310 – Conductors for General Wiring

Change at a Glance:

Article 310 was extensively reorganized to increase the usability of the article.

Part I. General
310.1 Scope
310.2 Definitions
310.3 Conductors

Part II. Construction Specifications
310.4 Conductor Constructions and Applications
310.6 Conductor Identification
310.8 Marking

Part III. Installation
310.10 Uses Permitted
310.12 Single-Phase Dwelling Services and Feeders
310.14 Ampacities for Conductors Rated 0-2000 Volts 310.15 Ampacity Tables
310.16 Allowable Ampacities of Insulated Conductors in Raceway, Cable, or Earth (Directly Buried)
310.17 Allowable Ampacities of Single-Insulated Conductors in Free Air.
310.18 Allowable Ampacities of Insulated Conductors in Raceway or Cable
310.19 Allowable Ampacities of Single-Insulated Conductors in Free Air
310.20 Ampacities of Conductors Supported on a Messenger
310.21 Ampacities of Bare or Covered Conductors in Free Air
310.60 Conductors Rated 2001 to 35,000 Volts

(See NEC for complete text)

310 – Ampacity Tables

Change at a Glance:

The ampacity tables will simply be titled as Table 310.16 through Table 310.21 (Example: Table 310.15(B)(16) will now be simply Table 310.16).

Article 310 Ampacity Tables.

Table 310.16 310.15(B)(16) (formerly Table 310.16) Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through 194°F), with– Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)*
[was Table 310.16(B)(16)]

Table 310.17 310.15(B)(17) (formerly Table 310.17) Allowable Ampacities of Single-Insulated Conductors Rated Up to and Including 2000 Volts in Free Air, Based on Ambient Temperature of 30°C (86°F)*
[was Table 310.16(B)(17)]

Table 310.18 310.15(B)(18) (formerly Table 310.18) Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 150°C Through 250°C (302°F Through 482°F). with Not More Than Three Current-Carrying Conductors in Raceway or Cable, Based on Ambient Air Temperature of 40°C (104°F)*
[was Table 310.16(B)(18)]

Table 310.19 310.15(B)(19) (formerly Table 310.19) Allowable Ampacities of Single-Insulated Conductors, Rated Up to and Including 2000 Volts, 150°C Through 250°C (302°F Through 482°F), in Free Air, Based on Ambient Air Temperature of 40°C (104°F)*
[was Table 310.16(B) (19)]

Table 310.20 310.15(B)(20) (formerly Table 310.20) Ampacities of Not More Than Three Single Insulated Conductors, Rated Up to and Including 2000 Volts, Supported on a Messenger, Based on Ambient Air Temperature of 40°C (104°F)*
[was Table 310.16(B)(20)]

Table 310.21 310.15(B)(21) (formerly Table 310.21) Ampacities of Bare or Covered Conductors in Free Air, Based on 40°C (104°F) Ambient, 80°C (176°F) Total Conductor Temperature, 610 mm/sec (2 ft/sec) Wind Velocity
[was Table 310.16(B)(21)]

310 – Ampacity for Conductors

Change at a Glance:

Revision throughout Article 310 occurred by removing the term “allowable” from allowable ampacities for conductors.

Example: 310.15 Ampacity Tables.

(A) General. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.15(B) (16) through Table 310.15(B)(19), and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21) Table 310.16 through Table 310.21, as modified by 310.15(B)(1) through (B)(7) 310.15(A) through (F) and 310.12. Under engineering supervision, ampacities of sizes not shown in ampacity tables for conductors meeting the general wiring requirements shall be permitted to be determined by interpolation of the adjacent conductors based on the conductor’s area. (B) Tables.

310.10 – Uses Permitted of XHHN, XHWN, and XHWN

Change at a Glance:

Type XHHN, XHWN, and XHWN-2 were added to the “Uses Permitted” locations and the ampacity tables based on appropriate temperature ratings.

310.10 Uses Permitted

The conductors described in 310.104 310.4 shall be permitted for use in any of the wiring methods covered in Chapter 3 and as specified in their respective tables or as permitted elsewhere in this Code.

(A) Dry Locations. Insulated conductors and cables used in dry locations shall be any of the types identified in this Code.

(B) Dry and Damp Locations. Insulated conductors and cables used in dry and damp locations shall be Types FEP, FEPB, MTW, PFA, RHH, RHW, RHW-2, SA, THHN, THW, THW- 2, THHW, THWN, THWN-2, TW, XHH, XHHW, XHHW-2, XHHN, XHWN, XHWN- 2, Z, or ZW.

(C) Wet Locations. Insulated conductors and cables used in wet locations shall comply with one of the following:
(1) Be moisture-impervious metal-sheathed
(2) Be types MTW, RHW, RHW-2, TW, THW, THW-2, THHW, THWN, THWN-2, XHHW, XHHW-2, XHWN, XHWN-2, or ZW
(3) Be of a type listed for use in wet locations

[These conductor types were also added to the ampacity tables (Table 310.16 through Table 310.21). See NEC for complete Code text.]

310.12 and Table 310.12 – Dwelling Unit Service and Main Power Feeder Conductors

Change at a Glance:

New dwelling unit service ampacity table from Informative Annex D, Example D7 added at 310.12 and text added indicating table permitted to be used if there are no temperature correction or adjustment factors needed.

310.12 Single-Phase Dwelling Services and Feeders.

For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single- phase, 120/240-volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (4) 310.12(A) through (D).
For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of 2 two ungrounded conductors and the neutral conductor from a 208Y/120-volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (3) 310.12(A) through (C).

(A) Services. For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

(B) Feeders. For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

(C) Feeder Ampacities. In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2) 310.12(A) or (B).

(D) Grounded Conductors. Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors, if the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.
Where correction or adjustment factors are required by 310.15(B)(2) or (3) 310.15(B) or (C), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor.
Informational Note No. 1: The service or feeder ratings addressed by this section are based on the standard ampere ratings for fuses and inverse time circuit breakers from 240.6(A).
Informational Note No. 2: See Example D7 in Annex D.

Table 310.12 Single-Phase Dwelling Services and Feeders

(See NEC and illustration provided for complete text)