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HOW FULL CAN YOU FILL IT?

The NEC Requires Air Circulation Among Tray Cables.

• Published in Southwire's Power Cable Update newsletter in March 1997
• Reprint permission granted

Here is a guide to NEC tray fill.

You can't just dump all the cables that will fit into a cable tray. It's not good engineering practice - and the National Electrical Code (NEC) doesn't allow it. "The primary consideration is cooling," says David Cooper, applications engineer. "NEC cable tray requirements are designed to allow adequate air circulation among the cables. That prevents undesirable hot spots in the cable tray."

NEC Divides Tray Applications

Here are some NEC cable tray facts:

• Section 31806 (f) says you can't mix cables rated at over 600V with cables rated at less than 600V, unless the higher-voltage cable is type MC or there is a fixed, solid barrier between cable types.
• Section 318-9 says you can put single-conductor cables and MV cables in trays only in industrial establishments, and then only in ventilated or ladder-type trays.
• Where all the cables in a tray are multiconductor cables larger than 4/0 — or singe-conductor cables larger than 1,000kcmil — tray fill is determined by the sum of the diameters of the cables in the tray. For other cases, fill is determined by the cross-sectional area of the conductors.
• Individual cables operating at over 2,001V are always laid in a single layer. Triangular bundles of single conductors are permitted, also laid in a single layer.
• Single conductors can be bundled into triangular groups if the groups are spaced at least 2.15 times the OD of the conductor apart. For cables 2,000V or less, the reference is 318-11 (b) (4). For cables 2,001V or more, the references are 318-8 (e) and 318-11 (b) (4).

Actual Ampacity May Vary

"Remember that as you load cables into trays, you may have to adjust ampacity ratings," says Cooper. For cables 2,00W or less, check paragraph 318-11. That will refer you to ampacity tables in section 310. For cables 2,001V or more, look at paragraph 318-13 for the ampacity references.