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Code Quiz

Nov 7, 2005 4:09 PM, By Steven Owen

Q. What is the minimum cable tray width required for an installation that consists of six sets of 1,000 kcmil conductors (rated 600V) bundled in six sets of three-each conductors in an uncovered ladder type tray? The bundles are configured in the form of a triangle – two side-by-side on the bottom with the third sitting on top of the two. The cable tray is not listed for grounding and will not be used as the equipment grounding conductor. As such, there are a total of 19 conductors in the tray. The outer diameter of all 19 conductors is 1.44 inches. Do you know the proper name of this 19th conductor?

A) 17.28 inches minimum / equipment bonding conductor

B) 18.72 inches minimum / equipment grounding conductor

C) 25.92 inches minimum / grounded conductor

D) 27.36 inches minimum / grounding electrode conductor

Answer: B

Explanation: Per a change in the 2005 NEC, 392.10(A)(1) was revised for single conductors (1,000 kcmil and larger) installed in cable trays. Where all of the cables are 1,000 kcmil or larger, the sum of the diameters of all single conductor cables shall not exceed the cable tray width, and the cables shall be installed in a single layer. It is permitted to install conductors that are bound together to comprise a circuit group in other than a single layer.

Therefore, the solution to this problem is as follows: Six bundled groups of three conductors, plus one equipment grounding conductor make a total of nineteen conductors. The bundling arrangement for these conductors ― installed in parallel, as a single feeder ― leaves us with a total of 12 conductors on the bottom row. Add to this the equipment grounding conductor and you end up with a grand total of 13 conductors on the bottom row of the tray.

13 X 1.44 = 18.72 inches, minimum.

Note that you do not need to account for the six conductors that are stacked (i.e., bundled) on top of the six sets of two conductors on the bottom row when making your final calculations.

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.