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Article 310 Conductors for General Wiring |
Article 310: Conductors for General Wiring
Part I. General
310.1 Scope.
This article covers general requirements for conductors rated up to and including 2000 volts and their type designations, insulations, markings, mechanical strengths, ampacity ratings, and uses. These requirements do not apply to conductors that form an integral part of equipment, such as motors, motor controllers, and similar equipment, or to conductors specifically provided for elsewhere in this Code.
Informational Note: For flexible cords and cables, see Article 400. For fixture wires, see Article 402.
Part II. Construction Specifications
310.4 Conductor Constructions and Applications.
Insulated conductors shall comply with #Table 310.4(A) Conductor Applications and Insulations Rated 600 Volts and #Table 310.4(B) Thickness of Insulation for Nonshielded Types RHH and RHW Solid Dielectric Insulated Conductors Rated 2000 Volts.
Informational Note: Thermoplastic insulation may stiffen at temperatures lower than −10°C (+14°F). Thermoplastic insulation may also be deformed at normal temperatures where subjected to pressure, such as at points of support.
Table 310.4(A) Conductor Applications and Insulations Rated 600 Volts
Table 310.4(B) Thickness of Insulation for Nonshielded Types RHH and RHW Solid Dielectric Insulated Conductors Rated 2000 Volts
Part III. Installation
310.10 Uses Permitted.
The conductors described in #310.4 Conductor Constructions and Applications. 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.
310.14 Ampacities for Conductors Rated 0 Volts – 2000 Volts.
310.14(A) General.
310.14(A)(1) Tables or Engineering Supervision.
Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15 or under engineering supervision, as provided in 310.14(B).
Informational Note No. 1: Ampacities provided by this section do not take voltage drop into consideration. See 210.19(A), Informational Note No. 4, for branch circuits and 215.2(A), Informational Note No. 2, for feeders.
310.15 Ampacity Tables.
310.15(A) General.
Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 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. The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).
Informational Note No. 1: Table 310.16 through Table 310.19 are application tables for use in determining conductor sizes on loads calculated in accordance with Part II, Part III, Part IV, or Part V of Article 220. Ampacities result from consideration of one or more of the following:
- (1) Temperature compatibility with connected equipment, especially the connection points.
- (2) Coordination with circuit and system overcurrent protection.
- (3) Compliance with the requirements of product listings or certifications. See 110.3(B).
- (4) Preservation of the safety benefits of established industry practices and standardized procedures.
Informational Note No. 2: For conductor area see Chapter 9, Table 8, Conductor Properties. Interpolation is based on the conductor area and not the conductor overall area.
Informational Note No. 3: For the ampacities of flexible cords and cables, see 400.5. For the ampacities of fixture wires, see 402.5.
Informational Note No. 4: For explanation of type letters used in tables and for recognized sizes of conductors for the various conductor insulations, see Table 310.4(A) and Table 310.4(B). For installation requirements, see 310.1 through 310.14 and the various articles of this Code. For flexible cords, see Table 400.4, Table 400.5(A)(1), and Table 400.5(A)(2).
310.15(B) Ambient Temperature Correction Factors.
310.15(B)(1) General.
Ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(1) or Table 310.15(B)(2), or shall be permitted to be calculated using Equation 310.15(B).
I\prime = I \sqrt{\frac{ T_c -T_{a}\prime }{ T_c - T_a }}
where:
I\prime= ampacity corrected for ambient temperatureI= ampacity shown in the tablesT_c= temperature rating of conductor (°C)T_{a}\prime= new ambient temperature (°C)T_a= ambient temperature used in the table (°C)
310.15(B)(2) Rooftop.
For raceways or cables exposed to direct sunlight on or above rooftops where the distance above the roof to the bottom of the raceway or cable is less than 23 mm (7⁄8 in.), a temperature adder of 33°C (60°F) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(1) or Table 310.15(B)(2).
Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment.
Informational Note: One source for the ambient temperatures in various locations is the ASHRAE Handbook -- Fundamentals.
Table 310.15(B)(1) Ambient Temperature Correction Factors Based on 30°C (86°F)
For ambient temperatures other than 30°C (86°F), multiply the ampacities specified in the ampacity tables by the appropriate correction factor shown below.
Table 310.15(B)(2) Ambient Temperature Correction Factors Based on 40°C (104°F)
For ambient temperatures other than 40°C (104°F), multiply the ampacities specified in the ampacity tables by the appropriate correction factor shown below.
310.15(C) Adjustment Factors.
310.15(C)(1) More than Three Current-Carrying Conductors.
The ampacity of each conductor shall be reduced as shown in Table 310.15(C)(1) where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables not installed in raceways are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
Table 310.15(C)(1) Adjustment Factors for More Than Three Current-Carrying Conductors
| Number of Conductors* | Percent of Values in Table 310.16 Through Table 310.19 as Adjusted for Ambient Temperature if Necessary |
|---|---|
| 4–6 | 80 |
| 7–9 | 70 |
| 10–20 | 50 |
| 21–30 | 45 |
| 31–40 | 40 |
| 41 and above | 35 |
*Number of conductors is the total number of conductors in the raceway or cable, including spare conductors. The count shall be adjusted in accordance with 310.15(E) and (F). The count shall not include conductors that are connected to electrical components that cannot be simultaneously energized.
310.15(C)(2) Raceway Spacing.
Spacing between raceways shall be maintained.
310.15(D) Bare or Covered Conductors.
Where bare or covered conductors are installed with insulated conductors, the temperature rating of the bare or covered conductor shall be equal to the lowest temperature rating of the insulated conductors for the purpose of determining ampacity.