Many designers and/or installers overlook the short circuit current rating (SCCR) of downstream contactors when designing and installing electrical systems. As used here, the term “contactor” refers to a single- or multiple-pole power-switching device (relay) located in a branch circuit – connected in series – used to switch one or more pieces of utilization equipment. This oversight may cause unnecessary downtime in the event of a high-level destructive fault above the rating of the equipment. The result can be loss of equipment, extensive damage to the conductors and raceway system cause by fire and arcing, and even personal injury.
The National Electrical Code (NEC) addresses current carrying capacities of both conductors and components. Unfortunately, some designers fail to recognize the limited SCCR of some lighting contactors. They may mistakenly place this equipment in a circuit with higher fault currents than the product listing allows. Electrical design safety of the lighting control system is instrumental in developing the foundation of the system before details of the control scheme are established.
Complying with the Code. In the 1999 Code, 110.10 was changed by substituting the word current for withstand. This change has remained in the 2002 Code.
Section 110.10 of the 2002 National Electrical Code states:
“Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.”
Overcurrent protective devices, such as fuses or circuit breakers, should be selected to prevent the SCCRs of the electrical components from being exceeded should a short-circuit or high-level ground fault occur. Electrical components include wire, switches, contactors, relays, and distribution equipment. Merely providing overrcurrent protection with an adequate interrupting rating won’t necessarily meet the requirements outlined in the Code. The combination of the circuit and the overrcurrent device must limit the available let-through current to a value that doesn’t exceed the SCCR of each listed device.
To minimize confusion and simplify compliance, the inspector only needs to verify that the equipment is listed and installed where the available fault current doesn’t exceed its rating. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.
Design considerations. Using a lighting control as an example, consider the application of contactors and remotely operated circuit breakers. It’s the designer’s responsibility to specify the properly rated device. The designer must know the available fault current at the service equipment and the SCCR of the manufacturer’s products that will be specified. The available fault current at the transformer that supplies the installation can usually be obtained from the serving utility. The procedure for determining fault current is found in the Institute of Electrical and Electronic Engineers (IEEE) Standard 141 or in IEEE Standard 241. The contactor or remotely operated circuit breaker manufacturer should either display the rating on the product in accordance with its listing, such as UL Listing, or provide rating data with the product.
Contactors were one of the first solutions for automated lighting control applications. In the past, contactors were generally placed near the end-use equipment. This was usually a substantial distance from the service equipment/panelboard that supplies the lighting load. The length of the conductor between the branch circuit overcurrent protection and the contactor provided sufficient impedance to reduce the available fault current at the contactor. Today, for convenience and maintainability, contactors (relays) are commonly located in a central cabinet in the electrical equipment room next to the panelboard. Therefore, knowing the impedance in the branch circuit wiring is critical to making sure the contactor meets the SCCR requirements. The shortened conductor length between the panelboard and relay cabinet decreases the impedance of the circuit and significantly increases the available fault current at the relay. Consequently, relay-based systems require additional design considerations to ensure compliance with the Code.
How to select components. UL standards have been established to determine the SCCR for electrical components. Contactors are typically listed under UL 508 Standard for Industrial Control Equipment. The standard defines a set of tests that established a performance level at specific fault current levels with a specified branch circuit protection device. The standard current test is 1,000A for relays and contactors rated not more than 1.5 horsepower. Some devices tested to UL 508 aren’t suitable for use as lighting contactors.
Generally, devices tested to UL 916 Standard for Energy Management Equipment aren’t suitable for use as lighting contactors. UL 916 energy management equipment is intended primarily for control panels used with energy management equipment fed by power-limited circuits. This standard also includes a short circuit test but is limited to 1,000 symmetrical amperes for control relays less than 1.5 hp. UL 916 doesn’t require markings for this rating to be on the product or literature.
Designers and electrical inspectors should be aware of design limitations when using UL 916 rated products for lighting controls. For example, the recent trend for pre-assembled electrical distribution systems reinforces the need to confirm that all components of the assembly have an adequate SCCR. For pre-assembled systems that use UL 916/UL 508 devices, the lowest rated device governs the SCCR of all the devices mounted inside an enclosure. The aforementioned issues indicate the high probability that the devices installed in these pre-assembled systems aren’t rated for the applications for which they’re commonly used.
Listing a device to UL 508 often requires additional protection for the product to pass the test sequence. The overcurrent device, branch circuit breaker, or fuse is then required to be marked on the Listed UL 508 device. As noted earlier, the device must have a minimum of 1,000A SCCR to meet UL 508. Some relays have a 5000A SCCR when used with and RK5 fuse. In order to ensure a NEC-compliant installation, the contactor must be installed per the manufacturer’s instructions with the required circuit breaker or fuse for protection.
The NEC and UL 489 Standard for Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures both indicate that the primary function of a circuit breaker is the protection of the conductors of a circuit. Remotely operated circuit breakers are designed to switch branch circuits while providing overcurrent protection and are Listed under UL 489. UL 489 requires the maximum interrupting rating be displayed on the circuit breaker.
The use of remotely operated circuit breakers is another control option. The remotely operated circuit breaker fits into any existing standard panelboard enclosure. No additional wiring or enclosures are required. The breakers are designed to handle high-inrush current loads and today’s high short circuit requirements.
Published ratings for remotely operated circuit breakers and lighting contactors that have been tested and listed are usually marked on the equipment or are readily available from the manufacturer. You must know the short-circuit current rating of the relay/contactor or the interrupting rating of the circuit breaker to determine whether the circuit complies with the requirements of 110.10 of the Code. If the ratings aren’t marked on the device or published with it, you must contact the manufacturer to verify the ratings.
Installations that deserve a second look to confirm compliance with the Code include:
A) Relay/contactor not protected by a branch short circuit protective device, such as a fuse
B) Lighting controls mounted in enclosure mounted near the panelboard
C) Branch circuit protection that doesn’t match markings of the relay/contactor
D) Equipment listed as UL 916 installed in a circuit that exceeds 1,000A of available fault current
E) Lighting contactors not specifically rated for lighting, such as “Definite Purpose”
It’s not enough that individual devices of your lighting control system meet the appropriate test standards. All elements of the branch circuit must be considered. The component SCCR, the overcurrent protective device(s), and the total circuit impedance must be selected to permit the protective devices to clear a fault without extensive damage to the circuit components.
If any component of the branch circuit or electrical assembly can’t meet the available fault current, the circuit doesn’t meet Code requirements.
Jordan is PowerLink product manager with the Power Management Organization of Schneider Electric in LaVergne, Tenn. Loyd is the president of R&N Associates, Sun Lakes, Ariz.
