When removing cables from a cable tray, maintain compliance with NEC fill limits and consider leaving 20% spare space for future additions.

NEC Fill Limits

According to NEC Article 392, cable trays have maximum fill limits based on the type of cables and tray construction. For power cables, the fill must not exceed 40% of the tray's cross-sectional area, while control or instrumentation cables can go up to 50% . These limits ensure proper ventilation, prevent overheating, and maintain ampacity.

• Ladder trays: Best ventilation, can handle higher fill percentages.
• Ventilated trough trays: Moderate fill, some protection against debris.
• Solid-bottom trays: Most restrictive fill due to poor heat dissipation .

Spare Space Considerations

While NEC does not mandate spare space for future cable additions, many companies adopt a 20% spare space quota to allow for future expansion, especially for low-power instrumentation cables . This is separate from the NEC fill requirement and is primarily for operational flexibility rather than thermal management.

• For high-power cables, NEC recommends up to 50% spacing to prevent ampacity derating due to heat.
• For low-power or instrumentation cables, 20% spare space is generally sufficient because these cables generate minimal heat .

Practical Guidelines for Cable Removal

1. Assess tray fill: Before removal, calculate the current fill using the sum of cable cross-sectional areas relative to the tray's usable area.
2. Leave spare space: Maintain at least 20% of the tray area free for future cable additions, especially for low-voltage or control cables.
3. Follow NEC limits: Ensure that the remaining cables do not exceed the NEC-specified fill percentages.
4. Document changes: Label removed cables and update tray schematics to maintain compliance and facilitate future installations .

Summary

For cable tray removal, use the NEC fill limits as the maximum allowable quota (40% for power cables, 50% for control cables) and leave 20% spare space for future additions if dealing with low-power or instrumentation cables. This approach balances safety, compliance, and operational flexibility without conflicting with NEC requirements .