310.15 Conductor Ampacity
(A) General Requirements
The ampacity of a conductor can be determined either by using the tables in accordance with 310.15(b), or under engineering supervision as provided in 310.15(C).
FPN No. 1: Ampacities provided by this section do note take voltage drop into consideration. See 210.19(A) FPN No. 4, for branch circuits and 215.2(D) FPN No. 2, for feeders.
If a single length of conductor is routed in a manner that two or more ampacity ratings apply to a single conductor length, the lower ampacity must be used for the entire circuit. See 310.15(B).
Exception: When different ampacities apply to a length of conductor, the higher ampacity is permitted for the entire circuit if the reduced ampacity length does not exceed 10 ft and its length doesn’t exceed 10% of the length of the higher ampacity.
(B) Ampacity Table
The allowable conductor ampacities listed in Table 310.16 are based on conditions where the ambient temperature isn’t over 86°F and no more than three current-carrying conductors are bundled together.
When conductors are installed in an ambient temperature other than 78 to 86°F, ampacities listed in Table 310.16 must be corrected in accordance with the multipliers listed in Table 310.16.
When correcting conductor ampacity for elevated ambient temperature, the correction factor used for THHN conductors is based on the 90°C rating of the conductor, based on the conductor ampacity listed in the 90°C column of Table 310.16 [110.14(C)].
When adjusting conductor ampacity, the ampacity is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)].
Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled in lengths exceeding 24 in., the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(A).
Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor.
The grounded neutral conductor is considered a current-carrying conductor, but only under the conditions specified in 310.15(B)(4). Equipment grounding (bonding) conductors are never considered current carrying, but they are not designed to be used for this purpose [310.15(B)(5)].
When adjusting conductor ampacity, the ampacity is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)].
Where more than three current-carrying conductors are present and the ambient temperature isn’t between 78 and 86°F, the ampacity listed in Table 310.16 must be adjusted for both conditions.
When adjusting conductor ampacity, the ampacity of THHN conductors is based on the 90°C rating of the conductor [110.14(C)].
FPN 2: See 376.22 for conductor ampacity adjustment factors for conductors in metal wireways.
Conductor ampacity adjustment only applies when more than 30 current-carrying conductors are installed in any cross-sectional area of a metal wireway.
Exception 3: The conductor ampacity adjustment factors of Table 310.15(B)(2)(a) do not apply to conductors installed in raceways not exceeding 24 in. in length.
Exception 5: The conductor ampacity adjustment factors of Table 310.15(B)(2)(a) do not apply to Type AC or MC cable when: (1) Each cable has not more than three current-carrying conductors. (2) The conductors are 12 AWG copper, and (3) No more than 20 current-carrying conductors (ten 2-wire cables or six 3-wire cables) are bundled.
When 11 or more 2-wire cables or 7 or more 3-wire cables (more than 20 current-carrying conductors) are bundled or stacked for more than 24 in., an ampacity adjustment factor of 60% must be applied.
Neutral Conductor
The neutral conductor of a 3-wire single-phase 120/240V system, or 4-wire 3-phase 120/208V or 277/408V wye-connected system isn’t considered a current-carrying conductor.
The neutral conductor of a 3-wire circuit from a 4-wire 3-phase 120/208V or 277V/480V wye-connected system is considered a current-carrying conductor.
When a 3-wire circuit is supplied from a 4-wire 3-[phase wye-connected system, the neutral conductor carries approximately the same current as the ungrounded conductors.
The neutral conductor of a 4-wire 3-phase circuit is considered a current-carrying conductor where the major portion of the neutral load consists of nonlinear loads. This is because harmonic currents will be present in the neutral conductor, even if the loads on each of the 3 phases are balanced.
Nonlinear loads supplied by 4-wire 3-phase 120/208V or 277/480V wye-connected systems can produce unwanted and potentially hazardous triplen harmonic currents (3rd, 9th, 15th, etc.) that can add on the neutral conductor. To prevent fire or equipment damage from excessive harmonic neutral current, the designer should consider increasing the size of the neutral conductor or installing a separate neutral for each phase. For more information, visit www.MikeHolt.com and see 210.4(A) FPN, 220.61 FPN 2, and 450.3 FPN 2.
Grounding (earthing) and bonding conductors aren’t considered current carrying.
For individual dwelling units or one-family, two-family, and multifamily dwellings, Table 310.15(B)(6) can be used to size 3-wire single-phase 120/240V service or feeder conductors (including neutral conductors) that serve as the main power feeder. Feeder conductors are not required to have an ampacity rating greater than the service conductors [215.2(A)(3)].
Warning: Table 310.15(B)(6) doesn’t apply to 3-wire single-phase 120/208V systems, because the grounded neutral conductor in these systems carries neutral current even when the load on the phases is balanced [310.15(B)(4)(6)]. For more information on this topic, see 220.61(C)(1).
Grounded Neutral Conductor Sizing. Table 310.15(B)(6) can be used to size the grounded neutral conductor of a 3-wire single-phase 120/240V service or feeder that serves as the main power feeder, based on the feeder calculated load in accordance with 220.61.
Because the grounded neutral service conductor is required to serve as the effective ground-fault current path, it must be sized so that it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the grounded neutral conductor in accordance with Table 250.66, based on the total area of the largest ungrounded conductor [250.24(C)(1)].
Editor’s note: This information was extracted from Mike Holt’s textbook, Understanding the National Electrical Code