100 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. Goodheart-Willcox Publisher Figure 6-10. The allowable percentage of filled conduit area varies with the number of conductors. with conductors. The area that can be fi lled is fi based on the number of conductors. The dif- ferent percentages are a result of the geometry involved with circular conductors within a cir- cular conduit. See Figure 6-10. Once the allowable percentage of fill is fi known, Table 4 in Chapter 9 of the Code is used to fi nd the minimum allowable size for the type of fi raceway being used. Raceways larger than the minimum required size can be used, but this will normally be uneconomical. Allowable Conduit Fill Number of Conductors 1 1 2 Over 2 Allowable Fill 53% 31% 40% 40 % Diagram SAMPLE PROBLEM 6-8 Problem: One 12 AWG THHN, one 10 AWG TH T HH N, and three 8 AWG THHN conductors are to be ru un through a rigid metal conduit. Determine the smallest conduit size that can be used. Solution: The total conductor area is 0.1442 2 in2 (see Sample Problem 6-7). Table 1 shows that the c co n- ductor area must be 40% (or less) of the conduit n cross section. Under Rigid Metal Conduit in Table 4, whe en fi lled with more than two conductors, 3/4″ fi condui it can ″ carry conductors with a total area of 0.220 0 in2. This is the smallest size that can carry at least 0.1442 4 in2, so 3/4″ conduit is the best size. ″ Table 1 Percent of Cross Section of Conduit and nd Tubing for Conductors Number of All Conducto or Conductors Types 1 53 2 31 Over 2 40 Table 4 Continued Article 344 — Rigid Metal Conduit (RMC) — — — — — — — — — — — 204 0.314 63 0.097 81 0.125 108 0.166 . 122 0.188 353 0.549 109 0.170 141 0.220 187 0 0.2 91 212 0.329 573 0.887 177 0.275 229 0.355 303 0 0. 470 344 0.532 984 1.526 305 0.473 394 0.610 522 0.809 591 0.916 1333 2.071 413 0.642 533 0.829 707 1.098 800 1.243 2198 3.408 681 1.056 879 1.363 1165 1.806 1319 2.045 3137 4.866 972 1.508 1255 1.946 1663 2.579 1882 2.919 4840 7.499 1500 2.325 1936 3.000 2565 5 3.974 2904 4.499 6461 10.010 2003 3.103 2584 4.004 3424 4 5.305 3877 6.006 8316 12.882 2578 3.994 3326 5.153 440 08 6.828 4990 7.729 13050 20.212 4045 6.266 5220 8.085 69 916 10.713 7830 12.127 18821 29.158 5834 9.039 7528 11.663 99 97 5 15.454 11292 17.495 0.632 0.836 1.063 1.394 1.624 2.083 2.489 9 3.09 90 3.570 4.050 0 5. .073 6 6.0 93 12 16 21 27 35 41 53 63 78 91 103 129 155 16.1 21.2 27.0 35.4 41.2 52.9 63.2 78.5 90.7 102.9 128.9 154.8 3/8 /2 /4 1 1 /4 1 /2 2 2 /2 3 3 /2 4 5 6 Metric Designator Trade Size mm in. mm² in.² mm² in.² mm² in.² mm² in.² ² mm² in.² i n .² Nominal Internal Diameter T Tot al Area 100% 2 Wires 31% Over 2 Wires 40% 1 Wire 53% 60% 353 0 549 109 0 170 141 — 12 2 2 Copyright © 2016 NFPA