Article 310 - Conductors for General Wiring, based on the 2017 NEC^®

Article 310 contains the general requirements for conductors, such as insulation markings, ampacity ratings, and conditions of use. It doesn’t apply to conductors that are part of flexible cords, or fixture wires. Nor does it apply to conductors that are integral parts of equipment [90.7 and 300.1(B)].

Uses permitted
Insulated conductors typically used in dry and damp locations include THHN, THHW, THWN, or THWN-2 [310.10(B)]. See Table 310.104 for a complete list of conductors you can install in dry or damp locations.

Insulated conductors typically used in wet locations include types THHW, THWN, THWN-2, XHHW, or XHHW-2 [310.10(C)]. See Table 310.104 for a complete list of conductors you can install in wet locations.

Insulated conductors and cables exposed to the direct rays of the sun must be [310.10(D)]:
(1) Listed as, or marked as, sunlight resistant. SE cable and the conductors contained therein are listed as sunlight resistant.
(2) Covered with insulating material, such as tape or sleeving materials that are listed as (or marked as) being sunlight resistant.

Conductor insulation must be suitable for any substance to which it may be exposed that may detrimentally affect the conductor’s insulation (e.g., oil, vapor, gases, fumes, liquids) [310.10(G)]. If wire-pulling compounds are used, make sure it’s compatible with the insulation on the conductors being installed. See 110.11 and Informational Note 2.

Parallel conductors
Ungrounded and neutral conductors can be connected in parallel only in sizes 1/0 AWG and larger where installed per 310.10(H)(2) through (H)(6).

When circuit conductors are installed in parallel, all circuit conductors within each parallel set must [310.10(H)(2)]:
(1) Be the same length.
(2) Consist of the same conductor material.
(3) Have the same circular mil area.
(4) Have the same type of insulation.
(5) Terminate using the same method (set screw fitting versus compression fitting).

When installed in raceways or enclosures, paralleled conductors must be grouped to prevent inductive heating.

Raceways or cables containing parallel conductors must have the same electrical characteristics. Conductors of one phase, neutral, or EGC are not required to have the same physical characteristics as those of another phase, neutral, or EGC [310.10(G)(3)].

Parallel conductor sets must have all circuit conductors within the same raceway [300.3(B)(1)]. Parallel sets of conductors aren’t required to have the same physical characteristics as those of another set to achieve balance.

Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor for the purpose of conductor ampacity adjustment, per 310.15(B)(3)(a) 310.10(G)(4).

The equipment conductors for circuits in parallel must be sized per 250.122(F). Sectioned equipment conductors smaller than 1/0 AWG are permitted in multiconductor cables, if the combined circular mil area of the sectioned equipment conductor in each cable complies with 250.122 [310.10(G)(5).

The minimum 1/0 AWG parallel conductor size rule of 310.10(H) doesn’t apply to equipment grounding conductors (EGCs).

Size equipment bonding jumpers and supply-side bonding jumpers per 250.102. The equipment bonding jumper isn’t required to be larger than the largest ungrounded circuit conductors supplying the equipment 310.10(G)(6)].

Ampacity
Article 100 says “ampacity” means the maximum current, in amperes, a conductor can carry continuously, where the temperature of the conductor won’t rise in excess of its insulation temperature rating.

People often make mistakes in applying the ampacity tables in Article 310. If you study the explanations carefully, you’ll avoid common errors such as applying Table 310.15(B)(17) when you should be applying Table 310.15(B)(16).

Why so many tables? Why does Table 310.15(B)(17) list the ampacity of 6 THHN as 105A, while Table 310.15(B)(16) lists the same conductor as having an ampacity of only 75A? To answer that, go back to Article 100 and review the definition of ampacity. Notice the phrase “conditions of use.” These tables set a maximum current value at which premature failure of the conductor insulation shouldn’t occur during normal use, under the conditions described in the tables.

The designations THHN, THHW, RHH, and so on, are insulation types. Every type of insulation has a limit as to how much heat it can withstand. Current flowing through a conductor creates heat. How well the insulation around a conductor can dissipate that heat depends on factors such as whether the conductor is in free air or in a raceway.

Conductor insulation deteriorates over time. That’s why (among other measures) we perform cable testing to predict failure and replace conductors while they’re still within design specifications. Conductor insulation failure takes decades under normal use and it’s a maintenance issue. However, if a conductor is forced to exceed the ampacity listed in the appropriate table, and thus its design temperature is exceeded, insulation failure happens much more rapidly. The failure can be catastrophic, thus exceeding the allowable ampacity of a conductor is a serious safety issue.

