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NEMA and CSA Span/Load Classes Span/Load Class Designations Commonly called the Load Class, this defines the load-carrying capability of the tray for a specific support span distance. The design and cost of the cable tray is greatly affected by this designation. In order to determine the most appropriate and economical system, a class should be selected that reflects the actual total working load and support span for each application. Concentrated Static Loads Some applications may require the cable tray to support the weight of a single, dead object in addition to the cable loads. Specifications typically require this to be applied at the mid-point of the span between the tray supports, which is the worst-case location. To incorporate this in the tray design the following formula can be used to convert the concentrated static load in pounds to an equivalent uniform load (W) in pounds per foot. That equivalent load can then be combined with the weight of cables, tray contents, and other loads that are also expressed in lbs/ft to determine the total Working Load. Formula
3 loads, expressed in lbs/ft: Refer to the chart below.
Cables Snow
40 lbs/ft +15 lbs/ft
Concentrated Load
+25 lbs/ft Total Working Load: 80 lbs/ft
With a support span of 20’ and a total working load of 80 lbs/ft, a NEMA Class 20B tray rated at 75 lbs/ft will not be adequate. A NEMA Class 20C tray, rated at 100 lbs/ft, will be required. Working Load The total load supported by the cable tray, uniformly distributed. This will be the combined weight of all of the cables or tray contents, any environmental loads (snow, ice, dust) and any concentrated static loads. Support Span The maximum distance between the tray supports. NEMA Load Classes A number (support span in feet) combined with a letter (load capacity: A=50, B=75 and C=100 Lbs/Ft). Defined in the National Electrical Manufacturers Assoc. NEMA Standard VE1. CSA Load Classes A single letter (A,B,C,D,E) designates the support span and load capacity per Canadian Standards Association CSA Standard C22.2, No. 126. Span/Load Class Designations Per NEMA Standard VE1 / CSA C22.2 No. 126
2 x (concentrated static load, lbs) support span length, ft
W e =
Note: This calculation is based on the load being applied at the center of the tray in the middle of a rung or bottom. If the specification requires the concentrated static load to be applied on top of one side rail, then the equivalent uniform load will be 2 times the result of this calculation. Example A cable tray is to be supported on 20’ spans. It shall contain 50 lbs/ft of cables and support 15 lbs/ft of snow load. It is also required to support a 250 pound concentrated static load applied in the center of the tray width. Using the calculation above, the concentrated static load is converted to an equivalent uniform load as follows: 2 x 250 lbs / 20 ft = 25 lbs/ft. The Total Working Load will be the combination of all
Working Load lbs/ft (kg/m)
Support Span, Feet (Meters)
5 (1.5)
8 (2.4)
10 (3.0)
12 (3.6)
16 (4.9)
20 (6.0)
25 (37) 45 (67) 50 (74) 65 (97)
5AA 8AA 10AA 12AA
20AA
D*
5A
8A
12A 16A
20A
C
75 (112) 100 (149) 120 (179) 200 (299)
8B 8C
12B 16B E* or 20B
12C 16C
20c
D* E*
* CSA Classes D and E are applicable to both 3.0 and 6.0 meter support spans. The appropriate support span should also be included when specifying these classes (example: D-3M).
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