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How to Connect Motor Leads

How to Connect Motor Leads
The Art of Connecting Motor Leads
Practical tips and best practices for motor terminal box
connection methods and procedures
Jim Bryan | Jun 18, 2018
When it comes to terminating motor leads, there are many schools of thought —
each of which has a following that thinks its camp has the best method. This
article does not consider motor starting methods or internal connections.
Instead, it describes some procedures for connecting motor leads to incoming
power and a few advantages and disadvantages of each. It also explains how to
insulate joints and splices without using epoxy or tape kits.
Types of terminations. Acceptable methods of connection include both
mechanical and crimp compression lugs. Connections that use twist-on
connectors are only acceptable for wire sizes no larger than 10 AWG. [See the
National Electrical Code, 2017 (NFPA 70-2017), Sec.110.14A.]
Related: Practical Advice for Motor Protection
How to Connect Motor Leads
Photo 1. Mechanical compression lug.
Mechanical compression lugs. These connectors secure the conductors with
set screws and are available in configurations having one to six or more barrels
(Photo 1). The lugs are installed on both the motor leads and the power supply
leads and then bolted together. Parallel motor leads and power supply
conductors should use lugs that have the same number of barrels as there are
Related: Is a Failing Bearing Causing the Vibration?
Table 1. Recommended torque values for compression lug set screws.
Tighten the set screws to secure the wire using the NEC recommended torque
values (Table 1). The bolt holding the lugs together should also be torqued to the
correct value (Tables 2, 3, and 4). If the lug and bolt are made of different
materials, they may expand and contract at different rates. In that case, use a
Belleville washer to maintain torque and help keep the bolt from stretching.
How to Connect Motor Leads
Table 2. Use these torque values when bolting together SAE Grade 5 metal-to-metal contact motor and power lead compression
Table 3. Torque values to join North American Grade 5 or 8 coarse-thread, unlubricated compression lugs for motor and power
Table 4. These torque values apply when joining compression lugs for ISO Class 8.8 and 10.9 coarse-thread, unlubricated
Note: The torque values in Tables 2, 3, and 4 are approximate and should only be
used in the absence of manufacturer’s specific tightening values. Indeterminate
factors such as surface finish, plating and lubrication preclude the publication of
How to Connect Motor Leads
factors such as surface finish, plating and lubrication preclude the publication of
accurate values for universal use.
Oxidation will increase the resistance and heating in the termination, so it’s a
good practice to coat the conductors and mating surfaces of lugs with an antioxidant formulated for this purpose. The same applies to all other connections
discussed in this article.
Advantages and disadvantages. Mechanical compression lugs are easy to use
and require no special tools — typically, only a torque wrench with a hex head
(Allen) or slotted head socket are required. These connections also can be used
where the motor is terminated on a bus bar in the terminal box. One
disadvantage is that the secureness of the connection relies on the tightness of
the set screws and the bolts that hold the lugs together.
Photo 2. Crimp compression lug.
Crimp compression lugs. These lugs (Photo 2) attach to the conductors using
specially designed mechanical or hydraulic crimping tools. Since the lugs come in
various sizes and profiles (e.g., indented, hexagonal, or lobed), it’s critical to use
the right crimping tool to avoid loose connections, damaged conductor strands,
or misshapen crimps (Fig. 1).
How to Connect Motor Leads
Fig. 1. Improperly crimped lugs.
Some crimp compression lugs are formed from a sheet of conductor material, so
the barrels have a seam. To prevent split seams and loose joints, position the
crimping tool so that it will indent the opposite side of the lug.
Crimped lugs bolt together in the same way as set screw lugs (which was
discussed earlier); but, for obvious reasons, they don’t have multiple barrels.
Therefore, each parallel conductor requires a lug, and a bolt holds all the lugs
Advantages and disadvantages. The crimp connection will securely join the
conductor and lug if the proper tool is used. It also can be used where the motor
is terminated on a bus bar in the terminal box.
One disadvantage of crimped connections is that each different lug profile and
size requires a special tool. One way to minimize this problem is to standardize
on the products of one lug manufacturer. That way, only one set of crimping tools
may be needed. Smaller tools often accommodate three or more wire sizes.
Larger hydraulic tool sets can have multiple dies for different lugs.
Insulating joints. Motors with voltages up to 2kV. Splice insulation kits with
explicit instructions are readily available to make well protected joints for motors
operating at less than 2kV Some use epo
resins others use arious tapes Both
How to Connect Motor Leads
operating at less than 2kV. Some use epoxy resins; others use various tapes. Both
have the same objective — to electrically insulate the joint from its surroundings
while mechanically protecting it from damage due to vibration, heat or impact.
Photo 3. Taping a joint electrically insulates it from its surroundings.
To insulate a joint without a kit (Photo 3), proceed as follows:
• Obtain adequate supplies of varnished cambric, rubber or rubber mastic, and
vinyl tapes.
• Ensure the connection is free of dirt, oil, moisture, or other contamination.
• Wrap the joint with two half-lapped layers of varnished cambric tape to prevent
any sharp edges from piercing subsequent tape layers. A half-lapped layer is one
where the second lap of tape around the joint covers one-half of the lap below it.
When finished, there will be two thicknesses of tape per side in each half-lapped
layer. If the varnished cambric has adhesive, face that outward.
• Next, wrap with four half-lapped layers of rubber tape. Rubber tape doesn’t
usually have an adhesive, but it will bond to itself if wrapped with constant
• Finish with at least two half-lapped layers of vinyl electrical tape. This will hold
the rest of the tape together, while adding electrical and mechanical protection.
Extend the tape a couple of inches (about 51 mm) along the wires to help seal out
contaminants and reduce the possibility of shorting.
Motors with voltages above 2kV. These motors require shielded cable. This will
distribute the voltage stress along the length of the cable and prevent localized
partial discharge at points where the cable is adjacent to metal structures.
• To be effective, the shielding should be continuous from the power supply to the
motor termination.
• To avoid voltage buildup, ground the shield at each point where the conductor
terminates, including splices. This will cause a circulating current in the shield
that produces heat. If the current in the shield circuit exceeds 5% of the current
in the conductor, reduce the current carrying capacity of the conductor. Multiple
How to Connect Motor Leads
variables determine the heating effect, so consult the wire vendor for the
appropriate derating factors.
Fig. 2. Cutaway view of a stress cone.
Medium voltage stress cones. Proper termination of the shielded cable includes
stress cones to help the transition from insulated conductor to conductor in air
(Fig. 2 and Fig. 3). Partial discharge can occur at any transition point — e.g.,
where a lead emerges from a stator core, a pointed section of winding like a stub
connection in a form coil or, as discussed here, a break in the insulation and
shielding system. In the case of the latter, a stress cone mitigates its effect.
Fig. 3. Stress cones in motor terminal boxes.
Conclusion. Although there are many ways to terminate motor leads, it’s
important to use recommended methods like mechanical or crimp compression
lugs to prevent failures. Connections and joints must also be insulated properly,
using readily available epoxy or tape kits or the application of protective and
insulating tapes (as outlined above).
Bryan recently retired from EASA, St. Louis, Mo., where he was a technical
support specialist. Contact EASA at [email protected].