The ampacity of a conductor can be determined either by using the tables per 310.15(B), or under engineering supervision as provided in 310.15(C) [310.15(A)(1)]. Ampacities provided by this section don’t take voltage drop into consideration. See 210.19(A) Informational Note 4, for branch circuits and 215.2(A)(1) Informational Note 2, for feeders.

Where more than one ampacity applies for a given circuit length, use the lowest ampacity value [310.15(A)(2)]. An exception applies where different ampacities apply to portions of a circuit (see the Exception and Informational Note to 310.15(A)(2)).

Conductors can’t be used where the operating temperature exceeds that designated for the type of insulated conductor involved [310.15(A)(3)]. For the temperature limitation of terminations, see 110.14(C)(1).

The insulation temperature rating of a conductor [Table 310.104(A)] is the maximum temperature a conductor can withstand over a prolonged period without serious degradation. The main factors to consider for conductor operating temperature include [310.15(A)(3) Informational Note 1]:
(1) Ambient temperature, which may vary along the conductor length and over time [Table 310.15(B)(2)(a)].
(2) Heat generated in the conductor by load current flow.
(3) The rate at which generated heat dissipates into the ambient medium.
(4) Adjacent load-carrying conductors, which raise the ambient temperature and impede heat dissipation [Table 310.15(B)(3)(a)].

The allowable conductor ampacities listed in Table 310.15(B)(16) are based on conditions where the ambient temperature is between 78 DegrF and 86DegrF, and no more than three current-carrying conductors are bundled together in a raceway, cable, or directly buried in earth.

The temperature correction [310.15(B)(2)(a)] and adjustment factors [310.15(B)(3)(a)] apply to the conductor ampacity, based on the temperature rating of the conductor insulation per Table 310.15(B)(16).

When conductors are installed in an ambient temperature other than 78 DegrF to 86 DegrF, the ampacities listed in Table 310.15(B)(16) must be corrected using the multipliers listed in Table 310.15(B)(2)(a).

Where four or more current-carrying power conductors are within a raceway longer than 24 in. [310.15(B)(3)(a)], or where cables are bundled for a length longer than 24 in., the ampacity of each conductor must be adjusted per Table 310.15(B)(3)(a).

“Number of conductors” is the total number of conductors, including spare conductors, adjusted per 310.15(B)(5) and (B)(6). It doesn’t include conductors that can’t be energized at the same time [Note for Table 310.15(B)(3)(a)].

Conductor ampacity adjustment of Table 310.15(B)(3)(a) doesn’t apply to conductors [310.15(B)(3)]:
• Installed in cable trays (392.80 applies). [310.15(B)(3)(a)(1)]
• In raceways having a length not exceeding 24 in. [310.15(B)(3)(a)(2)]
• Within Type AC or Type MC cable under the following conditions: [310.15(B)(3)(a)(4)]
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.

Neutral conductors
The neutral conductor of a 3-wire, single-phase, 120/240V system, or 4-wire, three-phase, 120/208V or 277/480V wye-connected system, isn’t considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(A)(5)(a)].

The neutral conductor of a 3-wire circuit from a 4-wire, three-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(A)(5)(b)].

The neutral conductor of a 4-wire, three-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a), but only if more than 50 percent of the load is nonlinear [310.15(A)(5)(c)].

Conductors
Only conductors in Tables 310.104(A) though 310.104(G) can be installed, and only for the application identified in the tables [310.104].

The smallest conductor permitted for branch circuits for residential, commercial, and industrial locations is 14 AWG copper, except as permitted elsewhere in the NEC [310.106(A)].

Conductors 8 AWG and larger must be stranded when installed within a raceway [310.106(C)].

Grounded [neutral] conductors must be identified in accordance with 200.6 [310.110(A)]. EGCs must be identified per 250.119 [310.110(B)]. Ungrounded conductors must be clearly distinguishable from neutral and EGCs [310.110(C)].

Correct application
To determine which conductor type is allowed for a given installation, identify the conditions of use. For example, is it a wet location? Instead of THHN, you’ll want to look at THHW.

To determine the minimum required size of that conductor, review the conditions of use and select the relevant table. Then apply Table 310.15(B)(2)(a) ampacity correction factors for the expected temperature, and/or Table 310.15(B)(3) ampacity adjustment factors for the number of conductors, where applicable.