Annex B Application Information for Ampacity Calculation

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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
ANNEX BAnnex B: AmpacitiesTable B.310.1ContinuedTemperature Rating of Conductor. (See Table 310.13.)60°C (l40°F) 7SoC (167°F) 90°C (194°F) 60°C (140°F) 7SoC (167°F) 90°C (194°F)Types THHN,TypesTHHN,THHW,THHW,THW·2,THW-2,THWN-2,THWN·2,RHH,RHH,Types RHW, RWH·2,RWH-2,THHW, USE-2, Types RHW, USE·2,THW, XHHW, THHW,THW, XHHW,THWN, XHHW-2, THWN, XHHW-2,Types TW, UF XHHW,ZW ZW-2 Type TW XHHW ZW-2Size (AWG or ALUMINUM OR COPPER-CLAD Size (AWG orkcmil)COPPERALUMINUMkcmil)I600 343 413 468 279 335 378 600700 376 452 514 310 371 420700750 387 466529 321 384 435 750800 397479 543 331 397450800900 415 500 570 350421 477 9001000 448 542 617 382 460 5211000Correction FactorsAmbient For ambient temperatures other than 30°C (86°F), multiply the ampacities shown above by the AmbientTemp. (0C) appropriate factor shown below. Temp. eF)21-25 1.08 1.05 1.04 1.081.05 1.04 70-7726-30 1.00 1.00 1.001.00 1.001.00 79-8631-350.91 0.94 0.96 0.91 0.940.96 88-9536-400.820.88 0.91 0.82 0.88 0.91 97-10441-450.71 0.82 0.87 0.710.82 0.87 106-11346-50 0.58 0.750.82 0.58 0.750.82 115-12251-55 0.410.67 0.76 0.410.67 0.76 124-13156-60- 0.58 0.71- 0.58 0.71 133-14061-70 -0.330.58 - 0.330.58 142-15871-80 - -0.41 -- 0.41 160-176*Unless otherwise specifically permitted elsewhere in this Code, the overcurrent protection for theseconductor types shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for10 AWG copper; or 15 amperes for 12 AWG and 25 AWG amperes for 10 AWG aluminum andcopper-clad aluminum.2008 Edition NATIONAL ELECTRICAL CODE 70--691
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Annex B Application Information for Ampacity Calculation

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