. The second is due to its location in the well; the splice is at the lowest portion of the drop cable in the pump installation.

Its location places it in the annulus between the drop pipe and well casing or borehole and immediately above the pump. The splice lies against and is secured to the drop pipe with the casing or open borehole often only an inch or so away. The well pump and therefore the splice are often placed in undersized, lined, crooked, tight, or misaligned wells or boreholes in which the installer has no control over if the splice will contact, scrape, or abrade against welds, screen wires, or casing or liner joints. This means the splice must be kept as compact as possible, while retaining the needed mechanical and electrical strength and continuity.

Although centering guides are often helpful to preventing abrasion of the drop cable or splice, there is never a guarantee of this. One of the best ways I have found to provide a compact splice is by staggering the individual splices.

Once each individual conductor has been physically joined and soldered (if applicable), the final step of the splice begins, the heat shrink or taped covering. Many pump installation firms now use heat shrink as the exclusive covering over a splice.

When heat shrink tubing is used for well pump splices, the material should conform to the following general guidelines: Shrink tubing should be heavy duty, thick walled, rated as flame retardant, and rated as water resistant for continuous submersion (not just immersion) in water (500 feet of water head recommended as the minimum value).

It should also possess high-impact and abrasion resistance with a thermoplastic adhesive liner to facilitate shrinkage and sealing under water head. The tubing should be rated for 600 VAC at 90°C continuous service between operating temperatures of –55°C to 110°C.

Application should occur at a shrink temperature of 120°C. Finally, heat shrink tubing should meet UL 486D and IEEE 383 for a vertical flame test and NEMA insulation thickness requirements for heavy-duty service and NSF 61-approved for potable water service.

Although I do not disagree with using heat shrink as a primary electrical wrapping, I do not agree with using it as the sole means of protection. I think the manufacturing variables and consistency in heat shrink material create situations where any of these separate variances could result in leakage through or up into the material and into the butt connector—particularly under high head. Using a low-cost, secondary tape wrapping helps to lessen or remove these potential failure causes.

I also think it is not inconceivable that a well pump may separate from the suspending drop pipe and hang from the drop cable. Adding a few more layers of tape to the splice will physically reinforce the connection, which may be extremely useful if the pump and motor were needed to be retrieved using the drop cable.

I also think that just as with the material itself, the possibility of different heat application and procedures between workers can result in excessive or inadequate distribution of heat across a splice, resulting in leakage into the splice. Generally, I suggest applying two half-lapped applications of electrical tape 1 inch past the end of the heat shrink tube, reversing direction to 1 inch past the other end, and then reversing again to cover the entire tubing.

An alternative to a heat shrink splice, and one I recommend on very deep or large horsepower sets, is the use of at least two layers of half-lapped, Scotch 3M #70, self-fusing, silicon rubber tape.

Begin over the center of the butt connector to 1 inch past each end, followed by two more half-lapped wraps of Scotch 3M #33 adhesive tape as an outer wrap. The combination of the pressure-sensitive, rubber-resin adhesive and the PVC backing of the #33 outer tape wrap over the excellent water sealing advantages of the #70 silicone rubber offers superior electrical and mechanical protection.

I personally used this system for many years and never had a splice-induced failure. In fact, twice, we were able to retrieve a motor from a well that had dropped from a broken pump, simply by the drop cable and this type of splice!


That wraps up this month’s edition of Engineering Your Business. Until next month, work safe and smart.

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 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

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