Impact of Short Circuit Current Rating on UL1995 Equipment
Son Dinh
Application Design Engineer
OEM Technology and Solutions Center
Schneider Electric
What is this Short Circuit Current Rating (SCCR) requirement?
The National Electrical Code (NEC) is primarily concerned with preventing fires, not damage to electrical
apparatuses. NEC’s interest in the SCCR is the amount of momentary electrical current that a device
can withstand before it will produce an uncontainable fire and/or explosion. If the fault current is less
than the equipment’s SCCR, then the equipment should mitigate, the preceding hazards.
The SCCR rating (rated in kA) is different than the current rating (Amps) of a device. The current rating
is the designed continuous current that the device can handle without reduction of the anticipated life (or
tripping, in the case of a circuit breaker).
The NEC, Article 409, is a code article that specifies the minimum requirements for an industrial control
panel. Article 409 was created in response to panel failures and general misuse. The requirements are
a consolidation of requirements applicable to all control panels that were previously contained in the
other various articles of the NEC. NEC 409 is titled “Industrial Control Panels”, however, the
requirements are also applicable to other specialty panels such as Crane and Hoist, Elevator, Industrial
Machinery, and Air Conditioning and Refrigeration control panels as stated in NEC 409.3 and Table
409.3. There are three sentences in NEC 409.110(3) that require a control panel to have a marked
short circuit current rating (SCCR).
(Extracted from NFPA 70 – National Electrical Code 2008 edition)
Currently the NEC only recognizes two methods to determine a panels’ SCCR rating.
a. Short-circuit current rating of listed and labeled assembly,
b. Short-circuit current rating established utilizing an approved method,
Impact of Short Circuit Current Rating on UL1995 Equipment
Method (a) would require 3rd party testing and certification and for method (b) the only recognized
approved method by the NEC is UL 508A, Supplement SB as stated in the Fine Print Notation (FPN) of
the requirement.
Furthermore, NEC Article 440 specifies the installation and construction requirements for Air-Condition
and Refrigeration Equipment. Article NEC 440-4(B) requires HVAC equipment with multiple motors
and/or loads to be marked with the SCCR rating of the motor controllers or industrial control panel.
Satisfying this requirement will require a major shift in control panel design for most manufacturers.
Therefore, the driver of this SCCR requirement is the NEC, specifically Articles 409 and 440.
What is UL 508A, Supplement SB?
Underwriters Laboratories (UL) standard for Industrial Control Panels is UL 508A. It is one of the most
popular methods used by manufacturers for ensuring a quality industrial control panel construction and
for satisfying the requirements of the NEC. Some of the requirements contained in UL 508A are
enclosure environmental rating, electrical spacings, component usage, wire ampacity sizing, sizing of
overcurrent protection, and markings to name a few of the topics covered by the standard. However, a
full discussion of UL 508A is well beyond the scope of this paper.
Within UL 508A, section Supplement SB contains the requirements for designing and rating a control
panel for a short circuit current rating (SCCR). These requirements allow the engineering of a panel’s
SCCR rating without performing short circuit testing on the panel assembly which is destructive and cost
prohibitive for most control panel builders. This method is commonly termed “the weakest-link” method
since the panel’s overall SCCR rating is based on the lowest SCCR rating of any power component or
power assembly that forms the power circuitry of the control panel. For the majority of panel builders,
the most practical means to assign a SCCR rating to a control panel is through the use of UL 508A,
Supplement SB. Currently UL 508A, Supplement SB is the only approved method other than
performing short circuit testing on the panel to comply with the SCCR requirement of NEC 409.110(3).
If compliance with UL 508A is met, then compliance with NEC 409 will also be met.
What is UL 1995?
Underwriters Laboratories (UL) standard UL1995 is the bi-national standard (US & Canada) for Heating
and Refrigeration equipment. Examples of equipment within the scope of UL1995 are heat pumps, air
conditioners, chillers, and air handlers. UL1995 currently does not require Heating Ventilation AirConditioning (HVAC) equipment or the control panel to have a marked SCCR rating. NEC Article 440,
specifies the installation and construction requirements for Air-Condition and Refrigeration Equipment.
NEC 440-4(B) requires HVAC equipment with multiple motors and combination loads to be marked with
a SCCR rating.
(Extracted from NFPA 70 – National Electrical Code 2008 edition)
In response to this NEC requirement, UL issued a Certification Requirement Decision (CRD) bulletin for
UL1995 in February of 2008. The CRD contains proposed new requirements that will require HVAC
equipment manufacturers to mark their equipment with a SCCR rating. One of the methods allowed is
using UL 508A, Supplement SB. The CRD is currently optional until the requirements are officially
added to the standard. Therefore, UL1995 now has optional requirements for the determination of a
SCCR rating for the HVAC equipment that allows 3rd party certified and labeled equipment to be
rejected (red-tagged) by the Authority Having Jurisdiction (AHJ) due to non-compliance with NEC 4404(B) if it is enforced.
2 of 6
Impact of Short Circuit Current Rating on UL1995 Equipment
What is the minimum SCCR level?
