2,

Facilities, NFPA 99-1990 Hazardous Area in Flammable Anesthetizing Location A-517-61(a)(1) Standard for Health Care Paragraph Facilities, NFPA 99-1990 12-4.1.3.2 A-517-61(a)(3) Standard for Health Care Paragraphs Facilities, NFPA 99-1990 3-4.1.2.1(e)(2) 7-5.1.2.4, 7-5.1.2.5 A-517-64 Standard for Health Care Paragraph Facilities, NFPA 99-1990 7-5.1.2.3 A-517-160(a)(2) Standard for Health Care Paragraph Facilities, NFPA 99-1990 3-4.3.1.2 A-517-160(b) Standard for Health Care Paragraph Facilities, NFPA 99-1990 3-4.3.3 70-855 Appendix B This appendix is not part of the requirements of this Code, but is included for information purposes only. B-310-15(b)(1). Formula Application Information. This appendix provides application information for ampacities calculated under engineering supervision. B-310-15(b)(2). Typical Applications Covered by Tables. Typical ampacities for conductors rated 0 through 2000 volts are shown in Tables B-310-1 through B-310-10. Underground electrical duct bank configurations, as detailed in Figures B-310-3, B-310-4, and B-310-5, are utilized for conductors rated 0 through 5000 volts. In Figures B-310-2 through B-310-5, where adjacent duct banks are used, a separation of 5 feet (1.52 m) between the centerlines of the closest ducts in each bank or 4 feet (1.22 m) between the extremities of the concrete envelopes is sufficient to prevent derating of the conductors due to mutual heating. These ampacities were calculated as detailed in the basic ampacity paper, "The Calculation of the Temperature Rise and Load Capability of Cable Systems," by J. H. Neher and M. H. McGrath, AIEE Paper 57-660. For additional information concerning the application of these ampacities, see Power Cable Ampacities, IEEE/ICEA Standard S-135/P-46-426. Typical values of thermal resistivity (Rho) are as follows: Polyethylene (PE) = 450 Concrete = 55 Polyvinyl Chloride PVC = 650 Very Dry Soil (rocky or sandy) = 120 Rubber and Rubber-like = 500 Damp Soil Paper Insulation = 550 (coastal areas, high water table) = 60 Average Soil (90 percent of USA) = 90 Thermal resistivity, as used in this appendix, refers to the heat transfer capability through a substance by conduction. It is the reciprocal of thermal conductivity and is normally expressed in the units °C-cm/watt. For additional information on determining soil thermal resistivity (Rho), see IEEE Guide for Soil Thermal Resistivity Measurements, ANSI/IEEE Standard 442-1981. B-310-15(b)(3). Criteria Modifications. Where values of load factor and Rho are known for a particular electrical duct bank installation and they are different from those shown in a specific table or figure, the ampacities shown in the table or figure can be modified by the application of factors derived from the use of Figure B-310-1. Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity can be used beyond the point of transition, a distance equal to 10 feet (3.05 m) or 10 percent of the circuit length figured at the higher ampacity, whichever is less. Where the burial depth of direct burial or electrical duct bank circuits are modified from the values shown in a figure or table, ampacities can be modified as shown in (1) and (2), which follow. (1) Where burial depths are increased in part(s) of an electrical duct run to avoid underground obstructions, no decrease in ampacity of the conduc- 70-856 NATIONAL ELECTRICAL CODE tors is needed, provided the total length of parts of the duct run increased in depth to avoid obstructions is less than 25 percent of the total run length. (2) Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per increased foot (305 mm) of depth for all values of Rho can be utilized. No rating change is needed where the burial depth is decreased. B-310-15(b)(4). Electrical Ducts. The term electrical duct(s) is defined in