Article 310 provides the general requirements for conductors, except those that are an integral part of equipment or are part of flexible cords or fixture wires. The most prominent feature of Art. 310 is its collection of ampacity tables. Why are there so many? Why does Table 310.15(B)(17) list the ampacity of 6 THHN as 105A, yet Table 310.15(B)(16) lists the same conductor as having an ampacity of only 75A?

The answers to such questions are in the definition of ampacity [Art. 100], specifically “conditions of use.” These tables set a maximum current value at which the conductor insulation shouldn’t prematurely fail during normal use, under the conditions described in the tables.

How well conductor insulation can dissipate the heat from current flow depends on several factors; read the table titles to see what these are. Designations such as THHN, THHW, and RHH are insulation types, each of which can withstand only so much heat before failing. Thus, ampacity varies by insulation type.

Article 310 underwent extensive changes with the 2011 revision process. What may be most noticeable are the many changes to section numbering. Other changes include making sizing requirements clearer and improving enforceability.


The 2011 revision adds the “uses permitted” section in 310.10. Section 310.10(H) now includes information previously contained in 310.4 regarding the installation of conductors in parallel. While 310.4 previously stated that paralleling of conductors was allowed for sizes 1/0 AWG and larger, the new wording in 310.10(H) is much more restrictive. The new language now states specifically that conductors for each phase, polarity, neutral, or grounded circuit are permitted to be connected in parallel only in sizes 1/0 and larger.

Another change to this section deals with equipment bonding jumpers. Many times, such as with a transformer secondary, an equipment bonding jumper (not an equipment grounding conductor) is installed alongside ungrounded and grounded conductors in parallel.

Although it seems obvious that these bonding jumpers don’t need to meet the minimum size requirements of 310.10(H) (1/0 AWG or larger), the NEC really didn’t address the issue. Now 310.10(H)(6) refers to 250.102 for conductor sizing, making this clearer.


Though still widely used by electricians and electrical engineers, the term “derating” has been mostly phased out over the last several NEC cycles. With the 2011 revision, this term is almost gone. The NEC now uses the term “ampacity adjustment” when referring to conductors that are bundled or installed so that there are more than three current-carrying conductors in the same raceway or cable. The Code uses the term “correction” when conductors are subjected to temperatures other than 86°F.

Conductors with insulation temperature ratings higher than the termination temperature rating can be used for conductor ampacity adjustment, correction, or both [110.14(C)]. This means you must base conductor ampacity 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 [310.15(B)].

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 per the provisions of 110.14(C). Put another way, 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 NEC. The fact that they were buried beneath the ampacity table — and the fact that the table itself was broken up into copper, aluminum, Fahrenheit, and Celsius — certainly didn’t help. The new replacement table provides a remarkably easier format, resulting in less confusion and making it more conducive to proper application.

An interesting addition to this table borrows from the Canadian Electrical Code — the allowance of smaller conductors when installed in an ambient temperature of less than 70°F. With this allowance, you can use up to 115% of the conductor’s ampacity in certain conditions. That can result in a smaller conductor. While previous editions of the NEC 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 Code editions used the term “nipple” to describe a raceway that’s 24 in. long (or less). This resulted in NEC users debating about the physical characteristics of the raceways, such as whether or not the raceway could contain bends. This change eliminates those debates by replacing the term “nipple(s)” with “raceway(s).” Consequently, you no longer need to guess at the intent of this section. You simply measure the length and determine the appropriate rules.

Previous revisions used the term “bundled” to describe when ampacity adjustment is required. Because the NEC doesn’t define this term, many people struggled in their attempts to determine when to apply the adjustment provisions of this section. That term has been replaced with “installed without maintaining spacing,” which, oddly enough, is also undefined. But it may be an easier phrase to understand and apply.

New to the 2008 NEC was a rule requiring that all conductors installed in conduits on rooftops have their ampacities adjusted dramatically. The term “conduit,” while not defined in Art. 100, doesn’t include raceways such as EMT, ENT, and FMT. With the 2011 change, which now uses the term “circular raceways,” conductors installed in these raceways will need to have their ampacities adjusted as well. Confused on conduit? Take a few minutes to look at the names of the Chapter 3 Articles, starting with Art. 342.

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

  • Copper conductors: 14 AWG, 12 AWG, 3 AWG, and 1 AWG, and 600kcmil, 1,500kcmil, and 2,000kcmil.
  • Aluminum conductors: 12 AWG, 8 AWG, and 6 AWG, and 300kcmil, 700kcmil, and 800kcmil.

Temperature correction factors

When installing conductors in an ambient temperature other than 78°F to 86°F, you must correct the ampacities listed in Table 310.15(B)(16) by using the multipliers listed in Table 310.15(B)(2)(a), as shown in Fig. 1 (click here to see Fig. 1). When correcting conductor ampacity for elevated ambient temperature, use the correction factor [310.15(B)(2)(a)] for THHN/THWN conductors, based on the:

  • 90°C rating of the conductor in a dry location.
  • 75°C rating of the conductor in a wet location.
  • Conductor ampacity listed in Table 310.15(B)(16) [110.14(C)].

When adjusting conductor ampacity, do so based on the temperature insulation rating of the conductor as listed in Table 310.15(B)(16), not the temperature rating of the terminal [110.14(C)].

Ampacity adjustment factors don’t apply to conductors within Type AC or Type MC cable under the following conditions (Fig. 2) (click here to see Fig. 2):

  • The cables don’t have an outer jacket,
  • Each cable has no more than three current-carrying conductors,
  • The conductors are 12 AWG copper, and
  • 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 more than 24 in.

Ampacity adjustment of 60% applies to conductors within Type AC or Type MC cable without an overall outer jacket, if:

  • The cable contains more than 20 conductors.
  • The conductors are stacked or bundled longer than 24 in. without spacing being maintained.

Ampacity adjustment of conductors installed in circular raceways exposed to direct sunlight on or above rooftops requires adding the ambient temperature adjustment in Table 310.15(B)(3)(c) to the outdoor ambient temperature. The value you get is the applicable ambient temperature to be used for ampacity correction factors by applying Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b). Note that the temperature adders in Table 310.15(B)(3)(c) are based on the results of averaging the ambient temperatures [310.15(B)(3)(c) Note].

Why this rule? The air inside circular raceways in direct sunlight is significantly hotter than the surrounding air. To comply with 310.10, you must account for that additional heat.

For example, a conduit with three 6 THWN-2 conductors with direct sunlight exposure that’s ¾ in. above the roof will require you to add 40°F to the correction factors on Table 310.15(B)(2)(a). Assuming an ambient temperature of 90°F, the temperature to use for conductor ampacity correction is 130°F (90°F + 40°F). The 6 THWN-2 conductor ampacity after correction will be 57A (75A 3 0.76), as shown in Fig. 3 (click here to see Fig. 3).

Properly sizing conductors is one of the most difficult aspects of using the NEC. It takes a thorough knowledge of Art. 240 and 310, plus knowledge of individual sections and subsections. For example, Sec. 110.14(C) contains the guidelines on how to size conductors based on the temperature ratings of the terminals of various electrical equipment. The fact that this is so critical is why it’s now referred to in the new Informational Note 2 of 310.15(A)(3). As you work through Article 310, don’t forget that Chapters 1-4 work together and apply generally to all installations [90.3].