{"id":56,"date":"2019-01-25T03:36:58","date_gmt":"2019-01-25T03:36:58","guid":{"rendered":"https:\/\/daviderice.wordpress.com\/?page_id=56"},"modified":"2026-01-07T14:10:40","modified_gmt":"2026-01-07T20:10:40","slug":"aerial-ampacity-calculations-and-the-nec-code","status":"publish","type":"page","link":"https:\/\/www.calcware.com\/?page_id=56","title":{"rendered":"Aerial Ampacity Calculations and the NEC Code"},"content":{"rendered":"\n

Introduction<\/strong><\/p>\n\n\n\n

The deficiencies of the Code in trying to provide ampacities\nfor what is essentially an infinite number of possible underground cable system\narrangements are apparent. However, one would think that the limited number of\npossible aerial cable applications are well covered by the Code Tables. While\nin some ways this is true, there are still some significant issues with the\nCode treatment of ampacity ratings for aerial systems, in particular for 0-2000\nVolt systems.<\/p>\n\n\n\n

1.            It\nis possible using the industry accepted Neher-McGrath calculation procedures to\nderive significantly higher ampacities for 0-2000 Volt cables than those listed\nin Code Tables 310.15(B)(16) and 310.15(B)(17). These tables are based on\nresearch performed in the 1930s and are very conservative. In fact, almost all\nthe other ampacity tables in the Code are based on the Neher-McGrath\ncalculation procedures.<\/p>\n\n\n\n

2.            The\nCode includes a number of derate\/uprate multipliers on the table ampacity\nvalues in order to provide for variations in ambient air temperature, quantity\nof conductors in a raceway or cable, and various cable tray configurations.\nHowever, even if the NEC table ampacity values are utilized, a program is very\nuseful in organizing the application, applying the proper multiplier and\nproviding a definitive organized report in electronic or printed format.<\/p>\n\n\n\n

3.            There\nare necessary aerial cable modifications to the NEC table values for which no\nderate\/uprate multipliers exist in the Code.<\/p>\n\n\n\n

The remainder of this paper examines the above points in\nmore detail.<\/p>\n\n\n\n

Calculation of Higher Ampacities for 0-2000 Volt\nCables<\/strong><\/p>\n\n\n\n

Using the industry accepted Neher-McGrath\ncalculation procedures, higher ampacities can be determined than provided in\nthe Code for 0-2000 Volt cables. The calculations procedures used to produce\nthe 0-2000 Volt NEC Code ampacity tables were developed in the 1930s and\ndocumented in a 1938 AIEE paper by S. J. Rosch1<\/sup>. In its time, this\nwork was excellent and greatly advanced the state of the art of ampacity\ncalculation procedures. However, the subsequent Neher-McGrath2<\/sup>calculations\nfurther advanced the state of the art and form the basis of all subsequent\ncable ampacity standards, including IEEE Standard S-135, (IPCEA P-46-426), and\nIEEE Standard 835-1994.<\/p>\n\n\n\n

The NEC Code itself contains a number of\nampacity tables which are based on the Neher-McGrath calculations as\nimplemented in the S-135 standard. A summary of the NEC tables shows that all\nampacities for cables greater than 2000 Volts are derived directly from\nStandard S-135, which in turn was based on the Neher-McGrath calculation\nprocedures. This results in some internal inconsistencies in the NEC Code\nbetween 0 \u2013 2000 Volt cable ampacities, (referred herein as low voltage), and\n2001 \u2013 35,000 Volt ampacities, (referred herein as high voltage.) The net result\nas demonstrated below, is that the Code specifies significantly higher\nampacities for high voltage cable applications than for low voltage\napplications.<\/p>\n\n\n\n

Refer to Tables 1 which compare various ratings for three 500 kcmil cables in conduit in air. Note that for various cable voltages from 2001 through 35,000 Volts for the NEC Code, and from 1 to 15 kV for Standard S-135, all ampacities for this installation stay in a relatively tight band of values from a minimum of 473 amps to a maximum of 481 amps. In contrast, the NEC Table 310.15(B)(16) rating, at 430 amps, is significantly lower. However, even this is not an equivalent comparison since the Table 310.16 values are based on a 30 Degree C ambient air temperature, whereas the other ratings are at a 40 Degree C ambient. Converting to a 30 Degree C ambient yields a Table 310.15(B)(16) ampacity of only 391 amps, which is 82% of the S-135 value for 1 kV class cable.<\/p>\n\n\n\n

Table 1<\/strong>
Comparative Ampacities for Three 500 kcmil Single Conductor Cables in Conduit in Air<\/strong>, Copper Conductors at 90 Deg C in 40 Deg C Ambient<\/strong> <\/p>\n\n\n\n

