Aerial Ampacity Calculations and the NEC Code


The deficiencies of the Code in trying to provide ampacities for what is essentially an infinite number of possible underground cable system arrangements are apparent. However, one would think that the limited number of possible aerial cable applications are well covered by the Code Tables. While in some ways this is true, there are still some significant issues with the Code treatment of ampacity ratings for aerial systems, in particular for 0-2000 Volt systems.

1.            It is possible using the industry accepted Neher-McGrath calculation procedures to derive significantly higher ampacities for 0-2000 Volt cables than those listed in Code Tables 310.15(B)(16) and 310.15(B)(17). These tables are based on research performed in the 1930s and are very conservative. In fact, almost all the other ampacity tables in the Code are based on the Neher-McGrath calculation procedures.

2.            The Code includes a number of derate/uprate multipliers on the table ampacity values in order to provide for variations in ambient air temperature, quantity of conductors in a raceway or cable, and various cable tray configurations. However, even if the NEC table ampacity values are utilized, a program is very useful in organizing the application, applying the proper multiplier and providing a definitive organized report in electronic or printed format.

3.            There are necessary aerial cable modifications to the NEC table values for which no derate/uprate multipliers exist in the Code.

The remainder of this paper examines the above points in more detail.

Calculation of Higher Ampacities for 0-2000 Volt Cables

Using the industry accepted Neher-McGrath calculation procedures, higher ampacities can be determined than provided in the Code for 0-2000 Volt cables. The calculations procedures used to produce the 0-2000 Volt NEC Code ampacity tables were developed in the 1930s and documented in a 1938 AIEE paper by S. J. Rosch1. In its time, this work was excellent and greatly advanced the state of the art of ampacity calculation procedures. However, the subsequent Neher-McGrath2calculations further advanced the state of the art and form the basis of all subsequent cable ampacity standards, including IEEE Standard S-135, (IPCEA P-46-426), and IEEE Standard 835-1994.

The NEC Code itself contains a number of ampacity tables which are based on the Neher-McGrath calculations as implemented in the S-135 standard. A summary of the NEC tables shows that all ampacities for cables greater than 2000 Volts are derived directly from Standard S-135, which in turn was based on the Neher-McGrath calculation procedures. This results in some internal inconsistencies in the NEC Code between 0 – 2000 Volt cable ampacities, (referred herein as low voltage), and 2001 – 35,000 Volt ampacities, (referred herein as high voltage.) The net result as demonstrated below, is that the Code specifies significantly higher ampacities for high voltage cable applications than for low voltage applications.

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.

Table 1
Comparative Ampacities for Three 500 kcmil Single Conductor Cables in Conduit in Air, Copper Conductors at 90 Deg C in 40 Deg C Ambient

NEC Table 310.15(B)(16)391x 0.91 derate applied to 430A for 40 Deg C ambient
S-135, (p. 264)4771 kV
NEC Table 310.60(C)(73)4752001 – 5000 Volts
NEC Table 310.60(C)(73)4805001 – 35000 Volts
S-135, (p. 264)4738 kV
S-135, (p. 264)48115 kV

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.

Table 2
Comparative Ampacities for One Single Conductor 500 kcmil Cable Isolated in Air, Copper Conductors at 90 Deg C in 40 Deg C Ambient

NEC Table 310.15(B)(17)637x 0.91 derate applied to 700A for 40 Deg C ambient
S-135, (p. 215)6951 kV
NEC Table 310.60(C)(69)6952001 – 5000 Volts
NEC Table 310.60(C)(69)6855001 – 35000 Volts
S-135, (p. 215)6888 kV
S-135, (p. 215)67815 kV

Does the NEC Code Allow Alternate Ampacity Calculations?

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?

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).”

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).

In paragraph 310.60(B) the following generalized formula is provided for use in these calculations:

ampacity calculations, Neher McGrath calculations, underground cable system, cable ampacities, ductbank, direct burial

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:

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

The determination of these parameters, especially the latter value, is generally where the differences between the various calculation procedures come into play.

Determining the Effects of Application Factors Not Accounted For in the Code

Not only are ampacity calculations permitted rather than the use of the NEC Code Tables, but sometimes application parameters dictate that calculations must be performed. Calculations can produce more precise answers when taking into account actual cable construction, conduit sizes and environmental characteristics. Some of the factors which can come into play which are not handled by the Code Tables include the following:

  • Conduit size – Larger conduits with lower fill factors than what is assumed in the Code Tables can significantly raise ampacities because of the greater heat radiating area of the larger conduit.
  • Conduit material – The Code tables utilize “Iron” conduit. Other materials, such as PVC, will result in more cable heating because of the higher thermal resistivity of non-metallic materials.
  • Conduit coatings – Fireproof conduit coatings intentionally will add thermal blanketing to an installation to add fire withstand capability. However, that thermal resistance works for heat flow in both directions and will result in derated ampacities not provided for in the Code.
  • Cable Insulation –  Differences in cable insulation thickness and thermal resistivity will result in different system ampacities.
  • Sun and Wind – There is no allowance for the effect of these environmental factors in the Code.
  • There are no tables for some aerial configurations.
  • Limitations on derate table for more than 3 cables in a conduit – Table 310.15(B)(3)(a) provides derate factors to adjust table ampacities for four our more current carrying conductors in a raceway. However, the table provides cable quantity ranges so that the derate for 4 conductors is the same as that for 6, 10 conductors as the same as 20, etc. Clearly a significantly different ampacity should apply in these cases for different quantities of cable.

Organization and Reporting of Circuit Ampacities

The NEC Code provides multipliers for application to the table ampacities in order to address changes in the following application parameters:

  • Ambient Temperature – All Code tables are based on either a 30 or a 40 Degree C ambient air temperature. Correction factor tables are supplied with many tables but provide a fixed multiplier for all temperatures within a 5 Degree C range, so that the ampacity for a 26 Degree C ambient is the same as for a 30 Degree C ambient. (There is a formula provided in the Code for a more precise calculation.)
  • More than three conductors in a conduit – As indicated above, Table 310.15(B)(3)(a) is provided for ampacity derating, in the event more than three current carrying conductors in a conduit are utilized.
  • Cable Tray Applications – Code Section 392 specifies various base table references and derate factors for cable installed in tray.

Even if it is decided that the Code tables will be utilized as the basis for cable ampacities, a computer program is very useful in organizing the application, applying the proper multiplier and providing a definitive organized report in electronic or printed format.

Calculated Ampacities and Local Governing Authorities

The legitimacy of utilizing Neher-McGrath calculation procedures to provide cable ampacity ratings for aerial cable systems has been clearly demonstrated. These calculation procedures are recognized by industry standards and practices and are in fact utilized in most of the NEC Code ampacity tables.

Nevertheless, it must be recognized that local governing authorities and inspectors may require strict adherence to the ampacities contained in Tables 310.15(B)(16) or 310.15(B)(17), regardless of what accepted industry calculations may establish. Consequently, serious attention must be given to this consideration when determining the basis for aerial system ampacities for a given project.

However, even where adherence to the cited Code Tables or others is absolutely required by the local governing authorities, the calculations can still often be utilized to address special concerns the authorities may have which are not addressed in the Code. For example, the effect on cable ampacities by the use of fireproofing insulation applied to electrical conduits.


1  “The Current-Carrying Capacity of Rubber-Insulated Conductors”, S. J. Rosch, AIEE Transactions, Vol. 57, April 1938, p. 155-167.

2  “The Calculation of the Temperature Rise and Load Capability of Cable Systems”, J. H. Neher, M. H. McGrath, AIEE Transactions, October 1957, p. 752 – 771.