Well, another three years has passed and
with it another revision cycle of the NEC has been
completed. As so often happens during code cycles, several
significant changes occurred during the 2005 NEC code
cycle. One of the more significant changes is the new
requirement for industrial machinery electrical panels,
industrial control panels, certain HVAC equipment, meter
disconnects, and certain motor controllers to be marked with a
short-circuit current rating. This article will discuss these
new marking requirements and equipment affected, provide
background information on the need for these changes, and
discuss the role that the inspector plays in enforcement of
these new requirements.
Summary of Change
What Changed?
The 2005 NEC has new requirements for the following
equipment to be marked with a short-circuit current rating:
· Industrial control panels [409.110]
· Industrial machinery electrical panels [670.3(A)]
· Certain HVAC equipment [440.4(B)]
· Meter disconnect switches [230.82(3)]
· Certain motor controllers [430.8]
Varying, but similar, reasons were provided
for these changes; however, they all had one goal, to ensure a
safe installation through compliance with 110.10 while
simplifying the inspection and approval process.
Significance of this change
The new 2005 NEC short-circuit current rating
marking requirements facilitate the inspection and approval
process. The marking of short-circuit current rating ensures a
key piece of information needed for a safe installation is
required to be provided (see figure
1). The change from marking the interrupting rating of the
main overcurrent protective device or no marking at all, to a
marking for the component or assembly short-circuit current
rating is a much needed revision. This change clarifies the
uncertainty associated with past short-circuit markings that
users incorrectly assumed were representative of the entire
assembly when, in fact, they were only an interrupting rating
of the main overcurrent protective device. By providing a
short-circuit current rating representative of the assembly,
the procedure for ensuring compliance with section 110.10 is
simplified. Short-circuit current ratings marked on components
and equipment make it easier to verify proper overcurrent
protection for components and equipment for specific
applications, whether it be the initial installation or
relocation of equipment.
Figure
2
Figure
3
Ensuring Compliance
For proper protection and compliance with NEC 110.10, the short-circuit current rating for a component or
equipment shall be equal to or greater than the available
short-circuit current where the equipment is being installed
in the system.
For the affected types of equipment, simply
require the following:
1. For the plan review process, the
engineer supplies the available short-circuit current at each
equipment installation point and the specific short-circuit
current rating for each piece of equipment or industrial
control panel. (The engineer must already be calculating the
available short-circuit current to assure compliance with
110.9, so this part of the requirements will not require any
new work. Obtaining the short-circuit current ratings for the
components or assemblies is new and will require additional
work.)
2. Upon site inspection, compare the actual
equipment marked short-circuit current rating to the submitted
data to ensure the rating is indeed as specified and
sufficient for the available short-circuit current available
at the point of installation.
New Assembly
Marking Requirements
Industrial Control Panels
Article 409 Industrial Control Panels is a
new article in the 2005 NEC. This new article provides
minimum requirements to facilitate the safe construction,
installation, and inspection of industrial control panels. A
definition for industrial control panels is provided in
409.2 to aid in determining the types of equipment to which
these requirements apply.
One of the more significant parts of new
article 409 is the new marking requirement for industrial
control panels. Section 409.110 provides a list of information
that is required to be marked in a location that is plainly
visible after installation. Included in this list of
information is a requirement that an industrial control panel
be marked with its assembly short-circuit current rating. The
short-circuit current rating used for compliance with Section
409.110 must be based on one of the following methods:
a. Short-circuit current rating of a listed
and labeled assembly
b. Short-circuit current rating established
utilizing an approved method
(Note: UL 508A-2001, Supplement SB, is an
example of an approved method.)
A major part of the substantiation for this
new requirement is that control panels are being misapplied in
a large number of applications because they have an inadequate
short-circuit current rating. These misapplications often
result because industrial control
panels have unique conditions surrounding them. Table 1
[pdf] shows
some of these conditions and how the new requirements help.
Air-Conditioning and Refrigeration
Equipment with Multimotor and Combination Loads
Section 440.4(B) was revised to require the nameplate of
this equipment to be marked with its short-circuit current
rating. This new marking requirement includes the following
exceptions: one- and two-family dwellings,
cord-and-attachment-plug-connected equipment, or equipment on
a 60A or less branch circuit. So for most commercial and
industrial applications, air conditioning and refrigeration
equipment with multimotor and combination loads must have the
short-circuit current rating marked on the nameplate.
