Annex B Application Information for Ampacity Calculation

Annex B: AmpacitiesANNEXBThe lower family of curves shows the relationship betweenRho and load factor that will give substantially thesame ampacity as the indicated value of Rho at 100 percentload factor.As an example, to find the ampacity of a 500 kcmilcopper cable circuit for six electrical ducts as shown inTable B.31O.5: At the Rho = 60, LF = 50, I} =583; for Rho= 120 and LF = 100, 1 2 = 400. The ratio IiII = 0.686.Locate Rho = 90 at the bottom of the chart and follow the90 Rho line to the intersection with 100 percent load factorwhere the equivalent Rho = 90. Then follow the 90 Rholine to Iillratio of 0.686 where F = 0.74. The desiredampacity = 0.74 x 583 = 431, which agrees with the tablefor Rho =90, LF = 100.To determine the ampacity for the same circuit whereRho =80 and LF =75, using Figure B.31 0.1, the equivalentRho = 43, F = 0.855, and the desired ampacity = 0.855x 583 = 498 amperes. Values for using Figure B.310.1 arefound in the electrical duct bank ampacity tables of this annex.Where the load factor is less than 100 percent and canbe verified by measurement or calculation, the ampacity ofelectrical duct bank installations can be modified as shown.Different values of Rho can be accommodated in the samemanner.Table B.310.1 Ampacities of Two or Three Insulated Conductors, Rated 0 Through 2000 Volts,Within an Overall Covering (Multiconductor Cable), in Raceway in Free AirBased on Ambient Air Temperature of 30°C (86°F)Temperature Rating of Conductor. (See Table 310.13.)60°C (140°F) 7SoC (167°F) 90°C (194°F) 60°C (140°F) 7SoC (167°F) 90°C (194°F)Types TW, UFTypes RHW,THHW,THW,THWN,XHHW,ZWTypes THHN,THHW,THW-2,THWN·2,RHH,RWH·2,USE·2,XHHW,XHHW-2,ZW-2TypeTWTypes RHW,THHW, THW,THWN,XHHWTypes THHN,THHW,THW·2,THWN·2,RHH,RWH·2,USE·2,XHHW,XHHW-2,ZW-2Size (AWG orkcmil)COPPERALUMINUM OR COPPER-CLADALUMINUMSize (AWG orkcmil)141210816*20*27*3618*24*33*4321*27*36*48-16*21*28-18*25*33-21*28*3714121086432I48667688102587990105121658910211913738515969804561708395516979931066432I1102/03/04/0121138158187145166189223163186214253941081241471131291471761271461671971102/03/04/025030035040050020523425527431524528130532837827631734537142716018520221825419222124226130321725027329534225030035040050070-690NATIONAL ELECTRICAL CODE2008 Edition