Fixture canopies
Q
What is your opinion of sealing outside fixture wall-mounted fixtures? I have enforced this for years and was just recently challenged by a contractor on the Code interpretation of this. I always used 370-25b and 410-13 as a guideline. Any help would be greatly appreciated.
A
From your question I assume that you are concerned that some fixtures do not have any insulating barrier in the canopy. My first response to your question was “If it's listed and no barrier is provided, it must not be needed.”
A call to UL confirmed what I suspected. I was told that the UL testing includes temperature testing in the area under the canopy. If the measured temperature exceeds listing limits, an insulating barrier must be supplied with the fixture. The installation instructions will also warn that the barrier must be installed.
If high temperatures are not found in testing, the thermal barrier is not required in order to obtain listing.
Section 110-3(b) applies here. Follow the directions. If no mention of the barrier is found in the instructions, it is not really needed.
Dann Strube
Separate structures?
Q
We have performed engineering services on a chain of small drive-in restaurants. The restaurants have two remote free-standing canopies where patrons park their cars and place food orders with an intercom system. Carhops then deliver the food to the patrons. The canopies are located about 20 feet from the main building, but are not physically connected to the main building. There are five 20A, 120V circuits to each canopy serving lighting, menu boards, signs and neon tubing. These circuits are fed from a contactor-controlled lighting panelboard located in the main building. After the building construction was completed, the local electrical inspector said that the canopies are “separate structures” and has required separate grounded electrical services to each canopy citing 1996 NEC 225-8. We have argued that thousands of existing service stations and convenience stores with similar canopies have not been required to have separate electrical services, but he will not relent on his interpretation. Is it the intent of the 1996 code to consider this type canopy as a separate structure and require a separate electrical service?
A
Two or more levels of GFP may be required, depending on how the various feeder branches are supplied. The intent of Section 517-17 is to enhance the reliability of the electrical system. Additional levels are required so that a fault in the normal branch (nonesssential loads) will not cause an interruption of power in the essential electrical system. Because the language used in 517-17(a) says “an additional step,” this may be interpreted to mean that only one additional level of GFP is required. However, the words “additional levels” in the second paragraph indicate that two or more levels may be required in order to provide the “100% selectivity” required by Section 517-17(b). In addition, more than one GFP device may be required at the second level, depending on the way the normal branch and the essential electrical systems are arranged. In your case, if the 400A device feeds the normal branch, and another device at the same level (also fed by the 1,600A device) supplies power to emergency equipment, then the 400A device may require GFP in order to meet the selectivity requirements of Section 515-17(b). However, you did not say what type of health care facility you are dealing with or what types of loads are served by the mentioned feeders. Normally, the essential and non-essential loads are separated at the first feeder level. Figures 517-30(a), (b) and (c) illustrate this arrangement. If your distribution scheme is similar to Figure 517-30(b), and the 3,000A device is at the service entrance level, only two levels would be needed. However, more than one device would likely be required at the second level.
Noel Williams
Continuous-load demand factors
Q
I have a Code question that has been bothering me. This question is not the result of a particular project, but rather just about general Code understanding. Article 220, paragraph 10(b) is reasonably clear as to the definition of continuous and non-continuous loads and how they affect the size of feeders and services. My question is, “How does the NEC expect the designer/engineer to treat the loads that have been decreased due to a demand factor elsewhere in the Code?”
A very clear example of my question comes from Table 220-20, Kitchen Equipment. After a demand factor of 65% is applied to six pieces of kitchen equipment, is this demand load considered continuous or non-continuous? I will admit that the equipment is probably non-continuous, but the load is computed with the idea that it could be in continuous use only with different pieces of equipment operating.
A
In the 1996 NEC, Section 220 10(b) did require that conductors and overcurrent devices be increased in size when subjected to continuous loads. This requirement is intended to address the overcurrent devices rather than the conductors, as ampacity is a continuous value by definition. However, overcurrent devices are tested under specific conditions that include connection to a wire with an ampacity essentially equal to the rating of the overcurrent device. Therefore, minimum sizes are imposed on conductors because of the overcurrent device they connect to rather than because of any risk of overheating the conductor itself. Now, in the 1999 NEC, the issue of continuous and non-continuous loads has been removed from Article 220 to clarify that it is not part of any load calculation, but is only a minumum sizing requirement for conductors and overcurrent devices. The relocated requirements are found in Sections 210-19 and 210-20 for branch circuits, in Sections 215-2 and 215-3 for feeders, and in Section 230-42 for services.
A load that is continuous is not permitted to be reduced by a demand factor. Even where demand factors are permitted for a specific type of load, such as lighting loads as covered in Section 220-11, the demand factors do not apply to loads “where the entire lighting is likely to be used at one time.” In the example you gave, the load is reduced by a demand factor because of intermittent operation. Because of intermittent operation, perhaps due to thermostatic controls, none of the individual kitchen loads would be continuous. The resulting demand load is generally not considered as continuous under the Code either. However, depending on the actual use of the equipment, the calculated load may be imposed on the feeder for three hours or more, and this condition would require an increase in the size of feeder conductors and overcurrent devices. In general, this condition would be unlikely, as the actual demand is likely to be even less than the calculated demand load, but it certainly is possible. Outside of the references given above, I see no specific language in the Code to cover this situation. However, the language of Section 220-35 requires that a measured demand load be increased by 25% and is thus treated as continuous in determining whether an existing feeder has spare capacity.
Section 220-20 is permissive. Higher demand factors could be used, and should be used if we know enough about the load to expect the scenario you describe. For example, motor loads are handled on a case by case basis under Section 430-26, and require substantial information about the nature of the load in order to apply a demand factor. Note that the continuous load requires a minimum conductor size that is selected independently of the actual load calculation under Article 220 and this minimum size is independent of the conditions of use of the conductor. Conditions of use might require ampacity corrections or adjustments due to ambient temperatures or more than three current carrying conductors run together.
Noel Williams
CODE GUIDE
Dann Strube, a nationally recognized NE Code expert, is a certified NE Code inspector in Indiana. He also teaches NE Code workshops.
Greg Bierals, president of the Electrical Design Institute, Davie, Fla., lectures nationwide on various NE Code subjects.
Noel Williams has taught the NFPA's NE Code Seminars for 10 years and is co-author of the NFPA's 1999 NE Code Changes. He's a licensed electrical inspector.