2011 National Electrical Code Changes

14. 310.15 Conductor Ampacity

This section, dealing with the ampacity of conductors, has been extensively revised.

310.15(B) Ampacity Table. The allowable conductor ampacities listed in Table 310.15(B)(16) are based on conditions where the ambient temperature isn’t more than 86°F, and no more than three current-carrying conductors are bundled together.

The temperature correction and adjustment factors apply to the ampacity for the temperature rating of the conductor, provided the corrected and adjusted ampacity doesn’t exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).

(2) Ambient Temperature Correction Factors. When conductors are installed in an ambient temperature other than 78°F to 86°F, the ampacities listed in Table 310.15(B)(16) must be corrected in accordance with the multipliers listed in Table 310.15(B)(2)(a). (click here to see Fig. 14)

(4) Ampacity adjustment factors don’t apply to conductors within Type AC or Type MC cable under the following conditions:

(a) The cables don’t have an outer jacket,

(b) Each cable has no more than three current-carrying conductors,

(c) The conductors are 12 AWG copper, and

(d) No more than 20 current-carrying conductors (ten 2-wire cables or six 3-wire cables) are installed without maintaining spacing for a continuous length longer than 24 in.

(5) Ampacity adjustment of 60% applies to conductors within Type AC or Type MC cable without an overall outer jacket under the following conditions:

(b) The number of current-carrying conductors exceeds 20.

(c) The cables are stacked or bundled longer that measure 24 in. without spacing being maintained.

Analysis: In this edition of the Code, the term “derate(ing)” isn’t used at all, except in a few instances. The term “ampacity adjustment” is used throughout the NEC when referring to conductors that are bundled or used on rooftops, and the term “correction” is used when conductors are subjected to temperatures other than 86°F.

Conductors with insulation temperature ratings higher than the termination’s temperature rating can be used for conductor ampacity adjustment, correction, or both [110.14(C)]. This means conductor ampacity must be based on the conductor’s insulation temperature rating listed in Table 310.15(B)(16), as adjusted for ambient temperature correction factors, conductor bundling adjustment factors, or both. This change clarifies that, after applying these adjustments and corrections, the resulting ampacity still can’t exceed the temperature limitations of the equipment termination.

The temperature correction factors formerly found at the bottom of (then) Table 310.16 in the 2008 NEC, were some of the least user-friendly in the Code. This new table provides a remarkably easier format, with less confusion and proper application being the end result. An interesting addition to this table is borrowed from the Canadian Electrical Code — the allowance of smaller conductors when installed in an ambient temperature of less than 70°F. With this allowance, the NEC user can use up to 115% of the conductor’s ampacity in certain conditions, which can result in a smaller conductor. While previous editions of the Code recognized colder environments, it allowed only for an increase to 104% of the conductor’s ampacity — a value that never really made the math worthwhile.

Previous editions of the NEC used the term “nipple” to describe a raceway that’s 24 in. or less in length. This resulted in Code users debating about the physical characteristics of the raceways, such as whether or not the raceway could contain bends. This change takes away that argument by removing the term “nipple(s)” and replacing it with “raceway(s).” One no longer needs to guess at the intent of this section and now needs only to measure the length and determine the appropriate rules.

The term “bundled” has been used for several Code cycles to describe when ampacity adjustment is required. Because the term isn’t defined in Art. 100, many people struggle in their attempts to determine when to apply the adjustment provisions of this section. While the phrase “installed without maintaining spacing” is also not defined, some NEC users may find it an easier phrase to understand and apply. This change isn’t intended to be a technical one, but rather an editorial one.

New to the 2008 Code came a rule requiring that all conductors installed in conduits on rooftops have their ampacities adjusted dramatically. The term “conduit,” while not defined in the NEC, doesn’t include raceways such as EMT, ENT, and FMT. With this change, conductors installed in these raceways will now have to have their ampacities adjusted as well.

The ampacity of some conductors in Table 310.15(B)(16) (formerly 310.16) didn’t match those found in the Canadian Electrical Code and were therefore changed. While no technical evidence was submitted showing insulation failure of the conductors, this proposal was passed. The end result was a change to the ampacities of:

Copper conductors: 14, 12, 3, and 1 AWG, and 600kcmil, 1,500kcmil and 2,000kcmil.

Aluminum conductors: 12, 8, and 6 AWG, and 300kcmil, 700kcmil, and 800kcmil.