2011 NEC CODE REFERENCE

CHAPTER:3 Wiring Methods and Materials

ARTICLE: 310Conductors for General Wiring

SECTION: II. Installation

310.15(B) Ampacities for Conductors Rated 0–2000 Volts

(3)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 single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.

Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the adjustment factors shown in Table 310.15(B)(3)(a) shall apply only to the number of power and lighting conductors (Articles 210, 215, 220, and 230).

The basis for the last paragraph of 310.15(B)(3)(a) is the assumption that the watt loss (heating) from any control and signal conductors in the same raceway or cable will not be enough to significantly increase the temperature of the power and lighting conductors. See 725.48 and 725.133 for limitations on the installation of control and signal conductors in the same raceway or cable as power and lighting conductors.

Informational Note No. 1: See Annex B, Table B.310.15(B)(2)(11), for adjustment factors for more than three current-carrying conductors in a raceway or cable with load diversity.

Informational Note No. 2: See 366.23(A) for adjustment factors for conductors in sheet metal auxiliary gutters and 376.22(B) for adjustment factors for conductors in metal wireways.

(1) Where conductors are installed in cable trays, the provisions of 392.80 shall apply.

(2) Adjustment factors shall not apply to conductors in raceways having a length not exceeding 600 mm (24 in.).
(3) Adjustment 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, rigid polyvinyl chloride conduit (PVC), or reinforced thermosetting resin conduit (RTRC) having a length not exceeding 3.05 m (10 ft.), and if the number of conductors does not exceed four.
Notes: Section 310.15(B)(3)(a)(3) defines conditions under which derating factors do not apply, as illustrated in Exhibit 310.3. The conductors have physical protection (conduit) that does not exceed 10 ft. in length, and the number of conductors does not exceed four.

Exhibit 310.3 An application of 310.15(B)(3)(a)(3)
(4) Adjustment factors shall not apply to Type AC cable or to Type MC cable under the following conditions:

a. The cables do not have an overall outer jacket.

b. Each cable has not more than three current-carrying conductors.

c. The conductors are 12 AWG copper.

d. Not more than 20 current-carrying conductors are installed without maintaining spacing, are stacked, or are supported on “bridle rings.”

CALCULATION EXAMPLE
A commercial office space requires fourteen 277-volt fluorescent lighting circuits to serve a single open office area. The office area lighting is assumed to be a continuous load, and the office ambient temperature will not exceed 30°C (86°F). Each circuit will be arranged so that it has a calculated load not exceeding 16 amperes. The selected wiring method is Type MC cable, 3-conductor (with an additional equipment grounding conductor), 12 AWG THHN copper. Each individual MC cable will contain a 3-wire multiwire branch circuit. To serve the entire area, this arrangement requires a total of seven Type MC cables bundled for a distance of about 25 ft., without maintaining spacing between them where they leave the electrical room and enter the office area.
Determine the ampacity of each circuit conductor in accordance with 310.15, applying 310.15(B)(3)(a)(4) to account for the bundled cables. Then determine the maximum branch-circuit overcurrent protection permitted for these bundled MC cables.
SOLUTION
Step 1. To apply 310.15(B)(3)(a)(4), first determine the quantity of current-carrying conductors. According to 310.15(B)(5), equipment grounding conductors are not counted as current-carrying conductors. According to 310.15(B)(4)(c), fluorescent lighting is considered a nonlinear load, so the grounded conductor of each Type MC cable must be counted as a current-carrying conductor:

Because the quantity of current-carrying conductors exceeds 20, a 60 percent adjustment factor is required by 310.15(B)(3)(a)(5).
Step 2. Determine the ampacity of each current-carrying conductor given the MC cables with more than 20 current-carrying conductors being bundled. From Table 310.15(B)(16):

Because the actual calculated load is 16 amperes of continuous load, 210.19(A)(1) is applicable. The conductors must have an ampacity equal to or greater than the load before the adjustment factor is applied. Because the ampacity of the conductors after the adjustment factor is applied is 18 amperes, no further adjustment is necessary and the conductors are suitable for this installation.
Step 3. Finally, determine the maximum size of the overcurrent protection device permitted for these bundled MC cable branch circuits. Section 240.4(B) permits the use of the next higher standard rating of overcurrent protection device. Therefore, although the conductors have a calculated ampacity of 18 amperes, a 20-ampere overcurrent protective device is permitted. In addition, and of significance, the 20-ampere overcurrent protective device is in compliance with 210.20(A), given that the actual 16-ampere continuous load would require a 20-ampere overcurrent protective device, based on the listing of the overcurrent device.
(5) An adjustment factor of 60 percent shall be applied to Type AC cable or Type MC cable under the following conditions:

a. The cables do not have an overall outer jacket.

b. The number of current carrying conductors exceeds 20.

c. The cables are stacked or bundled longer that 600 mm (24 in) without spacing being maintained.

