Machine Design

Control panels get new ratings

A new supplement to the UL codes dictates all low-voltage control panels be rated for short-circuit current.

Lori Tennant
Industry Standards Manager
Schneider Electric
Palatine, Ill.

The SCCR of a panel is determined by the individual ratings of the power-control equipment and other devices in the primary power path. Typically, the panel is rated to the value of the lowest SCCR component. By choosing pretested combinations of components, installers may benefit from a higher SCCR than would come from individual component ratings.

The three individual control panels each must carry an SCCR marking. The smallest rated panel determines the overall rating for the entire panel system, unless other means are used to limit current flow into the panel in the event of a short circuit.

Short-circuit current ratings (SCCRs) are marked on power-handling components such as this circuit breaker from Square D.

Those who work in the electrical industry always support one common goal — the safe installation and use of equipment. So when the National Electrical Code changes, it gets their attention.

One change in the NEC now under intense scrutiny is the new Article 409 covering electrical panels used for industrial controls, HVAC equipment, and industrial machinery. Article 409 dictates that low-voltage panel nameplates carry a short-circuit current rating (SCCR) for the entire panel. Low-voltage in this case means the panel works at 600 V or less. The new Article was added to the 2005 National Electrical Codes and took effect Jan. 1 of last year or when jurisdictions adopt this version of the code.

As reinforcement to Article 409, Underwriters Laboratories released Supplement SB for UL 508A that takes effect on April 25 of this year. The new supplement requires panels certified to UL 508A carry an overall SCCR and outlines the procedures to determine that rating. Article 409 references Supplement SB as an approved method to determine panel ratings for compliance with the Article.

States that have already adopted the 2005 NEC can carry on business as usual. Manufacturers in states still working under earlier versions of the Code must now put SCCRs on their panels to maintain UL compliance. But even in states working under the latest NEC, manufacturers get confused trying to understand and comply with this new requirement.

Many people don't realize the severity of hazards electrical faults create. Early control panels used only load demands when rating components. No consideration was given to the amount of power available to the panel from the source. Short circuits were known to literally blow panels off walls when electrical components could not handle the resulting power surge. Needless to say, personnel in the vicinity of those panels suffered the consequences.

Industrial facilities have a wide range of available fault currents based on the size of the incoming service, type of feed transformer, and location of electrical equipment inside the facility. And most electrical components already carry an SCCR listing how much fault current they handle. But does every component in the panel possess a rating sufficient to handle the power level of a short circuit?

Prior to Article 409 electrical control panels connected to machinery did not need a posted SCCR. Panel designs were based on general requirements from several different articles in the NEC. As the use of these panels expanded, so did misapplication and misuse of control equipment associated with their installation. Short circuits in equipment connected to these panels could create energy levels that exceeded what the lowest-rated component in the panel could stand.

Article 409 demands that the entire panel and all power components inside it meet the SCCR for the application. Placing an overall SCCR on the panel lets installers and inspectors easily verify whether the panel suits a given installation. The goal, of course, is to ensure that electrical safety is not compromised.

Article 409 covers industrial control panels intended for general use; but it also recognizes that panels may be constructed and installed for applications covered by other NEC articles. For example, Article 440 outlines airconditioning and refrigeration equipment, Article 610 targets cranes and hoists, and Article 670 covers industrial machinery. Article 409 supplements the provisions of those other Articles. It does not replace them.

Previous standards allowed the practice of listing a panel according to the ampere-interrupting rating (AIR) of its main overcurrent protective device. The new codes and standards now use the combined power circuit to determine the overall SCCR for the panel.

The final machine builder or assembler of equipment inside the panel has three methods for obtaining an SCCR. The first method tests each combination of devices in a panel and records the results in follow-up procedures for UL approval. With so many product combinations possible within a panel, this option calls for considerable testing and maintenance by the builder and may require third-party testing and certification.

For the second option the machine builder can purchase pretested combinations of devices from a major supplier. The machine builder tabulates the combined pretested ratings into the UL approval procedures for the panel. Use of approved combinations may permit higher SCCRs than those available using SCCRs from individual components. For example, the combination of a Square D Corp. KAL32150 circuit breaker, an 8502-SFO2V motor controller, and a 9065-SEO15 overload relay carries a 10-kA (10,000 A) SCCR at 240 V. Yet both the motor controller and over-load relay are individually rated for 5 kA — the typical limiting value as identified by the third option.

