Article 310 Conductors for General Wiring

Article 310 is to be used for general requirements for wiring, but not in areas where it is part of an integral device such as a motor, motor controller, or where covered in another part of the NEC®.

Conductors must be insulated and made of copper, copper-clad aluminum, or aluminum unless otherwise specified in the NEC®.

Any conductor installed in a raceway must be stranded if it is 8 AWG or larger. This is not required if it is permitted or required in another part of the NEC®.

Conductors 1/0 AWG or larger and having each phase, polarity, neutral, or grounded circuit conductor can be run in parallel if they are the same length, the same size, and of the same material with the same insulation and terminated the same way. They must be electrically joined at both ends. The cables or raceways must have the same physical characteristics if the conductors are run in separate cables or raceways. The same numbers of conductors have to be used in each raceway or cable. The conductors of one phase, neutral, or ground do not have to have the same physical characteristics as those of another to achieve balance. There are four exceptions to this:

1. Section 620.12(A)(1), Exception.

2. If smaller than 1/0 AWG, used for control power to indicating instruments, relays, and similar devices, and in the same raceway, each individual conductor can carry the entire load and the overcurrent rating is not larger than the ampacity of any single conductor.

3. Sizes smaller than 1/0 AWG are allowed for frequencies of 360 Hz and higher provided that Exception 2 is met.

4. With engineering supervision 2 AWG or larger grounded neutral conductors can be run parallel in existing installations.

Minimum sizes are shown in Table 310.5 except as permitted in other sections of the Code®. For voltages up to 2000, it is 14 AWG for copper and 12 AWG for aluminum or copper-clad aluminum.

In dry locations any type of insulated conductor or cable noted in the Code can be used. Use only Type RHW, TW, THW, MTW, RHW-2, THW-2, THHW, THHW-2, THWN, THWN-2, XHHW, XHHW-2, ZW, or a type listed as such for a wet location, or a moisture impervious metal-sheathed conductor. For dry and damp locations use Types FEP, FEPB, MTW, PFA, RHW, RHH, RHW-2, SA, XHH, XHHW, XHHW-2, THHN, THW-2, THW, THHW, THHW-2, THWN, THWN-2, TW, Z, or ZW. If conductors are exposed to direct sunlight the cables must be listed as sunlight resistant; or conductors must be listed and identified as sunlight resistant; or insulated conductors can be covered with tape, sleeving, or similar material that is listed for the application and identified as sunlight resistant.

All conductors and cables must have marked on them the maximum rated voltage, the proper letter type, the manufacturer's name or trademark, the AWG size or circular mil area, and cable assemblies where the neutral conductor is smaller than the ungrounded conductor. The size markings should be on the surface and spaced not more than 610 mm (24 in.) for eight types of cables. Other markings may be spaced 1.0 m (40 in.) apart. Certain cables can use marker tape inside the cable. Refer to this section in the NEC® for specific types where marker tape is permitted, which cables can have the size located elsewhere, and suffix designations for number of conductors. Special characteristics can be surface-marked on conductors listed in Chapter 3.

(A) Grounded Conductors. Insulated grounded conductors must be identified as noted in Section 200.6.

(B) Equipment Grounding Conductors. Equipment grounding conductors must be as noted in Section 250.119.

(C) Ungrounded Conductors. These must be distinguished from grounded and grounding conductors.

Insulated conductors must be in accordance with Tables 310.13 and 310.61 through 310.64.

(A) General

(1) Tables or Engineering Supervision. Ampacities can be determined by tables as provided in (B) or under engineering supervision as noted in 310.15(C).

(2) Selection of Ampacity. If more than one ampacity is determined from the tables or calculations, the lowest value must be used. There is one exception for adjacent portions of a circuit under certain circumstances.

(B) Tables. Ampacities are noted in the Allowable Ampacities Tables 310.16 through 310.19 and 310.20 through 310.23 for conductors rated 0 to 2000 volts. They can be modified by paragraphs (1) through (6).

[*] See Section 240.4(D).

(1) General. For explanation of Type Letters, and for recognized size of conductors for the various conductor insulations, see Section 310.13. For installation requirements, see Sections 310.1 through 310.10, and the various articles of this Code. For flexible cords, see Tables 400.4, 400.5(A), and 400.5(B).

(2) Adjustment Factors

(a) More than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where multi-conductor cables or single conductors are bundled or stacked longer than 600 mm (24 in.) without proper spacing and not in a raceway, the ampacities shall be reduced as shown in the following table Table 310.15(B)(2)(a). In a parallel set of conductors each conductor is counted as a current-carrying conductor. The exceptions to this are listed on the bottom of the table.

FPN Note 1: See Appendix B, Table B.310.11 for adjustment factors for more than three current-carrying conductors in a raceway or cable with load diversity.

FPN Note 2: For correction factors for conductors in sheet metal auxiliary gutters see 366.23(A) and 376.22 for conductors in metal raceways.

Exception No. 1. When conductors of different systems, as provided in Section 300.3, are installed in a common raceway or cable the derating factors shown in Table 310.15(B)(2)(a) shall apply to the number of power and lighting (Articles 210, 215, 220, and 230) conductors only.

Exception No. 2. For conductors installed in cable trays, the provisions of Section 392.11 shall apply.

Exception No. 3. Derating factors shall not apply to conductors in nipples having a length not exceeding 610 mm (24 in.).

Exception No. 4. Derating factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit or rigid nonmetallic conduit having a length not exceeding 3 mm (10 ft.) and the number of conductors does not exceed 4.

Exception No. 5. Adjustment factors shall not apply to Type AC cable or to Type MC cable without an overall outer jacket under the following conditions:

(a) Each cable has not more than three current carrying conductors.

(b) The conductors are 12 AWG copper.

(c) Not more than 20 current carrying conductors are bundled, stacked, or supported on "bridle rings."

A 60 percent adjustment factor shall be applied where the current carrying conductors in these cables that are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing exceeds 20.

(b) More than One Conduit, Tube, or Raceway. Spacing between conduits, tubing, or raceways shall be maintained.

(3) Bare Conductors or Covered Conductors

Where bare or covered conductors are used with insulated conductors, their allowable ampacities shall be limited to that permitted for the adjacent insulated conductors.

(4) Neutral Conductor

1. A neutral conductor that carries only the unbalanced current from other conductors of the same circuit need not be counted when applying the provisions of Section 310.15(B)(2)(a).
2. In a three-wire circuit consisting of two-phase wires and the neutral of a four-wire, three-phase wye-connected system, a common conductor carries approximately the same current as the line to neutral load currents of the other conductors and shall be counted when applying the provisions of Section 310.15(B)(2)(a).
3. On a four-wire, three-phase wye circuit where the major portion of the load consists of nonlinear loads there are harmonic curents present in the neutral conductor and the neutral shall be considered to be a current-carrying conductor.

(5) Grounding or Bonding Conductor

A grounding or bonding conductor shall not be counted when applying the provisions of Section 310.15(B)(2)(a).

(6) 120/240 Volts, Three-Wire, Single-Phase Dwelling Services and Feeders

For individual dwelling units of one family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted to be utilized as 120/240-V, three-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For applications of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch circuit panelboard(s). The feeder conductors to a dwelling unit shall not be required to be larger than its service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided that the requirements of Sections 215.2, 220.61, and 230.42 are met.

(C) Engineering Supervision.

With engineering supervision the following formula can be used to calculate ampacities:

where

FPN: See Annex B for examples of formula application.

(A) Definitions.

(B) Ampacities of Conductors Rated 2001 to 35,000 Volts.

(C) Tables.