Four multiconductor power cables, all 4/0 AWG and larger, are to be installed in a single layer in a ladder cable tray. Their outside diameters are 1.55 inches, 1.55 inches, 1.85 inches and 2.05 inches. For multiconductor cables of 4/0 AWG and larger in this type of tray, the sum of the cable diameters may not exceed the width of the tray. Standard tray widths available are 6, 9, 12 and 18 inches. What is the narrowest tray that may be used?
- A6 inches
This rounds the 7.00-inch sum down to the nearest available width. The sum is a minimum the tray width has to meet or exceed, so a narrower tray is not permitted.
- B12 inches
This adds roughly half again to the sum for spacing between cables. Cables of this size in a single layer are permitted to touch, so no spacing allowance belongs in the calculation.
- 9 inches
- D18 inches
This doubles the sum on the assumption that each cable needs a diameter of clear space beside it. That spacing requirement belongs to other installations, not to this single-layer rule.
Why C is correct
Add the diameters: 1.55 + 1.55 + 1.85 + 2.05 = 7.00 inches. The tray width must be at least that sum, so a 6-inch tray is too narrow and the next standard width, 9 inches, is the smallest that works. The cables also have to lie in one layer, which is what makes a simple sum of diameters the right measure.
What this question is testing
Tests whether you apply the single-layer sum-of-diameters rule that governs large multiconductor cables in a tray rather than reaching for the percentage fill thinking that applies to raceways, and whether you round up to the next available tray width.
On the job
Tray sizing is normally settled at the design stage, but electricians are the ones who discover during installation that a change order added a cable the tray cannot take in a single layer. Stacking large cables to make them fit is a real field temptation and it defeats the whole basis of the tray ampacity tables, since the middle cable can no longer shed heat. The fix is a wider tray or a second run.
Memory technique
Side by side, one layer, add the diameters.
Exam tip
Large multiconductor cables in a tray: add the diameters, and the tray has to be at least that wide.
Where to look it up
Article 392 covers cable trays; the number-of-cables provisions separate multiconductor cables by size and by whether the tray is ladder, ventilated or solid bottom.