Panelboard mysteries revealed
By Gerald Versluys, PE, LEED AP, TLC Engineering for Architecture, Jacksonville, Fla.
Switchboards and panelboards have been the backbone of our industry and haven't really changed for years. We've become accustomed to NEC Article 408 , which gave us the definitions for lighting and appliance branch panelboards and power panelboards. We use these terms freely in our conversations and, more importantly, in our specifications. But get ready to forget these terms—they will become obsolete with the adoption of NEC 2008.
Historically, NEC 2005 Articles 408.34 and 408.35 (and former Article 384) defined and segregated panel types. The traditional descriptions limited lighting and appliance branch panelboards to 42 overcurrent protective devices and power panelboards to a maximum of 10% branch circuits. All of that restrictive language has been eliminated from the NEC. The explanatory language in the NEC 2008 handbook makes great efforts to clarify the removal of these restrictions. Basically, there are panelboards—and that's it.
It didn't take long to confirm that our manufacturing brethren have noticed the change. They already have the products waiting for us, sometimes still listed under “lighting and appliance panelboards.” To the manufacturers' credit, there already has been a great deal of research and white papers addressing any potential thermal issues that might be caused by 84 overcurrent protective devices in the same enclosure. The new panels are the traditional 20 in. wide x 5.75 in. deep tubs; however, we must now plan for a 62 in. high tub (84 circuit panelboards equipped with a main circuit breaker).
Most engineers agree with the change, citing the benefit of electrical room wall space efficiency. Everyone has felt the pressure to do more with less, and the new products cut horizontal wall space requirements in half. Other engineers cited fewer “points of failure” in a listed 84 circuit panel versus a field-constructed “double tub panel,” the solution most of us provide when the circuit count exceeds 42 but the load doesn't justify another feeder. We also discussed the NEC Article 240 maximum mounting height (6 ft 7 in. above finished floor to device operator handle); this requirement places the bottom of the tub 12 in. above the floor. Most of the engineers I spoke with agreed it wouldn't be a problem, just something new.
The 2008 NEC also requires a separate equipment ground bus for every panel. This was always good design; now it is written in the code.
The last new item that is rippling through the industry involves arc flash labeling required by 2008 NEC 110.16 on all panelboards. Clients, electricians, and others always ask what they “need to do about that.” There doesn't seem to be a consensus yet on how this will be enforced. NEC 110.16 requires an owner to label his electrical equipment, but it isn't particularly clear on what the label should look like or what should be on it.
Overcurrent and shock protection
Closely related to the NEC 110.16 requirement is NFPA 70E-2004 Article 130 , which clearly requires the facility owner to define shock protection boundaries and corresponding protective clothing based upon a shock and flash hazard analysis. NFPA 70E does not require the client to put this information in the required NEC 110.16 label; however, most clients don't have the in-house capability to provide this information to their staff. Our engineering firm has been hired by clients to produce a wide variety of work based on this section, ranging from incident energy one-line diagrams to adhesive labels dedicated for each specific panelboard that identifies all the NFPA 70E required values. Personally, I feel that our code writers should establish a standard for owners and specifying engineers to follow on NEC 110.16. If the intent is to provide the NFPA information on the labels, then reference these two similar codes together in the code.
Selective overcurrent coordination has become a major issue since the NEC 2005 started requiring it for all emergency distribution. It is the engineer's responsibility to ensure that the system is selectively coordinated. Engineers must remember to keep the required ratios between levels of distribution when dealing with a system that must coordinate. It is important to realize that in a selectively coordinated system, the panelboard ampacity is often determined by the necessary time-current curves of the overcurrent protective devices rather than the actual load on the panel.
Even when experienced engineers follow the required overcurrent protection device ratios, they still specify panelboards (say, 600 amps and larger) with static trip circuit breakers in order to give the client as much flexibility within the electrical system as possible. Trying to coordinate an electrical distribution system after it's been specified and installed usually ends in disaster.
Always develop the coordination studies before the panelboard specifications are written to assure that proper overcurrent protective devices are specified (static trip breakers are incredibly expensive to retrofit) and included in the cost of the original bid.
Despite these recent changes, most panelboard specifications are the same as they were 20 years ago. Withstand ratings, interrupting rating, and integrated equipment short circuit rating are constantly mixed and muddled when specifying panelboards. The goal is to ensure that the equipment exceeds the calculated available short circuit current and is able to prevent a catastrophic failure.
Two other minor things often trip up the specifying engineer: terminations and wire bending space. All panelboard termination lugs are rated at 75 degrees and rely on the feeder as a heat sink; 90-degree C lugs are not available. Yes, this means you can't use the 90-degree column of NEC Table 310.16 when sizing feeders to a panelboard. Few engineers consider wire bending space when specifying a panelboard; typically that is considered a manufacturer's issue. When specifying an oversized feeder, it's essential to realize that there are limitations to what the panelboard enclosure can contain (normally just one bend in the nominal feeder). Often, specifying a gutter alongside a panelboard tub is the only logical solution.
Last, but certainly not least for our clients: Will this product last? There is very little published data available on how panelboards endure various environmental challenges. A representative from one manufacturer offered these general guidelines: a panelboard should last 50 years if it's cleaned, maintained, and climate conditioned; 25 years if it's not cleaned but indoors; and 15 years maximum in an exterior environment. The representative went on to enlighten me about the real challenge facing electrical equipment longevity: obsolescence of breaker parts.
Also, exercise your breakers annually. The contacts of a circuit breaker quickly establish a preferred path for the current flow; this preferred path deteriorates the contact surface over time. Annual exercising of the contacts will shift the contact area and the corresponding current path, and minimize carbon accumulation, dramatically extending the overall life of the contacts.
Finally, who doesn't think thermal scans are a good idea? They find all the flaws, field connections and factory manufactured. Scans present a complete, real-world picture of the installed design and set a baseline for all future scans. I'm convinced that thermal scan reports are the easiest and most cost-effective insurance available to our industry.
Versluys is a senior electrical engineer and principal at TLC Engineering for Architecture. He is a member of Consulting-Specifying Engineer's editorial advisory board.
Knowing the terms
The withstand rating (WSR) refers to the level of fault current a panelboard/switchboard can withstand without sustaining damage. The WSR describes the robustness of the busing, insulation, and structural construction of a product. Often the WSR for a panelboard can be 100 to 200,000 amps, but this is only for the non-active part of the panelboard/switchboard.
It is more useful to reference the actual interrupting capacity (AIC) of the individual circuit breakers mounted within a panelboard. AIC refers to the current rating a protective device can safely interrupt. It is common within the industry to list the AIC for the circuit breakers that will be installed in a panelboard and assume that the WSR of the panel itself will exceed these values.
Integrated equipment short circuit rating is sometimes used to describe the lowest AIC rating in a panelboard; it is more commonly used to describe a switchboard's listed rating resulting from a series rated combination of circuit breakers. This term is easily confused with others and should be used with discretion. Series rating refers to a combination of multiple circuit protective devices that is listed by UL to interrupt a larger short circuit current than the lowest individual rated breaker. Engineers must be cautious about specifying series rated equipment. Series rated breaker combinations are not accepted by all authorities having jurisdiction. Engineers also must be careful to use the exact breakers (listed by UL) when modifying a series rated panelboard in order to preserve the series rating. If a panel will be modified frequently in the future, series rated equipment is probably not a prudent selection.