A major part of the substantiation for this
change was that there is an increasing number of installations
where HVAC equipment installed in commercial, industrial, and
multifamily dwellings is subject to high levels of
short-circuit current. The majority of HVAC equipment used in
these installations was not marked with short-circuit current
rating. Without this marking, the installer and AHJ have
limited ability to determine if the equipment being supplied
is suitable for the application. These new requirements ensure
that the equipment is marked with the amount of short-circuit
current the assembly can withstand. This information can be
used by the installer and AHJ by verifying that the marked
short-circuit current rating is less than the available
short-circuit current and facilitating a safe installation.
Industrial Machinery Electrical Control
Panel
The machine nameplate marking requirements in 670.3(A)
were revised. The existing requirements for marking of the
machine overcurrent protective device interrupting rating were
removed and a new requirement for the industrial control panel
nameplate of industrial machinery to include its assembly
short-circuit current rating was added. In the past, the NEC required that the machine nameplate include only the
interrupting rating of the machine overcurrent protective
device, if one was furnished. This marking could be misleading
as it did not represent the short-circuit current rating of
the entire assembly, but could be misinterpreted as such. Two
options were provided for establishment of this short-circuit
current rating:
1. Machine control enclosures that are
listed and labeled and provided with a short-circuit current
rating established during the listing
2. Machine control enclosures that are not
listed are required to provide a short-circuit current rating
established using an approved method.
A fine print note was added to provide
users an example of a method, UL 508A Supplement SB, that
could be used to determine the short-circuit current rating.
New Component
Marking Requirements
Motor Controllers
The marking requirements in 430.8 were
revised and now require that motor controllers be marked with
their short-circuit current rating. Motor controllers cover
anything that is used as an on/off device for a motor circuit
such as contactors, drives, combination controllers, etc.
There are some exceptions to this new requirement which
· Exempt certain controllers for small
motors as allowed in Part VII, such as clock motors,
attachment plugs and receptacles, and snap switches.
· Permit controller short-circuit current
ratings to be marked elsewhere on the assembly.
· Exempt certain controllers used in
assemblies which have a short-circuit current rating, as a
short-circuit current rating on an assembly covers all of the
internal components.
Meter Disconnect Switches (rated up to 600
V)
The list of equipment allowed ahead of the service
disconnecting means in 230.82(3) permits a meter disconnect
switch, provided the equipment into which it is installed is
grounded. This was revised to include a requirement that the
meter disconnect switch has a short-circuit current rating
adequate for the available short-circuit current.
Frequently Asked
Questions
What Is a Short-Circuit Current Rating?
Short-circuit current ratings on components
and equipment represent the maximum level of short-circuit
current that the component or equipment can withstand and are
used for determining compliance with 110.10. This rating can
be marked on individual components or assemblies. The new
short-circuit current ratings required by 409.110, 440.4(B),
and 670.3(A)(4) represent the maximum amount of fault current
that the assembly can withstand under fault conditions.
Assembly ratings take into account all components contained
within the equipment rather than just the main overcurrent
protective device.
CAUTION: Short-circuit current ratings are
different from interrupting ratings marked on overcurrent
protective devices. A common mistake is to assume that the
interrupting rating of the overcurrent protective device
protecting the circuit represents the short-circuit rating for
the entire circuit (see figure
5). Interrupting ratings, used for compliance with 110.9,
apply solely to the overcurrent device. The characteristics of
the overcurrent device (e.g., opening time, let-through
energy, etc.) need to be used in determining compliance with
110.10, not the interrupting rating (see figure
4).
Why is the short-circuit current rating
marking needed?
Inspectors and installers need this information in order
to ensure compliance with Section 110.10. Equipment installed
where fault current levels exceed their short-circuit current
limit can be hazardous to persons and property. Short-circuit
current ratings marked on components and equipment make it
easier to verify proper protection for components and
equipment for specific applications, whether it be the initial
installation or relocation of equipment.
How is it determined?
For meter disconnect switches and motor controllers, this
withstand level or short-circuit current rating is often
determined by product testing. For assemblies, the marking can
be determined through product listing or by an approved
method. With the release of the Industrial Control Panel
Standard (UL 508A), an industry method is now available. Any
method used, whether UL 508A or another approved method,
should be based on the "weakest link" approach. In
other words, the assembly should be limited to installation
where fault levels do not exceed the withstand rating of
devices with the lowest short-circuit current rating (see figure
6). The marking determined should represent the limits of
the assembly for a safe installation. Current-limiting
overcurrent protective devices can be used in the feeder or
branch circuits to increase the assembly short-circuit current
rating where lower rated components are used. When
current-limiting devices that limit fault-current levels to
within the lower rated components rating are used, they
provide protection for the assembly provided the
current-limiting OCPD is used within its ratings.
