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Industrial Control Panels for Roof Top Units

Posted By Michael D. Fontaine, Thursday, January 01, 2009
Updated: Thursday, February 07, 2013

The electrical inspector has to verify the adequacy of SCCR of heating and cooling equipment at the location where it is installed; the electrical engineer designing the electrical distribution system has to assure that the engineer specifying heating and cooling equipment specifies equipment (integral industrial control panel) with an adequate SCCR; and the manufacturer needs to provide heating and cooling equipment that meets the requirements of today’s electrical distribution systems, which may have high available fault currents at the location of the heating and cooling equipment.


Control panel

Three Basic Issues

There are three basic issues regarding short-circuit current ratings (SCCR) of heating and cooling equipment that an electrical inspector, the authority having jurisdiction (AHJ) or an electrical engineer/designer has to deal with. They are as follows:

1. What is the SCCR of the heating and cooling equipment being applied;

2. How is the SCCR documented or established (i.e., proved); and

3. Is the SCCR of the equipment greater than the short-circuit current available at the point of application (connection) of the equipment to the electrical distribution system?


Photo 1. RTU integral industrial control panel as originally manufacturered

The National Fire Protection Association (NFPA®) has been trying to make dealing with these issues easier. However, many manufacturers of equipment are not on board when it comes to meeting these new requirements. The last several revisions of theNational Electrical Code(NEC®), which is produced and published by the NFPA, have been revised to make it easier for the electrical inspectors and engineers/designers to assure that items number 1, 2, and 3 above have been complied with, thereby increasing electrical safety. I find it amazing that there still is some kicking, screaming, and fighting the progress of the twenty-first century regarding electrical safety issues. Underwriters Laboratories, Inc. (UL®) is still in the process of catching up with some of the changes made to theNEC. One of these changes is the marking of SCCR on utilization or control equipment so that it is plainly visible, as can be seen from the following incident.

I recently worked on the electrical design of a new electrical building/room for an ethanol production facility in Lexington, Nebraska. Since the electrical building/room required cooling and heating, a new roof top unit was purchased to provide cooling and heating for the new electrical building/room, which contains a considerable amount of electronic equipment including smart motor control centers. The electrical distribution equipment in the building/room was designed for an available fault current of 65,000 A symmetrical. Since, the RTU is located beside the new electrical building/room, the cabling to the unit did not decrease the designed fault current value below 35,000 A symmetrical. See photo 1 for a picture of the RTU industrial control panel as originally manufactured.


Photo 2. RTU integral industrial control panel as upgraded by CSE Services, LLC

The RTU was purchased from a major HVAC equipment manufacturer. The manufacturer was asked what the SCCR rating of the equipment was. It was the first time that the local sales engineer was asked the question, "What is the short-circuit current rating of the RTU you are proposing?” Since he had not been asked the question before, the sales engineer passed the question along to the factory. This major manufacturer of roof top units (RTUs) stated that based on Section 90.7 of the NEC, their "equipment is exempt from inspection as long as the equipment is not damaged or altered from the design in which the equipment was listed by a qualified electrical testing laboratory (such as UL, CSA, ETL, etc…..).” They further stated that their "units are listed under UL 1995, which is for Heating and Cooling Equipment” and that "UL 1995 does not require the marking of a short-circuit rating on the panel.” Next they stated that their "”equipment meets all of the listing requirements of UL 1995″” and is "exempt from the inspection that requires the marking.” Additionally, they state that "[u]nfortunately the NEC . . . is subject to local inspector interpretation and 90.7 is often over-looked as it has been a repeated tendency to turn straight to Article 440 which specifically addresses Air conditioning and Refrigeration Equipment.”