The third option uses a method described by UL in its UL 508A Supplement SB. This method takes the approach of using the "least-common denominator" SCCR for all components within a panel. For all power-circuit components in the panel — such as circuit breakers, motor controllers, and overload relays — the panel marking becomes the same as the device with the lowest SCCR rating. For a simplified example: A panel contains a circuit breaker with an SCCR of 25 kA, a motor controller at 10 kA, and an overload relay rated at 5 kA. The overall SCCR for the panel becomes 5 kA, based on the lowest-rated component.

In reality, the third option is much more involved then the simplified example may indicate. The overall process is further detailed in Supplement SB of UL508A. For example, the first step on the road to determining a panel SCCR is to get the SCCR of every power component in the panel. Most manufacturers give the SCCR of their devices on the product spec sheet. Likewise, component markings or instruction markings may indicate specific overcurrent protections a device requires. Table SB4.2 in Supplement SB lists standard fault SCCRs for unmarked devices. In many cases the default SCCR is 5 kA. Or the rating can be determined by testing the component or combination of components per UL 508A.

When current-limiting protective devices are included in a feeder circuit, the rating is modified based on the let-through values they have. This is how certain combinations may receive an SCCR higher than the lowestrated component.

Facility engineers and inspectors must identify which components are in the power circuit of the control panel. Basically, all components between the main disconnect and the control transformers require SCCRs. The standard specifically describes SCCR requirements for motor starters, controllers or contactors, drives, circuit breakers, fuses, and terminal blocks. Devices such as PLCs or logic components sitting downstream from a control transformer or power supply are not included.

Unfortunately, many industrial facilities see fault currents exceeding 5 kA. Most electrical systems have fault currents higher than 10 kA while some exceed 65 kA. A low SCCR on the control panel may limit the places that equipment may be installed. On the other hand, overspecifying the rating can make equipment bigger and more expensive.

Supplement SB allows for higher SCCRs. The current-limiting provision lets a current-limiting circuit breaker or fuse be part of a feeder circuit to modify the SCCR. Current-limiting devices applied in feeder circuits provide high SCCRs for devices in downstream branch circuits.

Use of current-limiting fuses or circuit breakers with peak currents lower than those of all downstream branch devices modifies the SCCR rating for that feeder circuit. Current-limiting devices added to the feeder circuit do not supersede the NEC requirement that individual branch circuits have short-circuit protection. Though additional components are involved, this approach creates an easy way for low-horsepower controllers to get high short-circuit feeder ratings.

The standard describes the use of products that have already been tested for a specific SCCR in combinations. This includes combination circuit protection, contactor, and overload relay pretested for an SCCR. The combination is then listed in the UL approval procedure for the control panel. The panel can be designed using manufacturer-tested SCCRs for all branch circuits and, thus, carry a high SCCR. Again, the panel is rated to the lowest branch or modified feeder SCCR it contains.

Control panels in commercial buildings and other locations continue to reside near service entries or large transformers. Panels close to these sources of power raise questions about safe installation.

Active industrial environments seldom keep equipment at the same location for long. The level of potential fault currents can vary dramatically by location — even at different points within the same building. Equipment clearly marked with its SCCR lets engineers and inspectors quickly assess if equipment can handle the potential fault current where it is installed. Such markings can also show where installers should take additional precautions, such as hooking up a Class-2 transformer to the feeder line to limit short-circuit current.

All in all, it's valuable to know what's likely to happen at a potential location for equipment. An adequate panel SCCR avoids untimely delays and costly re-work. Once the new rules take effect, inspectors and facilities engineers can weed out control panels that don't comply with these new requirements.

State Version State Version State Version
AK 2002 LA Local OH 2005
AL Local MA 2005 OK 2005
AR 2002 MD Local OR 2005
AZ Local ME 2005 PA 2002
CA 2002 MI 2002 RI 2005
CO 2005 MN 2005 SC 2005
CT 2005 MO Local SD 2002
DE 2002 MS Local TN 2002
FL 2002 MT 2002 TX 2005
GA 2005 NB 2005 UT 2005
HI Local NC 2005 VA 1999
IA 2005 ND 2005 VT 2005
ID 2005 NH 2005 WA 2005
IL Local NJ 2002 WI 2005
IN 2005 NM 2005 WV 1999
KS Local NV Local WY 2005
KY 2005 NY 1999    

Local: No state agency, adoption by local jurisdictions
Information collected by Square D/Schneider Electric.

NEC adoption by state

Though the NEC is the standard for electrical practices throughout the U.S., each state or local jurisdiction must ratify adoption of the current version. Some state regulations specify the "current standard" as the official state version. Others require approval by their local governing body. These are the current levels of the Code adopted by each state as of March 2006. UL approval for industrial control panels demands compliance with UL 508A Supplement SB in all states as of April 25, 2006.

Schneider Electric, (847) 397-2600,

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