Parallel shifting of cable trays

Parallel shifting of cable trays involves arranging multiple conductors in parallel to balance current, minimize heating, and comply with electrical standards.

Purpose of Parallel Cable Arrangements

When high current is required, multiple cables are connected in parallel for each phase to meet ampacity requirements without oversizing individual conductors. Proper parallel arrangement ensures equal current distribution, prevents overheating, and extends cable lifespan by avoiding insulation damage due to imbalanced currents ( ).

Key Considerations

1. Phase Sequence and Cable Arrangement

• Correct phase sequencing is critical to reduce circulating currents and magnetic interference between parallel conductors.
• Common configurations, such as A B C C B A in flat arrangements, help minimize mutual heating and improve current balance ( ).
• Single-core cables require careful arrangement to maintain equal current sharing and prevent localized overheating ( ). 2. Cable Spacing and Tray Layout
• Maintain adequate clearance between cables to reduce mutual heating and allow proper heat dissipation.
• Cable trays should be sized to accommodate parallel conductors without overcrowding, following NFPA 70 Section 392.20(C) ( ). 3. Ampacity and Derating
• Each conductor in a parallel set must be sized according to the total current requirement, considering ambient temperature and grouping factors.
• For multiconductor cables in trays, derating rules differ from conduit installations; NFPA 70 Section 392.80(A)(1)(a) clarifies that derating applies only to cables with more than three current-carrying conductors ( ). 4. Connections and Taps
• Any taps, extensions, or connections must connect to all conductors in the parallel set to ensure equal current sharing, as required by NEC 300.3(B)(1) ( ).

Benefits of Proper Parallel Shifting

• Improved current balance and reduced risk of overheating.
• Material savings, as smaller conductors can be paralleled instead of using oversized single conductors ( ).
• Compliance with standards, ensuring safety and reliability in high-current installations.

Summary

Parallel shifting of cable trays is a critical design practice for high-current systems. It requires attention to phase sequence, cable arrangement, spacing, ampacity, and proper connections. Following NEC and international standards ensures balanced currents, reduced heating, and long-term reliability of the electrical installation.