Introduction of Cable Tray System
What is a Cable Tray System? Per the National Electrical Code, a cable tray system is "a unit or assembly of units or sections and associated fittings forming a rigid structural system used to securely fasten or support cables and raceways." What does this mean?
? Cable trays support cable the way that roadway bridges support traffic.
? A bridge is a structure that provides safe passage for traffic across open spans.
? Cable tray is the bridge that allows for safe transport of wires across open spans.
? Therefore, think of cable tray as the structural component of a building's electrical system.
Description and Selection
Since Cable Tray is used in a wide variety of applications and under widely varying conditions, it is important that you gain an understanding of material specifications and structural design, and apply that knowledge when selecting trays and specifying fittings, parts, and accessories. Cable Tray systems provide rigid structural support for cables in a variety of commercial and industrial applications. The basic styles of cable tray are: Ladder, Perforated and Solid Bottom Channel Tray Type and Wire Basket
Ladder
Ladder consists of two longitudinal side members (side rail) connected by individual traverse (rung
support) members. It is intended for use as a power cable or control cable support.
Channel Tray
Channel is a one piece support with either ventilated perforated or solid bottom sections. These sections are used with a single power cable, multiple control, or signal circuit cables, which would provide better electrical shielding and better insulation mechanical protection from Foreign Object Damage (FOD) .
Quick List Selection Process
Some of the considerations for Cable Tray selection are
1. Support Span Issues are: Strength and Length : Very important to first consider the support span as it affects the strength of the system and the length of the straight sections required..
2. Working Load Issues are: Size (Width, Loading Depth, and Strength) Cable Load Types and numbers of cables to support-Total cable load in kg per linear meter (kg/meter), Power - is single layer - issue width (refer to local electrical code), Low Voltage - is stacked - issue loading depth and width (refer to affecting code)
Select a Cable Tray system that meets the working load for the support span required and a straight section length that fits the installation. NEMA VE 2 - Straight sections equal to or larger than span.
3. Installation Environment Issues are: Material and Finish :
a. Indoor Dry - Institutional, Office, Commercial, Light Industrial use Aluminum, Pre- Galvanized Steel, Electro Galvanized Steel
b. Indoor Industrial - Automotive, Pulp and Paper, Power Plants use Aluminum, Pre-Galvanized Steel, Possibly Hot-Dipped Galvanized After Fabrication (HDGAF)
c. Outdoor Industrial - Petrochemical, Automotive, Power Plants use Aluminum, Hot-Dipped Galvanized After Fabrication (HDGAF)
d. Outdoor Marine - Off Shore Platforms use Aluminum, Stainless Steel.
NEMA LOAD CLASSIFICATION
The National Electrical Manufacturers Association (NEMA) has standardized the classification of cable tray based on the load to be carried per foot, and the distance between span supports. Maximum Design Load for Maximum Associated Support Spacing CSA Load Classification TSA manufacturers Cable Tray in accordance with NEMA Standards Publication VE1-2005 and CSA Standard C22.2 No. 126.1
NEMA SUPPORT SPAN LOAD Standards Publication VE1-2005 CLASS (in feet) (lbs./foot) equal to meter (kg/meter)
8A = 8 feet span 50 lbs/foot equal to 2.40 meters span 74 kg/meter run
8B = 8 feet span 75 lbs/foot equal to 2.40 meters span 112 kg/meter run
8C = 8 feet span 100 lbs/foot equal to 2.40 meters span 149 kg/meter run
12A = 12 feet span 50 lbs/foot equal to 3.66 meters span 74 kg/meter run
12B = 12 feet span 75 lbs/foot equal to 3.66 meters span 112 kg/meter run
12C = 12 feet span 100 lbs/foot equal to 3.66 meters span 148 kg/meter run
16A = 16 feet span 50 lbs/foot equal to 4.88 meters span 74 kg/meter run
16B = 16 feet span 75 lbs/foot equal to 4.88 meters span 112 kg/meter run
16C = 16 feet span 100 lbs/foot equal to 4.88 meters span 149 kg/meter run
20A = 20 feet span 50 lbs/foot equal to 6.00 meters span 74 kg/meter run
20B = 20 feet span 75 lbs/foot equal to 6.00 meters span 112 kg/meter run
20C = 20 feet span 100 lbs/foot equal to 6.00 meters span 149 kg/meter run
Class Design Load CSA Standard C22.2 No. 126.1
A 25 lbs/foot, 10 feet span = 37 kg/meter, 3 Meter Span
C 65 lbs/foot, 10 feet span = 97 kg/meter, 3 Meter Span
D 45 lbs/foot, 20 feet span = 67 kg/meter, 6 Meter Span
E 75 lbs/foot, 20 feet span = 112 kg/meter, 6 Meter Span
Why Limit Deflection?
The primary reason to limit deflection in cable tray systems is appearance. Engineers and owners take pride in the appearance of their installations. So rigid restrictions on deflection of cable trays installed at eye level or in a prominent location are common. However, it is neither economical nor good engineering practice to restrict deflection of a cable tray system in less prominent areas.
Methods of Decreasing Deflection
There are various ways to limit deflection of a cable tray. If the objective is minimal installed cost, they should be considered in this order:
? Increasing depth of the tray.
Deflection in any location can be reduced by increasing the depth of the load-carrying side members and/ or by adding to their cross-sectional area. Adding to the depth generally utilizes the material most economically.
? Decreasing span length.
For economic reasons, this method of reducing deflection should be a last resort, since it increases field labor considerably. However, it can be an effective means to improve the appearance of an installation when the number of spans to be reduced is small in comparison to the number in the entire installation.
Summary
As a guide, a span-deflection ratio of around 1/200 satisfies most owners. This ratio provides an allowable deflection of 15 mm in a 3 meter span, 18 mm in a 3.66-meter span, and 30 mm in a 6 - meter span under the actual loads encountered.