Q. What is the maximum number of multi-conductor cables (for control circuits only) permitted in an 18-inch wide by 4-inch deep ladder type cable tray? The outer diameter of each conductor is 0.5 inches and there will be no equipment-grounding conductor installed within the cable tray. Does the number of allowed cables change if the cable tray has a solid bottom? If so, what is the new number?
A) 144 / 144
B) 183 / 146
C) 184 / 147
D) 366 / 366
Answer: B
Explanation: First, convert the outer diameter of the ½-inch cable to square inches. Why? Because the NEC calculations for determining maximum raceway and cable tray fill are based on the square inch areas of conductors. A simple, but easy formula to use in converting an outer diameter dimension to square inch area is:
Per 392.9(B) of the 2005 NEC, a maximum allowable fill of 50% of the total cross sectional area of a ladder or ventilated cable tray is permitted. The total cross sectional area of cable tray in this example is:
18 inches x 4 inches = 72 inches
50% of 72 inches = 36 inches
36 inches ÷ 0.19635 square inches = 183.34
The 0.34 must be dropped. Rounding up to 184 would be slightly more than 50% fill. Therefore, the total number of cables allowed in the tray is 183.
Per 392.9(D) of the 2005 NEC, a maximum allowable fill of 40% of the total cross sectional area of a solid bottom cable tray is permitted.
40% of 72 inches = 28.80 inches
28.80 inches ÷ 0.19635 = 146.67
Even though 0.67 is greater than 0.50, we still cannot round up. The 0.67 must be dropped. Rounding up to 147 would be slightly more than 40% fill. Therefore, the total number of cables allowed in the tray is 146.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at [email protected].
