As I discussed in last month’s column, Article 310, covering conductors for general wiring, has been totally rewritten for the 2011 National Electrical Code (NEC). This article is now divided into three parts: Part I covers general requirements, Part II covers installations of conductors, and Part III covers construction specifications for conductors.

The introduction text to the previous 310.4, involving installation of conductors in parallel, has been modified to clearly indicate that aluminum, copper-clad aluminum or copper conductors shall be permitted to be connected in parallel only in sizes 1/0 AWG and larger; this text now appears in 310.10(H). Adding the word “only” will leave no doubt that smaller conductors are not permitted to be installed in parallel, unless the installation complies with the stated requirements in the two exceptions to this general rule. There was a minor change to the second exception as follows: “Exception No. 2: Under engineering supervision, grounded neutral conductors in sizes 2 AWG and larger 1 AWG grounded neutral conductors shall be permitted to be run installed in parallel for existing installations.”

This minor change clarifies the already obvious fact that 2 AWG and 1 AWG conductors are the only conductors smaller than 1/0 AWG that can be installed in parallel and then only under engineering supervision to alleviate any overheating of neutral conductors for existing installations due to high content of triplen harmonic currents. A new (6) has been added to address parallel equipment-bonding jumpers installed in raceways; it directs the user to comply with the installation and sizing requirements in 250.102.

Former Section 310.13 has been relocated to a new Part III in the 2011 NEC. It is titled “Construction Specifications” with former Tables 310.13(A) through (E) relocated to Tables 310.104(A) through (E). In Table 310.104(A) [formerly Table 310.13(A)], Cable Construction (B) columns were added under MTW for thickness of insulation where a nylon jacket or equivalent is added. This column was inadvertently dropped from the 2008 NEC and has been added back into the table. Note 4 to Table 310.104(A), stating that “some rubber insulations do not require an outer covering” for “RHW” and “XHHW”, has been deleted since the information in the note was redundant. Former 310.5, covering the minimum size of conductors, has been relocated to new 310.106(A) and (C), with the former Table 310.5 now located as Table 310.106(A). Section 310.14, covering aluminum conductor material, has been relocated to new 310.106(B) with no change to the existing text.

Tables 310.16 through 310.20 have been identified as Tables 310.15(B)(16) through 310.15(B)(20) to clearly identify the tables are aligned to the specific information from 310.15(B). The ambient-temperature correction factor portion of these tables have been removed from the former location in Tables 310.16 and 310.17 and relocated to Table 310.15(B)(2)(a) covering 30°C (86°F) for Table 310.15(B)(16) and Table 310.15(B)(17). The same action was taken for the ambient-temperature correction factor portions for Tables 310.18 through 310.20 by establishing a new ambient-temperature correction factor in Table 310.15(B)(2)(b) for 40°C (104°F) with some of the ambient-temperature ranges modified to 5°C increments. Text has been added as a new second paragraph to 310.15(B) stating, “the temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).” There are asterisk notes at the bottom of each new Table 310.15(B)(16) through 310.15(B)(20), indicating which ambient--temperature table to use—either Table 310.15(B)(2)(a) or (b). In addition, many of the allowable ampacities for conductors in Tables 310.15(B)(16) and (17) have been modified slightly to harmonize with Tables 2 and 4 of the Canadian Electrical Code. For example, 14 AWG and 12 AWG were changed in the 60°C Column of Table 310.15(B)(16) from 20 and 25 amperes, respectively, to 15 and 20 amperes, respectively. Before installing a conductor and connecting it to a breaker or fuse, check the new ampacity values in Tables 310.15(B)(16) and (17), since other changes may affect your installation.

Finally, the previous 310.15(B)(2)(c) and Table 310.15(B)(2)(c) have been changed to 310.15(B)(3)(c) and Table 310.15(B)(3)(c) with a new text as follows: “Ambient Temperature Adjustment for Conduits Circular Raceways Exposed to Sunlight on or Above Rooftops.” The temperature adjustment only applied to conduits for the 2008 NEC and now applies to any circular raceway for the 2011 NEC.


ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and mark.c.ode@us.ul.com.