xcc

the size of the conductor. Where the cross-sectional area of a conductor increases 50 percent (e.g., from 1000 to 1500 kcmil), a Type THW conductor ampacity increases only 80 amperes (less than 15 percent). A 100-percent increase (from 1000 to 2000 kcmil) causes an increase of only 120 amperes (appr...

the size of the conductor. Where the cross-sectional area of a conductor increases 50 percent (e.g., from 1000 to 1500 kcmil), a Type THW conductor ampacity increases only 80 amperes (less than 15 percent). A 100-percent increase (from 1000 to 2000 kcmil) causes an increase of only 120 amperes (approximately 22 percent). Generally, where cost is a factor, installation of two (or more) paralleled conductors per phase could be beneficial. The parallel connection of two or more conductors in place of using one large conductor depends on compliance with 310.10(G)(2) to ensure equal current division to prevent overloading any of the individual paralleled conductors. Where individual conductors are tapped from conductors in parallel, the tap connection must include all of the conductors in parallel for that phase. Tapping into only one of the parallel conductors would result in unbalanced distribution of tap load current between parallel conductors, resulting in one of the conductors carrying more than its share of the load, which could cause overheating and conductor insulation failure. For example, if a 250-kcmil conductor is tapped from a set of two 500-kcmil conductors in parallel, the splicing device must include both 500-kcmil conductors and the single 250-kcmil tap conductor. Informational Note to Exception No. 2. The word triplen refers to a third-order harmonic current, such as the third, sixth, ninth, and so on. The concern is limited to odd-number triplen harmonic currents, such as the third, ninth, and fifteenth, since they are additive currents in the neutral conductor and do not cancel. For more information on power quality and harmonics, see Chapter 10 of NFPA 70B, Recommended Practice for Electrical Equipment Maintenance. Conductor and Installation Characteristics. The paralleled conductors that comprise each ungrounded conductor, grounded conductor, neutral conductor, equipment grounding conductor, equipment bonding jumper, or supply-side bonding jumper shall comply with all of the following: Be the same length Consist of the same conductor material Be the same size in circular mil area Have the same insulation type Be terminated in the same manner ENHANCED CONTENT Collapse To avoid excessive voltage drop and to ensure equal division of current, different phase conductors must be located close together. Each phase conductor, grounded conductor, and the grounding conductor (if used) must also be grouped together in each raceway or cable. However, isolated phase installations are permitted underground where the phase conductors are run in nonmetallic raceways that are in close proximity. All conductors of the same phase or neutral are required by 310.10(G)(2) to be of the same conductor material. For example, if 12 conductors are paralleled for a 3-phase, 4-wire, 480Y/277- volt ac circuit, four conductors could be installed in each of three raceways. The NEC does not intend that all 12 conductors be copper or aluminum but does intend that the individual conductors in parallel for each phase, grounded conductor, and neutral be the same material, insulation type, length, and so forth. For example, the conductors in phases A and B might be copper, and those in phase C might be aluminum. Also, the three raceways are intended to have the same physical characteristics (e.g., three rigid aluminum conduits, three steel IMCs, three EMTs, or three nonmetallic conduits), not a mixture (e.g., two rigid aluminum conduits and one rigid steel conduit). See also 300.3(B)(1) for more on paralleled installations. Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors composing one paralleled set shall not be required to have the same physical characteristics as those of another paralleled set. ENHANCED CONTENT Collapse All parallel raceways or cables for a circuit are required to be of the same size, material, and length. In this case, “cables” means wiring method-type cables such as Type MC. The impedance of a circuit in an aluminum raceway or aluminum-sheathed cable differs from the impedance of the same circuit in a steel raceway or steel-sheathed cable; therefore, separate raceways and cables must have the same physical characteristics. Also, the same number of conductors must be used in each raceway or cable. See also 300.20 regarding induced currents in metal enclosures or raceways. Ampacity Correction or Adjustment. Conductors installed in parallel shall comply with 310.15(B)and (C). Equipment Grounding Conductors. Where parallel equipment grounding conductors are used, they shall be sized in accordance with 250.122. Sectioned equipment grounding conductors smaller than 1/0 AWG shall be permitted in multiconductor cables, if the combined circular mil area of the sectioned equipment grounding conductors in each cable complies with 250.122. Bonding Jumpers. Where parallel equipment bonding jumpers or supply-side bonding jumpers are installed in raceways, they shall be sized and installed in accordance with 250.102. ENHANCED CONTENT Collapse The equipment bonding jumper size requirements might be different from the requirements for equipment grounding conductors (EGCs). On the supply side of the service, the size of the bonding jumper is based on 250.102(C)(1), which is the same as the requirement in 250.66 for grounding electrode conductors (GECs). On the load side of the service, the size is based on 250.122, which is also the size requirement for EGCs. The 1/0 AWG minimum size limitation on paralleled conductors does not apply to the equipment bonding jumper. 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 310.12(A) through (D). For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of two ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.12(A) through (C). 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(A) shall be permitted to be applied. Table 310.12(A) Single-Phase Dwelling Services and Feeders Conductor (AWG or kcmil) Service or Feeder Rating (Amperes) Copper Aluminum or Copper-Clad Aluminum 100 4 2 110 3 1 125 2 1/0 150 1 2/0 175 1/0 3/0 200 2/0 4/0 225 3/0 250 250 4/0 300 300 250 350 350 350 500 400 400 600 Note: If no adjustment or correction factors are required, this table shall be permitted to be applied. 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(A) shall be permitted to be applied. Feeder Ampacities. In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B). 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) or (C), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor. Informational Note No. 1: See 240.6(A) for standard ampere ratings for fuses and inverse time circuit breakers. Informational Note No. 2: See Informative Annex D, Example D7. ENHANCED CONTENT Collapse The main service or feeder to a dwelling unit is permitted to be sized at 83 percent of the disconnect rating. The calculation is not based on the rating of the overcurrent device protecting the main feeder. The minimum disconnect rating for a dwelling unit is 100 amperes according to 225.39 and 230.79. This calculation applies only to conductors carrying 100 percent of the dwelling unit’s diversified load. If a 120/240-volt single-phase service supplies a one-family dwelling or an individual unit of a two-family or multifamily dwelling, the reduced conductor size is applicable to the service-entrance conductors or feeder conductors that supply the dwelling unit. The feeder conductors to a dwelling unit are not required to be larger than its service-entrance conductors. The exhibit below illustrates where Table 310.12(A) could be applied. The reduced conductor size permitted is applicable only to the service-entrance conductors run to each apartment from the meters. The reduced conductor size permitted is also applicable to the feeder conductors run to Apartment 3 in the exhibit from the service disconnecting means because that feeder carries the entire load. 310.14 Ampacities for Conductors Rated 0 Volts – 2000 Volts. (A) General. 