Table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable

Number of Conductors* / Percent of Values in Table 310.15(B)(16) through Table 310.15(B)(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 adjusted in accordance with 310.15(B)(5) and (6).

(b) More Than One Conduit, Tube, or Raceway

Spacing between conduits, tubing, or raceways shall be maintained.

Spacing is normally maintained between individual conduits in groups of conduit runs from junction box to junction box because of the need to separate the conduits where they enter the junction box, to allow room for locknuts and bushings.

(c) Circular Raceways Exposed to Sunlight on Rooftops

Where conductors or cables are installed in circular raceways exposed to direct sunlight on or above rooftops, the adjustments shown in Table 310.15(B)(3)(c) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b).

Informational Note: One source for the average ambient temperatures in various locations is the ASHRAE Handbook — Fundamentals.

The conductors in outdoor conduits installed on or near the surface of the roof are subject to a significant increase in temperature when the roof is exposed to direct sunlight. The closer the conduit is to the roof, the greater the ambient temperature adjustment. See the following example and Exhibit 310.4.
CALCULATION EXAMPLE
A feeder installed in intermediate metal conduit, Type IMC, supplies a panelboard inside a mechanical room on top of a commercial building in the St. Louis, MO, area, as shown in Exhibit 310.4. The calculated load on the feeder is 175 amperes. The lateral portion of the raceway is secured to elevated supports, crosses the rooftop, and is exposed to sunlight. The elevated supports are not less than 15 in. above the finished rooftop surface. Determine the minimum size circuit conductor using aluminum 90°C XHHW-2 insulation, taking into consideration only the exposure to sunlight. None of the loads are continuous, and the neutral is not considered a current-carrying conductor. The design temperature is based on the averaged June, July, and August 2 percent design temperature from the 2005 ASHRAE Handbook.
SOLUTION
Step 1. Determine the ambient temperature (compensated for proximity of conduit to the rooftop exposure to sunlight):
a. Ambient temperature (compensated for proximity of conduit to the rooftop exposure to sunlight) = design temperature + value from Table 310.15(B)(3)(c).
b. Design temperature for St. Louis area = 94°F.
c. Temperature adjustment from Table 310.15(B)(3)(c) for a raceway elevated 15 in. above rooftop = 25°F
d. Compensated ambient temperature:

Step 2. Determine the temperature correction factor for this application.
From Table 310.15(B)(16) for aluminum 90°C XHHW-2 insulated conductors, select the proper temperature correction factor. Using the aluminum 90°C column and the temperature correction factor row for 119°F, the temperature correction factor is 0.82.
Step 3. Using aluminum 90°C XHHW-2 insulated conductors, determine the proper conductor size to be used in the IMC to supply the 175-ampere load.
a. Because the load is calculated at 175 amperes noncontinuous, and the neutral conductor is not considered to be a current-carrying conductor, the conductor ampacity is calculated as follows:

b. Now, moving back to the 90°C column of Table 310.15(B)(16), select a conductor not less than 213 amperes, or a minimum size conductor of 250-kcmil aluminum XHHW-2:

c. Verify that the conductor ampacity at 75°C is sufficient for the calculated load to comply with terminal temperature requirements of 110.14(C): The 75°C aluminum column of Table 310.15(B)(16) ampacity equals 205 amperes, which is greater than the 175-ampere calculated load.
Using a very specific set of circumstances, this example demonstrates that roughly an 18 percent loss of usable conductor material occurred. This loss is due solely to high ambient heat present where a cable or raceway is subjected to sunlight and is installed within a specific proximity to the rooftop.

Exhibit 310.4 An IMC conduit crossing a rooftop and exposed to sunlight. (Courtesy of the International Association of Electrical Inspectors)

Table 310.15(B)(3)(c) Ambient Temperature Adjustment for Circular Raceways Exposed to Sunlight on or Above Rooftops

Temperature Adder
Distance Above Roof to Bottom of Conduit / °C / °F
0–13 mm (½ in.) / 33 / 60
Above 13 mm (½ in.)–90 mm (3½ in.) / 22 / 40
Above 90 mm (3½ in.)–300 mm (12 in.) / 17 / 30
Above 300 mm (12 in.)–900 mm (36 in.) / 14 / 25

Informational Note to Table 310.15(B)(3)(c): The temperature adders in Table 310.15(B)(3)(c) are based on the results of averaging the ambient temperatures.

Rev. 1/25/12 2011 NEC Code Reference 310.15(B)(3)

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