The SCCR requirement does not specify a minimum short circuit current value. It only requires that the
equipment or panel be marked with a SCCR value. The intent is that the panels’ SCCR value will be
used to judge its’ suitability and acceptance based on the available short circuit fault current to the
equipment or panel at the installation site. In order to obtain the available short circuit fault current, a
short circuit fault study would need to be conducted by a qualified engineer.
The following table in Appendix A illustrates an approximate maximum available fault current on the
secondary output (480V, 3 phase) of a power distribution transformer based on the transformers’ kVA
rating. For the higher available fault currents to be possible, it would require a very large power
distribution transformer. Furthermore, Ohm’s Law proves that this fault current will be reduced the
further downstream the equipment or panel is located relative to the power distribution transformer.
The figure in Appendix B illustrates an example of the reduction of available fault current as a result of
conductor impedance and the equipment’s distance relative to the power distribution transformer.
What is the impact on HVAC equipment?
Some or all of the following effects may be experienced by HVAC equipment manufacturers in
complying with the SCCR requirement.
•
•
•
•
Increases engineering time and labor to determine SCCR rating,
Short circuit testing may be needed to achieve desired SCCR rating,
Specific component selection and usage required to achieve the desired SCCR rating,
Redesign of equipment.
In order to achieve a desired SCCR rating by using UL 508A, Supplement SB, engineers will need to
verify and document the SCCR ratings of all of the power components. For the majority of
components, the marked SCCR rating is not the stand alone SCCR rating for the component. The
component is additionally marked to state that it is suitable for up to a certain SCCR rating and voltage
when protected by the specified short circuit protective device(s). The exceptions to this are short circuit
protective devices like circuit breakers and fuses and other devices that have been tested without any
additional upstream short circuit protection. The following is an example of how a component’s SCCR
rating may appear.
Therefore, to maintain the component’s SCCR rating, one must abide with the specified short circuit
protection type and stated maximum size. According to UL, the SCCR rating(s) for a component can
appear on the product label, on the product carton, on an instruction sheet that is provided with the
component, or on UL’s website (for some products). Additionally, components and assemblies can be
tested and specifically described in the equipment or panel manufacturer’s 3rd party certification file.
In some cases, the equipment or panel manufacturer may choose to have the equipment or assembly
short circuit tested for a desired SCCR rating. This may be the only option if the equipment or panel
uses custom or proprietary electrical components or assemblies that lack the desired SCCR rating. In
some cases, depending on several factors (number of variations, weakest-link, redesign costs, lack of
solutions, etc.), it may be more cost effective to test a current design instead of performing a redesign of
the equipment or panel.
3 of 6
Impact of Short Circuit Current Rating on UL1995 Equipment
Generally, in order to achieve a rating of more than 5kA, component selection is critical. It is very
difficult to inter-mix components from various manufacturers and expect to achieve a higher SCCR
rating for the power circuit. This is because most component manufacturers have no vested interest in
testing their component in conjunction with a competitor’s component if they offer a competing
component (i.e. contactor and overload relay). Furthermore, achieving a higher SCCR rating is often
more difficult because fewer product options may exist that meet these rating requirements. This is
analogous to NEMA environmentally rated components. There exists more product availability with a
NEMA 12 versus a NEMA 4X due to the difficulty in achieving a 4X rating.
UL 508A, Supplement SB is structured on the weakest-link method. Therefore, the overall panel SCCR
rating is determined based on the lowest SCCR rated power component. If component substitutions are
used from a different or the same manufacturer in order to achieve the desired SCCR rating, design
changes will be very likely since industrial components are not “plug-&-play” and often have different
dimensions and terminal connections that require updates to the panel layout, drawings, and bill of
materials to achieve the desired SCCR rating.
Designing a one size fits all panel with a high SCCR rating (for example, 65kA), will likely increase the
cost of the equipment. This is because 65kA solutions generally cost more than an equivalent solution
with a lower SCCR rating. Also, for a given component, it may have different SCCR ratings at different
voltages (for example, 65kA @240V and 25kA@480V). So at 480V, a different solution would be
required if the 65kA component rating is desired. Further more, some components may have the
desired SCCR rating but only at a Wye voltage rating (480/277 or 600/347) instead of a Delta voltage
rating, see example below
If using Wye voltage rated component(s), this automatically limits the panel for use on a Wye voltage
system because a component must be used within its marked and certified ratings.
In all cases, a panel will need to be minimally engineered and documented in order to substantiate the
panel’s marked SCCR rating. The degree of impacts previously discussed will vary depending on the
components currently used in the panel and their SCCR rating along with the overall desired SCCR
rating for the panel or equipment. For more information on UL 508A, Supplement SB, visit Schneider
Electric’s website at www.us.squared.com/UL 508A.
4 of 6
Impact of Short Circuit Current Rating on UL1995 Equipment
APPENDIX A
Approximate available short circuit fault current at the secondary terminals of a power
distribution transformer.
For most typical installations, far lower levels are the norm
particularly when the utility source fault level and impedance are taken into account.
5 of 6
Impact of Short Circuit Current Rating on UL1995 Equipment
APPENDIX B
Illustrates that short circuit fault current is a factor of the power distribution transformer size
(reference Appendix A) and the impedances of the power conductors and other equipment
upstream of the end equipment or control panel.
6 of 6