How to Connect Motor Leads

30/07/2018 How to Connect Motor Leads
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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
MAINTENANCE, REPAIR & OPERATIONS > MOTORS
The Art of Connecting Motor Leads
Practical tips and best practices for motor terminal box
connection methods and procedures
Jim Bryan | Jun 18, 2018

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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
leads.
Related: Is a Failing Bearing Causing the Vibration?
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.
Photo 1. Mechanical compression lug.
Table 1. Recommended torque values for compression lug set screws.

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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
Table 2. Use these torque values when bolting together SAE Grade 5 metal-to-metal contact motor and power lead compression
lugs.
Table 3. Torque values to join North American Grade 5 or 8 coarse-thread, unlubricated compression lugs for motor and power
leads.
Table 4. These torque values apply when joining compression lugs for ISO Class 8.8 and 10.9 coarse-thread, unlubricated
materials.

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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 anti-
oxidant 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.
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).
Photo 2. Crimp compression lug.

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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
together.
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
Fig. 1. Improperly crimped lugs.