Assume that an ampacity of 60Ais needed in a circuit with a 75 - PDF document

Assume that an ampacity of 60Ais needed in a circuit with a 75°C termination45°C. First, since one termination temperature rating is higher than the other,the lowest one must be used, which is 60°C. The first choice might be a 4AWG TW conductor with an ampacity of 70Aat 60°C. However, in the NEC®the Correction Factors table at the bottom of conductor ampacity Table 310.16reveals that the 70Aampacity must be derated, due to the 45°C ambient, by afactor of 0.71. This yields a new ampacity of 49.7, which is less than therequired 60. This is where a conductor with a higher temperature ratingbecomes useful. A4 AWG THHN conductor has a 90°C ampacity of 95A.Again, looking at the table at the bottom of Table 310.16, a factor of .87 must 57 ©2005 Cooper Bussmann Conductors & Terminations three current-carrying conductors in a raceway, cable, or earth (directlyTable310.16). The note for 14, 12, and 10 AWG conductors is a very important note ConductorAmpacity For Temperature RatedSize AWGCopper Conductors 60¡C75¡C90¡C14*20*20*25*12*25*25*30*10*30*35*40*8405055655657547085953851001102951151301110130150 AmbientFor ambient other than 30¡C, multiply conductor allowable AmbientTemp. ¡Campacities by factors below (NEC¨Table 310.16)Temp. ¡F 60¡C75¡C90¡C21-251.081.051.0470-7726-301.001.001.0078-8631-350.910.940.9687-9536-400.820.880.9196-10441-450.710.820.87105-11346-500.580.750.82114-12251-550.410.670.76123-13156-60–0.580.71132-14061-70–0.330.58141-15871-80––0.41159-176 conductors: 14 AWG - 15A, 12 AWG - 20A, 10 AWG - 30A. Depending on thecorrection factors may be less than the values in Table 310.16. In those cases,10AWG in 50°C ambient would have a derating factor of 0.75, which results inan ampacity of 26.25 (35Ax 0.75). So in this case, the ampacity would be240.4(B) would allow the next standard fuse, which is a 30Afuse. The 30Afor this same conductor is 0.94, so the new ampacity is 32.9A(35Ax 0.94).However, a 35Afuse can not be utilized because NEC® 240.4(D) limits the ambient greater than 30°C. The correction factors for the conductor allowableTable 310.16.are below. Conductor Ampacity Correction Factors For Ambient Temperatures# Of Current-% Values in NECAmpacity TablesCarrying310.16 to 310.19 As Adjusted for Ambient T emperature if Necessary 4 … 6807 … 97010 … 205021 … 304531 … 404041 & greater35Termination Ratingsrating and ampacity. Shown below are three common termination ratings andthe rules. Remember, from the example above, the conductor ampacity mayrated conductor, but the ampacity of the conductorconductor, but the ampacity of the conductor must bebased as if conductor is rated 75°C. A60°C conductors at 75°C ampacity. If 90°C or higher tem-