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: See 210.19, Informational Note, for voltage drop on branch circuits that this section does not take into consideration. See 215.2(A)(2), Informational Note No. 2, for voltage drop on feeders that this section does not take into consideration. Informational Note No. 2: See NFPA 79-2021, Electrical Standard for Industrial Machinery, Table 12.5.1, for the allowable ampacities of Type MTW wire. (2) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the lowest value shall be used. Exception: Where different ampacities apply to portions of a circuit, the higher ampacity shall be permitted to be used if the total portion(s) of the circuit with lower ampacity does not exceed the lesser of 3.0 m (10 ft) or 10 percent of the total circuit. Informational Note: See 110.14(C) for conductor temperature limitations due to termination provisions. ENHANCED CONTENT Collapse Temperature Limitation of Conductors. No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a way, with respect to type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded. Informational Note No. 1: See Table 310.4(1) and Table 315.10(A)for the temperature rating of a conductor that is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The ampacity tables of Article 310 and the ampacity tables of Informative Annex B, the ambient temperature correction factors in 310.15(B), and the notes to the tables provide guidance for coordinating conductor sizes, types, ampacities, ambient temperatures, and number of associated conductors. The principal determinants of operating temperature are as follows: (1) Ambient temperature — ambient temperature may vary along the conductor length as well as from time to time. (2) Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents. (3) The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation. (4) Adjacent load-carrying conductors — adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation. Informational Note No. 2: Refer to 110.14(C) for the temperature limitation of terminations. ENHANCED CONTENT Collapse Most terminations are designed for 60°C or 75°C maximum temperatures. The higher-rated ampacities for conductors of 90°C, 105°C, and so forth cannot be used unless the terminals at which the conductors terminate have comparable ratings. Ambient temperature must be considered in determining the allowable ampacity of conductors. Conductors should have a rating above the anticipated maximum ambient temperature. The operating temperature of conductors should be controlled at or below the conductor rating by coordinating conductor size, number of associated conductors, and ampacity for the conductor rating and ambient temperature. Tables 310.16 through 310.20 have ampacities based on a 30°C or 40°C ambient temperature, as indicated in the table heading. Where the ambient temperature is different, Table 310.15(B)(1)(1) or Table 310.15(B)(1)(2) is used to correct the ampacity. If more than three conductors are installed without spacing to allow the adequate dissipation of heat, the additional adjustment shown in 310.15(C)(1) must also be applied. The basis for determining the ampacities of conductors for Tables 310.16 and 310.17 was the NEMA Report of Determination of Maximum Permissible Current-Carrying Capacity of Code Insulated Wires and Cables for Building Purposes, dated June 27, 1938. The basis for determining the ampacities of conductors for Tables 310.18 and 310.19 and the ampacity tables in Informative Annex B was the Neher–McGrath method. Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of Equation 310.14(B). [310.14(B)] code book image: 8e59c35e-aa9c-11ec-a46d-e9dd887336cb where: Tc = conductor temperature in degrees Celsius (°C) Ta = ambient temperature in degrees Celsius (°C) Rdc = dc resistance of 305 mm (1 ft) of conductor in microohms at temperature, Tc Yc = component ac resistance resulting from skin effect and proximity effect Rca = effective thermal resistance between conductor and surrounding ambient 310.15 Ampacity Tables. 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 circular-mil 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 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. (4) Preservation of the safety benefits of established industry practices and standardized procedures Informational Note No. 2: See Chapter 9, Table 8, Conductor Properties, for conductor area. Interpolation is based on the conductor circular-mil area and not the conductor overall area. Informational Note No. 3: See 400.5 for the ampacities of flexible cords and cables. See 402.5 for the ampacities of fixture wires. Informational Note No. 4: See Table 310.4(1) and Table 310.4(2) for explanation of type letters used in tables and for recognized sizes of conductors for the various conductor insulations. See 310.1 through 310.14 and the various articles of this Code for installation requirements. See Table 400.4, Table 400.5(A)(1), and Table 400.5(A)(2) for flexible cords. ENHANCED CONTENT Collapse Ampacity tables, particularly Table 310.16, do not consider all factors affecting ampacity. However, experience has proven the table values to be adequate for loads calculated in accordance with Article 220 because not all the load diversity occurring in most installations is specifically provided for in Article 220. If loads are not calculated in accordance with the requirements of Article 220, the table ampacities, even if corrected in accordance with ambient correction factors and the notes to the tables, might be higher than needed for the actual load. See also 310.14(B) and Informative Annex B for more on ampacities calculated under engineering supervision. Ambient Temperature Correction Factors. General. Ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(1)(1) or Table 310.15(B)(1)(2), or shall be permitted to be calculated using Equation 310.15(B)(1). [310.15(B) (1)] code book image: 967d0aeb-aa9c-11ec-a46d-e9dd887336cb where: I’ = ampacity corrected for ambient temperature I = ampacity shown in the tables Tc = temperature rating of conductor (°C) Ta’ = new ambient temperature (°C) Ta = ambient temperature used in the table (°C) 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 19 mm (3⁄4 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)(1) or Table 310.15(B)(1)(2). Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment. Informational Note: The ASHRAE Handbook — Fundamentals is one source for the ambient temperatures in various locations. Table 310.15(B)(1)(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. Ambient Temperature (°C) Temperature Rating of Conductor Ambient Temperature (°F) 60°C 75°C 90°C 10 or less 1.29 1.20 1.15 50 or less 11–15 1.22 1.15 1.12 51–59 16–20 1.15 1.11 1.08 60–68 21–25 1.08 1.05 1.04 69–77 26–30 1.00 1.00 1.00 78–86 31–35 0.91 0.94 0.96 87–95 36–40 0.82 0.88 0.91 96–104 41–45 0.71 0.82 0.87 105–113 46–50 0.58 0.75 0.82 114–122 51–55 0.41 0.67 0.76 123–131 56–60 — 0.58 0.71 132–140 61–65 — 0.47 0.65 141–149 66–70 — 0.33 0.58 150–158 71–75 — — 0.50 159–167 76–80 — — 0.41 168–176 81–85 — — 0.29 177–185 Note: Table 310.15(B)(1)(1) shall be used with Table 310.16 and Table 310.17 as required. Table 310.15(B)(1)(2) Ambient Temperature Correction Factors Based on 40ºC (104ºF) For ambient temperatures other than 40°C (104°F), multiply the ampacities specified in the ampacity tables by the appropriate correction factor shown below. Ambient Temperature (°C) Temperature Rating of Conductor Ambient Temperature (ºF) 60°C 75°C 90°C 150°C 200°C 250°C 10 or less 1.58 1.36 1.26 1.13 1.09 1.07 50 or less 11–15 1.50 1.31 1.22 1.11 1.08 1.06 51–59 16–20 1.41 1.25 1.18 1.09 1.06 1.05 60–68 21–25 1.32 1.2 1.14 1.07 1.05 1.04 69–77 26–30 1.22 1.13 1.10 1.04 1.03 1.02 78–86 31–35 1.12 1.07 1.05 1.02 1.02 1.01 87–95 36–40 1.00 1.00 1.00 1.00 1.00 1.00 96–104 41–45 0.87 0.93 0.95 0.98 0.98 0.99 105–113 46–50 0.71 0.85 0.89 0.95 0.97 0.98 114–122 51–55 0.50 0.76 0.84 0.93 0.95 0.96 123–131 56–60 — 0.65 0.77 0.90 0.94 0.95 132–140 61–65 — 0.53 0.71 0.88 0.92 0.94 141–149 66–70 — 0.38 0.63 0.85 0.90 0.93 150–158 71–75 — — 0.55 0.83 0.88 0.91 159–167 76–80 — — 0.45 0.80 0.87 0.90 168–176 81–90 — — — 0.74 0.83 0.87 177–194 91–100 — — — 0.67 0.79 0.85 195–212 101–110 — — — 0.60 0.75 0.82 213–230 111–120 — — — 0.52 0.71 0.79 231–248 121–130 — — — 0.43 0.66 0.76 249–266 131–140 — — — 0.30 0.61 0.72 267–284 141–160 — — — — 0.50 0.65 285–320 161–180 — — — — 0.35 0.58 321–356 181–200 — — — — — 0.49 357–392 201–225 — — — — — 0.35 393–437 Note: Table 310.15(B)(1)(2) shall be used with Table 310.18, Table 310.19, Table 310.20, and Table 310.21 as required. ENHANCED CONTENT Collapse Conductors in outdoor conduits or cables installed less than ¾ inches above the surface of the roof are subject to a significant increase in temperature when the roof is exposed to direct sunlight. Adjustment Factors. 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. 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(C)(1) shall apply only to the number of power and lighting conductors. Informational Note No. 1: See Informative Annex B 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 for adjustment factors for conductors and ampacity for bare copper and aluminum bars in auxiliary gutters and 376.22(B) for adjustment factors for conductors in metal wireways. (a) Where conductors are installed in cable trays, 392.80 shall apply. Adjustment factors shall not apply to conductors in raceways having a length not exceeding 600 mm (24 in.). 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