Finally, they state "[i]n addition to 90.7, NEC 110.10 specifically states that ‘Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.’ NEC 440.4(B) requires marking only to ensure compliance with NEC 110.10 which was already obtained with the listing of the unit and the statements clearly expressed in NEC 90.7.” It would appear that they are implying that their equipment is suitable to be used on any electrical system of the proper voltage because it is listed regardless of the fault current available at the terminals of the unit and that the local electrical inspector (AHJ) need look no further. Nothing could be farther from the truth as explained below. The above appears to be an attempt to mischaracterize NEC requirements, or an inability to understand NEC requirements by the manufacturer of the RTU so as not to have to upgrade the heating and cooling equipment that they are manufacturing. Manufacturers of this type of equipment need to manufacture them such that they will have an adequate SCCR and can be installed on today’s modern electrical systems, which may have a high available fault current.

Finally, the manufacturer went on to say that "[i]t should be noted that all electrical components in UL 1995 listed products must have an individual short-circuit current rating equal to5,000 amps by default. However, the control panel as a whole must additionally be tested and listed to UL 508A in order to apply an official short-circuit current rating to UL 1995 listed equipment. UL Standard 508A is the standard for Industrial Control Panels. UL 1995 does not require UL 508A listing due to the fact that the control panel is an internal part of the HVAC equipment and is not a separate stand-alone panel. So, to make a long story short, we do not have a . . . [marked] short-circuit current rating for our equipment, but per UL 1995 the product has, by default, a 5000 amp short circuit rating.”

This shows a misunderstanding of supplement SB of UL 508A, which provides a procedure for determining SCCR of an industrial control panel based on use of tested components and a least common denominator approach to establish the industrial control panels SCCR. UL 508A does not require that the industrial control panel be tested as a unit. I don’t believe that the manufacturer understood the ramifications of what they were saying. The preceding did establish that the RTU had an SCCR of 5,000 A symmetrical and that unmarked heating and cooling equipment built in accordance with UL 1995 has an SCCR of 5,000 A symmetrical, and may not be applied on a electrical distribution system having an available fault current higher than 5,000 A symmetrical where the unit is applied.

The manufacturer refused to upgrade the SCCR of their equipment. The local salesman understood the situation and was helpful, and one person at the factory understood the situation. However, it appears that the management of the factory did not. In this case, as previously stated, an SCCR of at least 35,000 A symmetrical was required. Based on the refusal of the manufacturer to upgrade the SCCR of their equipment to a reasonable value, I had to have the unit shipped to a local UL 508A panel shop to have the industrial control panel that was part of the unit rebuilt so that it would have an SCCR rating that was plainly visible and adequate for the available fault current at the terminals of the unit. CSE Services, LLC of Omaha, Nebraska, was retained to upgrade the industrial control panel’s SCCR. They were able to upgrade it so that it has an SCCR of 65,000 A symmetrical by following the requirement and directions given in Supplement SB to UL 508A. This required that addition of approximately $500 of new components, and the labor and talent to properly engineer the changes and install the new components. The manufacturer of the RTU missed an opportunity to step into the twenty-first century and be a safety leader. This manufacturer also missed an advertising opportunity as well. He could have made the required changes and produced an advertisement stating that his units had adequate SCCR(s). See photo 2 for a picture of the RTU industrial control panel as re-manufactured.

Meaning for Electrical Inspectors

What does this mean to the local electrical inspector (AHJ)? It means that if an industrial or commercial heating and cooling unit is not marked with an SCCR and is built in accordance with UL 1995, it has an SCCR of only 5,000 A symmetrical, and that it may not be applied at a point in an electrical distribution system where the available fault current is greater than 5,000 A symmetrical, unless upgraded. The AHJ does not have to inspect the interior wiring of the heating and cooling equipment to perform this inspection, he only has to look at the nameplate of the equipment. If it does not have an indicated SCCR, it is 5,000 A symmetrical. It is the responsibility of the electrical inspector and the electrical engineer/designer to make sure that heating and cooling equipment having an integral industrial control panel has an adequate SCCR. It is the responsibility of manufacturers to produce equipment that has an adequate SCCR. In summation, one can ask the rhetorical question, "How many heating and cooling units are out there that do not have an adequate SCCR? And are they equivalent to ticking time bombs?”

© Michael D. Fontaine 2008.


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Tags:  Featured  January-February 2009 

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