Helpful Inspection
Hints
1. A simpler approach to compliance with 110.10.
The new marking requirements provide
important information about equipment that will be used in a
particular installation. Compliance is ensured by verifying
that the short-circuit current available at each piece of
equipment is less than the short-circuit current rating marked
on the equipment. This can become cumbersome for installations
that use a large quantity of equipment. This task can be
simplified for the installer, designer, or AHJ with the use of
equipment with high short-circuit current ratings. For
installations where all the equipment has high short-circuit
current ratings, a simpler method for ensuring compliance with
110.10 is:
a. Verify the maximum, worst case
short-circuit current available at the terminals of the supply
transformer.
b. If all the equipment in the system has
short-circuit current ratings greater than this maximum, worst
case available short-circuit current, then the detailed
short-circuit current study is not necessary.
2. For unlisted or unevaluated equipment
· Require a field evaluation of the
equipment by a certified testing laboratory.
· There is an option for using other
methods than listing for determination of short-circuit
current rating. The key is that the method used has to be
approved. There are many jurisdictions around the country that
have alternate methods for approval of equipment that is not
listed. The following steps are provided as an example of
regulations which could be implemented when an approved method
is used rather than listing:
1. Require a bill of material (BOM)
with component short-circuit current ratings in the detail
and a schematic of the internal circuitry of the equipment.
This documentation can be used for pre-authorization similar
to plan review.
2. Require documentation from a
professional engineer engaged primarily in the design or
maintenance of the equipment and licensed or certified in
the jurisdiction or state. The method shall be documented
and stamped by the professional engineer. This documentation
shall be available to those authorized to design, install,
inspect, maintain, and operate the system.
3. For final inspections, require
documentation for any changes to the BOM or schematics
4. If the short-circuit current rating
marked on the equipment matches the interrupting rating of the
overcurrent protective device in the supply disconnect, verify
that a proper method was used to determine an assembly
rating. Make sure that the marked short-circuit current
rating is not just based upon the interrupting rating of the
main overcurrent protective device. This is particularly
common on industrial machinery covered by Article 670, as
670.3 in the previous edition of the Code required this
misleading method of marking.
5. If there is a high short-circuit current
rating marked on the equipment, check to see if there are
current-limiting overcurrent protective devices in the feeders
and or branch circuits. High short-circuit current ratings for
assemblies can only be achieved by either (1) the use of
components and overcurrent protective devices with high
short-circuit current and interrupting ratings, or (2) by the
use of current-limiting overcurrent protective devices (see figure
7). While components with high short-circuit current
ratings exist, it is often very difficult or costly to find or
use them. Therefore, it is common to have a device with a high
interrupting rating in the supply circuit or main for the
panel and components with low short-circuit current ratings
downstream. The marking could inappropriately be based on the
interrupting rating of the main overcurrent protective device
and not represent what the entire assembly can withstand.
Current-limiting devices are the only way to increase an
assembly short-circuit current rating where devices with low
short-circuit current ratings are used.
Summary
The new short-circuit current rating requirements are a major
step towards ensuring the safe installation of equipment.
There now are mandatory requirements for the marking of
short-circuit current ratings of certain components such as
motor controllers and meter disconnects and complete
assemblies such as industrial machinery [670.3(A)], industrial
control panels [409.110], and HVAC equipment [440.4(B)]. These
changes simplify the approval process for the AHJ. In order to
enforce these new requirements, the AHJ should:
-
Ensure that the equipment is marked
with a short-circuit current rating according to one of
the methods listed above.
-
Ensure that the available short-circuit
current at the point of installation does not exceed the
assembly short-circuit current rating for compliance with
110.10.
The new short-circuit current rating
marking requirements are just one of several significant
changes that occurred during the 2005 NEC code cycle.
As with any requirement in the NEC, a rule is only a
rule if it is used and enforced. Hopefully, we as an
electrical industry, from designer to enforcer, can work
together to make sure that these changes do what they are
intended to do.
Todd Lottmann is an electrical
engineer employed by Cooper Bussmann, Inc. focusing on
Codes and Standards. Todd is a member of NEC ® Code Making Panel 12 representing NEMA, a member of the
UL508A standards technical Panel, member of NEMA 1IS
Industrial Controls section, and involved with the
NFPA79 technical committee. Todd is the Bussmann IAEI
representative participating in the national section
meetings and various chapter meetings around the
country. |
