What type of cable trays should be used for the mains of a cable trunking cabling system?
The types of cable trays available are stepped, ventilated trough or solid bottom. What are the reasons for choosing a specific type of cable tray?
The engineer or designer should select the type of cable tray with the features that best meet the requirements of the project. For several types of installation, the National Electrical Code (NEC) specifies the type of cable tray to be used.
Single core and MV type cables must be installed in ladders or ventilated trough cable trays. Single core and MV type cables are not permitted to be installed in solid bottom cable channels [1993 NEC Section 318–3(b)].
In Class II, Division 2 Hazardous (Classified) Locations (Dust), the type of cable permitted to be installed in a cable tray must be in a ladder or ventilated trough cable tray. Solid bottom cable bridges are not permitted in Class II, Division 2 locations [1993 NEC Section 02-(b)].
Ladder cable trays
The ladder cable tray is used in approximately 75% of cable tray cabling system installations. It is the main type of cable tray because it has many desirable features.
The ladder cable tray without cover allows maximum free flow of air through the cables. This allows the heat generated in the cable conductors to be effectively dissipated. In this case, the conductor insulation in the cables of a correctly designed cable tray cabling system will not exceed its maximum operating temperature. The cables do not deteriorate prematurely due to high operating temperatures.
The steps of the ladder cable tray provide convenient anchor points for bundling cables in non-horizontal cable tray lines, or where cables must be kept in position in horizontal cable tray lines. This capability is necessary for single-core cable installations. In the event of a fault (short circuit), the magnetic force generated by the fault current will pull the single core cable out of the cable trough if it is not securely fastened to the cable trough.
The cable can exit or enter the ladder cable trunking through the top or bottom of the cable trunking. Where cables enter or exit the conduit, the conduit-to-cable-bridge fixture can be mounted upright or upside down to terminate the conduit at the top or bottom of the cable tray side rails.
No moisture will accumulate in ladder cable bridges.
If installing cable bridges where working space is an issue, hand access through the bottom of the cable bridge may help facilitate the installation of small diameter cables: control instruments, signals, etc.
The most common spacing for ladder cable bridges is 9" across. This spacing can be used to support all sizes of cable. This spacing is ideal for small diameter PLTC and TC type cables, as the support distance allows for no significant sagging of small cables between the cross-ties. 12 or 18 inch cross-tie spacing provides adequate cable support, but the slight sagging of small diameter cables between the cross-ties may be aesthetically objectionable for some installations. the maximum allowable distance between supports for 1/0 to 4/0 AWG single core cables is 9 inches [1993 NEC Section 318–3(b) (1)].