Power-limited fire alarm (PLFA) cables can share a cable tray with low-voltage communications cables under NEC 760.139, but physical separation, shielding, and best practices are strongly recommended to ensure safety and signal integrity.

Regulatory Guidance

According to the National Electrical Code (NEC) Article 760.139, two or more PLFA circuits, communications circuits, or Class 3 circuits are permitted to share the same cable tray, raceway, or enclosure . For Class 2 circuits, the same rule applies if the insulation meets the level required by the PLFA circuits (e.g., 600V-rated insulation), . This means that, from a code perspective, sharing a tray is allowed, but compliance depends on proper insulation and adherence to the NEC specifications.

Safety and Best Practices

Even though code allows co-location, dedicated pathways are widely recommended to prevent mechanical damage and ensure the reliability of life-safety systems . Key considerations include:

• Mechanical Protection: Fire alarm cables must be secured with fire-resistant hardware and supported by the building structure to prevent damage during installation or maintenance of data cables .
• Electromagnetic Interference (EMI): Close proximity to data cables can introduce EMI, potentially degrading network performance or affecting fire alarm signaling. Using metallic barriers, partitions, or shielded trays can mitigate this risk .
• Separation Distance: While the NEC does not mandate a specific distance for PLFA and communications circuits, industry practice often recommends at least 6 inches of separation or the use of a physical divider within the tray .
• Fire-Rated Pathways: Fire alarm cables should be routed in fire-rated conduits or trays to maintain functionality during a fire event .

Practical Implications

• Inspection Compliance: Improper installation can lead to failed inspections by the Authority Having Jurisdiction (AHJ) or fire marshal, requiring costly rework .
• System Reliability: Bundling fire alarm and data cables without protection increases the risk of accidental cuts, pinching, or stress on the fire alarm conductors, potentially compromising life-safety signaling .
• Long-Term Performance: Using dedicated trays, metallic dividers, or shielded pathways ensures both fire alarm and data systems maintain performance and reliability over time .

Conclusion

While NEC 760.139 permits PLFA and low-voltage communications cables to share a cable tray, it is best practice to provide physical separation, shielding, or dedicated pathways to protect the fire alarm system from mechanical damage and EMI. Following these guidelines ensures compliance, system reliability, and occupant safety.