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PowerFlex Drives
Hot Tips
March 2003
True Love – Drives and the 140M
The question of short circuit protection for drive installations has come up frequently in the last
few years. The introduction of new products and changes in electrical codes has opened a
wider variety of options to the user. The Bulletin 140M with its small size and superior
price / performance ratio has become a logical and desirable device for use on the input of
ac drives.
But, with the new choices comes some misunderstandings of the UL and NEC
requirements. UL inspections or code inspections have raised questions and customers
have requested direct and clear answers. Many of these
questions are prompted by customers and / or inspectors who do
not fully understand the UL requirements or the new sections of the
To answer these questions directly, let’s first separate the issues: UL
from NEC.
All currently active Allen-Bradley drives are UL listed products. UL
witnesses a number of different tests conducted at our facility. They
also consider the design of the drive’s internal short circuit protection
scheme and circuitry. These factors combine to make the
determination for product listing.
Once a test is conducted and passed to the satisfaction of UL, that drive family is added, by
base catalog number, to the UL file (E59272) and no further restrictions are placed on it by UL.
If a new product line is released, UL will examine the new drive’s internal short circuit
protection scheme. If it is deemed similar or identical to existing product, that drive family is
added to the listing file without additional short circuit testing. If UL requests a new test, that
test will be performed. All listed A-B drives are covered by one file, E59272.
If restrictions are placed on an installation, either by UL or by Rockwell Automation, those
restrictions (i.e. Fuse size, type, CB setting) will be listed in the User Manual of each respective
The U.S. National Electric Code is intended to safeguard “persons and property from hazards
arising from the use of electricity” 1 . Compliance with the code results in a virtually hazard free
installation. The code applies to new installations with a provision to mandate the upgrade of
existing installations if there is imminent danger to personnel.
The area of confusion regarding the 140M in a drive installation focuses on the NEC
requirements for Branch Short Circuit Protection. Article 430, Section IV addresses this issue
for motor circuits. It states that the intent is “to protect the motor branch-circuit conductors, the
motor control apparatus and the motors against overcurrent due to short circuits or grounds.” 2
In the past, the acceptable short circuit protection devices were fuses or circuit breakers. Sizing
rules are very specific to insure proper protection.
PowerFlex Drives
Hot Tips
March 2003
Much like the Bulletin 140M, a drive is considered a self protecting device by UL. Because the
drive has its own electronic short circuit protection, UL does not restrict the type of device used
for branch short circuit protection. Combination testing (short circuit device and drive tested as
one unit) is unnecessary. Rockwell can also impose restrictions but this too is unnecessary for
any of our drive products. Therefore, the latest versions of the User Manuals list the data
needed to select any number of branch short circuit protection devices, including dual element
and non-time delay fuses, magnetic only and thermal magnetic breakers and Bulletin 140M Self
Protecting Motor Starters.
There are two keys to a complete installation:
For a UL installation and, in some cases for NEC inspections, both devices must be
UL listed. The Bulletin 140M is a UL508 listed device with file # E-205542. As stated
previously, File # E59272 covers the drives.
The NEC must allow the use of both devices. Article 430.2 and others specifically
recognize Adjustable Speed drives. Article 430.52 (C) (6) recognizes devices like the
Bulletin 140M (Self-Protected Combination Controller) as acceptable for branch short
circuit protection, placing limits only on sizing and the range of adjustability. To
points of confusion arise on this issue:
Previous versions of the U.S. NEC did not recognize the SelfProtected Combination Controller as an acceptable short circuit
protection device. Customers are, therefore, used to requiring a
UL489 listed Circuit Breaker or fuses as shown in Figure 1.
This is no longer the case.
b) Previous approval of the older Bulletin 140MN required fuses
upstream from the device to be acceptable as a branch short
circuit protection device as shown in Figure 2.
Today, there is no requirement for fuses
upstream from the Bulletin 140M, regardless of
the Rockwell Automation drive installed.
Figure 1
Since the NEC now recognizes the Self
Protecting Combination Controller as an
acceptable branch short circuit protection
device, the preferred layout is as shown in
Figure 3.
Figure 2
The sizing of the Branch Short Circuit
Protective device is determined by the NEC. The NEC does not specify a
minimum size. UL specifies a minimum size at 125% of the drive rating,
but we do not recommend this smaller device because nuisance tripping is
likely. The tables in the User Manuals reflect the maximum size allowed
by the NEC (including all exceptions).
Figure 3
PowerFlex Drives
Hot Tips
March 2003
The “Circuit Breaker” column refers to Inverse time or Thermal – Magnetic circuit breakers.
For drives less than 100 Amps the breaker size shown is the highest frame break below 400% of
the drive rating. For drives equal to or greater than 100 Amps, the breaker shown is the highest
frame break below 300% of the drive rating.
Our own configured and engineered products choose a breaker between 150% and 225% for the
Cutler-Hammer breakers currently used. Customers should size their breakers for the full load
amps of their system and de-rate according to the breakers specification if necessary. They must
not exceed the breaker maximum size listed in our User Manuals(see sample below).
The “Motor Circuit Protector” column refers to MCPs or instantaneous trip, magnetic only
devices. The values shown in the tables are the most likely choice allowed by the NEC and or
MCP manufacturer’s data based on our interpretation. MCPs are chosen at the next higher
frame break above 125% of the FLA of the typical motor that would be used on a particular
drive. In addition, the NEC requires that the minimum adjustment of the trip level on the MCP
must be equal to or below 800% of the motor FLA. The exceptions in the NEC allow for the
MCP to be adjusted up to but not greater than 1300% of the motor rating.
This means that there may be more than one choice for MCPs for the same drive and the
customer may choose the device based on his system FLA and the MCP manufacturer’s
The 140M column lists all available 140Ms that meet the current rating needed for that
particular drive. These devices, however, can have different AIC ratings and must be chosen
All Rockwell Automation drives User Manuals are being updated to reflect this information.
Printed manuals will take some time to cycle through to the new edition. Electronic manuals
will be available on The Automation Bookstore as soon as possible.
As a final note, it should also be stated that a Bulletin 140M should not be used on the output of
an ac drive (example shown in Figure 4) as an isolating disconnect switch or motor overload
PowerFlex Drives
Hot Tips
March 2003
device. The 140M is designed to operate on sine wave voltage, and the drive’s PWM waveform
does not allow it to operate properly. Damage to the device will occur.
Figure 4
National Electric Code 2002 Article 90.1 (A)
National Electric Code 2002 Article 430.51 (General)
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