Source<\/strong><\/td>Amps<\/strong><\/td>Notes<\/strong><\/td><\/tr>
NEC Table 310.15(B)(16)<\/td>391<\/td>x 0.91 derate applied to 430A for 40 Deg C ambient<\/td><\/tr>
S-135, (p. 264)<\/td>477<\/td>1 kV<\/td><\/tr>
NEC Table 310.60(C)(73)<\/td>475<\/td>2001 – 5000 Volts<\/td><\/tr>
NEC Table 310.60(C)(73)<\/td>480<\/td>5001 – 35000 Volts<\/td><\/tr>
S-135, (p. 264)<\/td>473<\/td>8 kV<\/td><\/tr>
S-135, (p. 264)<\/td>481<\/td>15 kV<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

In like manner, Table 2 summaries the various ratings for a single conductor 500 kcmil cable isolated in air. Again, all ampacity ratings are in a relatively tight band from 678 to 695 amps, except for the NEC Table 310.15(B)(17) value which at 637 amps, is 92% of the equivalent S-135 rating.<\/p>\n\n\n\n

Table 2<\/strong>
Comparative Ampacities for One Single Conductor 500 kcmil Cable Isolated in Air<\/strong>, Copper Conductors at 90 Deg C in 40 Deg C Ambient<\/strong> <\/p>\n\n\n\n

Source<\/strong><\/td>Amps<\/strong><\/td>Notes<\/strong><\/td><\/tr>
NEC Table 310.15(B)(17)<\/td>637<\/td>x 0.91 derate applied to 700A for 40 Deg C ambient<\/td><\/tr>
S-135, (p. 215)<\/td>695<\/td>1 kV<\/td><\/tr>
NEC Table 310.60(C)(69)<\/td>695<\/td>2001 – 5000 Volts<\/td><\/tr>
NEC Table 310.60(C)(69)<\/td>685<\/td>5001 – 35000 Volts<\/td><\/tr>
S-135, (p. 215)<\/td>688<\/td>8 kV<\/td><\/tr>
S-135, (p. 215)<\/td>678<\/td>15 kV<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Does the NEC Code Allow Alternate Ampacity Calculations?<\/strong><\/p>\n\n\n\n

While it is possible to compute higher cable ampacities with more refined and industry accepted calculation procedures, is this something that the NEC Code allows, or is strict adherence to the tables required?<\/p>\n\n\n\n

The Code itself clearly indicates that use of the tables is not required, and that engineering ampacity calculations can be undertaken for any specific application. In Section 310.15, “Ampacities for Conductors Rated 0-2000 Volts”, it is stated in paragraph (A)(1) that, “Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15.(B) or under engineering supervision, as provided in 310.15(C).”<\/p>\n\n\n\n

Similarly, for cables rated 2001 to 35,000 Volts, the following wording is offered in Section 310.60.(A), “Ampacities of Conductors Rated 2001 to 35,000 Volts. Ampacities for solid dielectric-insulated conductors shall be permitted to be determined by tables or under engineering supervision, as provided in 310.60(B) and (C).<\/p>\n\n\n\n

In paragraph 310.60(B) the following generalized formula is provided for use in these calculations:<\/p>\n\n\n

\n
\"\"<\/figure>\n<\/div>\n\n\n

The real complexity in the use of this formula is in the determination of the Rca’ factor, which is the overall thermal resistance between the conductor and ambient. For a cable in conduit, this thermal resistance is the sum of the following individual thermal resistances: <\/p>\n\n\n\n

    \n
  • Insulation and any other cable jacket or coverings. <\/li>\n\n\n\n
  • Air space between the cable outer diameter and the conduit inner diameter.<\/li>\n\n\n\n
  • Conduit cross section <\/li>\n\n\n\n
  • Air interface zone at the conduit surface to ambient air <\/li>\n<\/ul>\n\n\n\n

    \n\nThe determination of these parameters, especially the latter value, is generally where the differences between the various calculation procedures come into play.\n\n<\/p>\n\n\n\n

    Determining\nthe Effects of Application Factors Not Accounted For in the Code<\/strong><\/p>\n\n\n\n

    Not\nonly are ampacity calculations permitted rather than the use of the NEC Code\nTables, but sometimes application parameters dictate that calculations must be\nperformed. Calculations can produce more precise answers when taking into\naccount actual cable construction, conduit sizes and environmental\ncharacteristics. Some of the factors which can come into play which are not handled\nby the Code Tables include the following:<\/p>\n\n\n\n