**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 necexpert@aol.com.*