Cable Tray Types Overview
Cable trays are open cable management systems. NEC Article 392 recognizes these types:
- Ladder tray — Two side rails connected by rungs; best airflow, most common in industrial plants. Ideal for power cables ≥4 AWG.
- Solid-bottom tray — Full solid floor; provides physical protection and containment for oil drip environments. Requires ampacity derating.
- Perforated tray — Solid sides with perforated floor; compromise between ladder and solid-bottom. Common in commercial construction.
- Wire mesh tray — Lightweight, flexible, popular in data centers for low-voltage data cables.
- Channel tray — Small (4" or 6" wide) for small quantities of cables or instrument tubing.
Cable Duct/Wireway Overview
Cable duct (wireway, or cable trunk) is an enclosed sheet-metal or PVC raceway with a hinged or removable cover. NEC Article 376 covers metal wireways. Key differences:
- Enclosed on all six sides (three sides for cable tray)
- Conductors must be derated if fill exceeds 20% of cross-sectional area (NEC 376.22)
- Maximum 30 current-carrying conductors per section without derating
- Better dust and splash protection (NEMA 12 rated types available)
- Suitable for use in electrical rooms and panel areas where open trays may not be aesthetically acceptable
Ampacity Impact: The Most Misunderstood Difference
This is where engineers make costly mistakes. NEC 392.80 covers ampacity in cable trays. The key rule:
- Ladder or ventilated tray with 4 AWG or larger conductors: No ampacity derating required (cables treated as in free air)
- Solid-bottom or covered tray: Apply 95% of free-air ampacity for cables not in conduit
- Any tray type, cables ≤3 AWG: Apply 60% (conduit fill method) ampacity if bundled in groups of 4+
In contrast, cable duct/wireway with more than 30 current-carrying conductors requires derating even for large cables. For high-density power distribution, this makes open ladder tray significantly more efficient.
Head-to-Head Comparison
| Factor | Cable Tray | Cable Duct/Wireway |
|---|---|---|
| Heat dissipation | Excellent (open air) | Limited (enclosed) |
| Ampacity derating (power cables) | Minimal (ladder tray) | Significant beyond 30 conductors |
| Dust/moisture protection | Low | High (NEMA 12 option) |
| Future cable additions | Easy | Limited by fill rules |
| Material cost | Lower | Higher |
| Best for | Industrial, process plants | Electrical rooms, panels |
What Type of Cable Can Go in Cable Tray?
Not all cable types are listed for cable tray use. NEC 392.10 lists permitted cable types, including:
- Any cable listed and marked for cable tray use (TC — Tray Cable, UL 1277)
- Type MC cable (Metal Clad, listed for cable tray)
- Type MV cable (Medium Voltage)
- Conductors 4 AWG and larger in metallic conduit laid in cable tray
Standard THHN in cable tray is only permitted if it is installed in conduit that runs through the tray — THHN cannot be laid loose in cable tray without conduit.
The Right Cable for Cable Tray: Tray Cable (TC)
For factory-wiring the most economical and code-compliant solution for open cable tray is Tray Cable (Type TC, UL 1277). TC cable features:
- Multiple conductor configurations (2 - 37 conductors)
- Flame-retardant jacket rated for open tray installation
- Available with overall shielding for EMI-sensitive control circuits
- 600V rating standard; 1000V available for VFD circuits
- Oil-resistant jacket options for petrochemical and food processing
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