Cable Trays: NEMA Classifications

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In this Tech Talk article, we will discuss NEMA Classifications for Cope Cable Tray.  The NEMA Classifications for Cable Tray were established to simplify and standardize the specification of Cable Tray.

This classification is based on the working load (the total weight of the cables), and the support span (the distance between supports).  

Cable Load/Working Load--The Cable load or the working load is the total weight of the cables to be placed in the tray. The NEMA classes are based on cable loads of 50#, 75#, and 100# per lineal foot. This is the total weight of cables in the tray. For purposes of selecting a suitable tray, this weight should be rounded off to the next higher NEMA working (allowable) load.

Support Spans--Support span is the distance between the supports. The NEMA standard support spans are based on 8', 12', 16' and 20'.

NEMA Classes--The following table summarizes the NEMA classes based on cable/working load and support span described previously.

 cable tray 1

In cases where cable loads cannot be determined prior to specification or purchase an estimate of cable weight may have to be made. The following table represents the maximum weight of insulated copper conductors which can be contained in a lineal foot of tray of the widths and load depths given. The National Electrical Code (NEC) greatly limits cable fill area and actual loads will be less. For example, the weight of multiconductor control and/or signal cable is close to those in the table; however, Article 318-8(3)(b) limits fill to 50% of cross section of tray, with 6" the maximum depth usable for computation. A 6" deep x 36" wide cross section would only be permitted to be loaded to 130 pounds per linear foot, using the table below. As cables increase in size and interstices get larger between cables, the total weight decreases. Total weights of cable are rarely more than NEMA categories.

cable tray 2

Other Loading Considerations--It is important to note that when specifying loading requirements, there are other loading factors that may need to be considered over and above the actual cable loads.

Destruction Load Capacity--The total weight in the tray which causes the tray to collapse, is called the "destruction load capacity". When trays do collapse, they generally do so by premature lateral buckling (compression) of the top flange.

Concentrated Loads--A concentrated load is a static weight applied between the side rails at mid span. When specified, these concentrated static loads may be converted to an equivalent uniform load (We), in pounds per lineal foot, using the following formula: cable tray 3 This load (We) is then added to the static weight of the cable before selecting the appropriate NEMA load span designation. Please note per the NEMA VE-1 guidelines all Cope Cable Trays are labeled as follows:

 cable tray 4

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