Providing branch-circuit overcurrent protection and subdividing electrical resistance-heating elements in appliances and fixed electric space heating equipment has been a long- standing requirement in the National Electrical Code (NEC). Recently, a discrepancy was discovered between the subdivision requirements in the NEC and those found in NFPA 86, Standard for Industrial Ovens and Furnaces.
A proposal to alleviate this problem and fix this discrepancy has been submitted to NEC Panel 17 for the 2008 NEC process. An explanation of the issue may help provide information to an installer caught between the requirements in the NEC and NFPA 86.
Section 422.1 states that Article 422 covers electrical appliances used in any occupancy. Reviewing the definition of an appliance is a requirement before applying the rules in Article 422. The definition of an appliance is “utilization equipment, generally other than industrial, that is normally built in standardized sizes or types and is installed or connected as a unit to perform one or more functions, such as clothes washing, air conditioning, food mixing, deep frying, and so forth.”
Immediately, the question that is raised is the meaning of the phrase “generally other than industrial” when applying the requirements in Article 422. Everyone knows what an appliance is in a household application. Washing machines, clothes dryers, dishwashers, disposals and trash compactors immediately come to mind.
But, does this definition apply to industrial equipment or not? Obviously, appliances, such as dishwashers, trash compactors and similar equipment, can be used in an industrial facility for the same purposes as used in residences or a commercial facility, such as in a lunchroom or similar application.
The issue becomes more problematic when a large, one-of-a-kind piece of equipment is designed and installed in an industrial facility. For example, some industrial ovens or furnaces can be sized as large as 2.2 megawatts (MW) or 2,200 kilowatts.
Section 422.11(F) covers general electric heating appliances in (F)(1), commercial kitchen and cooking appliances in (F)(2), and water heaters and steam boilers in (F)(3). Section 422.11(F)(1) requires heating appliances employing resistance-type heating elements rated more than 48 amperes, other than household appliances with surface heating elements and commercial- type heating appliances with open-coil or exposed sheathed-coil heating elements, to have the heating elements subdivided.
Each subdivided load shall not exceed 48 amperes and shall be protected at not more than 60 amperes. With a 2.2 MW boiler or furnace, the size and number of heating elements would be huge, so subdividing the heating elements would be a major issue.
The reason for the 48-ampere subdivision rule is to minimize the amount of damaging energy released during a short circuit involving the heating elements. By reducing the available energy release, the risk of fire is also minimized. A secondary effect is to isolate and open only a small number of heating elements to provide for continuity of service, especially where the boiler or furnace is critical to the industrial facility.
In a short circuit, large amounts of energy are released in the form of both heat and magnetic energy. As stated in the commentary for the 2005 NEC Handbook, the subdivision size was selected to use a maximum size fuse holder of 60 amps or a 60-ampere circuit breaker since each 48-ampere load would be protected at 125 percent x 48 or 60 amps.
An example of the amount of energy that may be generated in a short circuit involving one of these boilers or ovens can be seen by using the formula I2t where I2 is the current squared multiplied by the time the energy is permitted to exist in the fault. The following is the example used in the 2005 NEC Handbook.
“By using the UL Electrical Construction Equipment Directory (Green Book), the energy let-through of a 350-ampere fuse can be compared to the energy let-through of a 60-ampere fuse. In the fuse section (JCQR), the let-through energy, approximated by the current squared and then multiplied by the time, or I2t, is provided for various fuse classes (UL).
“A 600-volt, 60-ampere Class T fuse could have a let-through, I2t, as high as 30,000 ampere squared seconds. But a 600-volt, 350-ampere Class T fuse could have a let-through, I2t, as high as 1,100,000 ampere squared seconds. That means the 350-ampere fuse could let through 36.67 times as much damaging energy as the 60-ampere fuse during a short circuit.”
As can be seen by this example, just by increasing the size of the fuse or circuit breaker from the 60-ampere subdivided heating element to a 350-ampere fuse protecting a much larger set of heating elements, the energy let-through and the potential for damage is much greater. This must be resolved in a reasonable fashion between NFPA 86 and the NEC. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at firstname.lastname@example.org.