92 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. 6.3 Allowable Ampacities in Conductors The tables found in Section 310.15 are used to compute the size and ampacity of conductors rated under 2000 volts. As we proceed, refer to the tables, which are included in the Reference Section of this text. Each table is used for a dif- ferent type of conductor: • Table 310.15(B)(16)—Used for up to three insulated conductors installed in raceway, as multiconductor cable within raceway, in air, or directly buried. This table is based on a surrounding temperature of 86°F (30°C). • Table 310.15(B)(17)—Ampacities for single insulated conductors installed in free air. The ambient temperature is also 86°F (30°C). Outside, overhead wiring is sized using this table. • Table 310.15(B)(18)—This table contains ampacities for three single insulated conductors in raceway or in cable whose insulation range temperatures are 302°F to 482°F (150°C to 250°C) and based on an ambient temperature of 104°F (40°C). Conductors used for general wiring have an insulation range of 140°F to 194°F (60°C to 90°C). • Table 310.15(B)(19)—Ampacities for single insulated conductors in free air having conductor insulation temperature ranging from 302°F to 482°F (150°C to 250°C). This table is based on an ambient temperature of 104°F (40°C). Section 310.15 further explains and modifiesfi the values found in these tables. To fi nd the ampacity of a conductor, simply fi read down the vertical column indicating the type of conductor, its temperature rating, and the insulation type to the horizontal row indi- cating its size. Normally, conductors are selected for a known load. The ampacity of the selected conductor must be greater than or equal to the specified load. fi SAMPLE PROBLEM 6-2 Problem: Determine the rated ampacity t of each of three 12 AWG type THW copper conductors o installed in conduit. Solution: In Table 310.15(B)(16), a value e of 25 amps is found in the 12 AWG row of the copper op THW column. The allowable ampacity of each h of the 12 AWG copper conductors is 25 amps. Table 310.15(B)(16) (formerly Table 310.16) Al ll owable Ampacities of Insulated Conductors Rated Up to and Inclu ud ing 2000 Volts, 60°C ° Through 90°C ° (140°F ° Through h 194°F), ° Not More than Three Current-Carrying Conductors in Ra ac eway, Cable, or Earth (Directly Buried), Based on Ambient Temperature T of 30°C ° (86°F)*° Temperatu ur e Rating of Conductor [See Table 310.104(A).] 60°C ° 75°C ° 90°C ° 60°C ° 75°C ° 90°C° (140°F) ° (167°F) ° (194°F) ° (140°F) ° (167°F) ° (194°F)° Types TBS, SA, SIS, FEP, FEPB, MI, RHH, Types TBS, SA, SIS, RHW-2, THHN, THHN, THHW HW, Types RHW, THHW, THW-2, Types RHW, THW-2, THWN WN- 2, THHW, THW, THWN-2, USE-2, XHH, THHW, THW, RHH, RHW-2, U US E-2, THW , Types TW, THWN, XHHW, XHHW, XHHW-2, Types TW, THWN, XHHW, XHH, XHH HW, XHHW UF USE, ZW ZW-2 UF USE XHHW-2, Z ZW- 2 COPPER E ALUMINUM OR COPPER-CLAD ALUMIN NU M COPPER 14 — — — — — 18 — — 18 — — — — — 16 — — 25 — — — — — 14** 15 20 30 15 20 2 25 12** 12** 20 25 12** 10** 30 35 40 25 30 3 35 10** 10** 35 8 40 50 55 35 40 45 8 Size AWG or kcmil Size AWG or kcmil 25 35 20 Copyright © 2016 NFPA