IEC-61537 – This International Standard specifies requirements and tests for cable tray systems (e.g. all metal cable trays, including wire mesh trays and non-metallic cable trays) to support, house cables and possibly other electrical equipment in electrical and/or communication system installations. However, the nominal loads of the tray are no longer limited to the four spans and three loads listed above. NEMA VE 1 now requires marking on cable trays to indicate the exact rated load on a specific span. For example, a tray can be designed for 150 lb/ft on a 30` wingspan. Curvatures or modifications of the ground shall be carried out in such a way as to effectively maintain the electrical continuity of the cable tray system and cable supports. Cable tray systems may have mechanically discontinuous segments between cable tray passages or between cable tray passages and equipment. The results of this test determine the NEMA load class(ies) into which a particular series of trays is grouped. These classes are defined by a combination of a number referring to the total span of the section under test and a letter indicating the weight capacity per foot in that range. FAQ: What is the right method to support cable glands? The most important part of the installation, according to NEC®, is that the wire tray system must have electrical continuity along its entire length and the cable support must be maintained.
Challenge: The absence of expansion joints or improper installation of these connections is the main cause of cable tray installation failures. Although this document is full of valuable information, there are some areas worth highlighting and raise most questions about cable tray installations. The second part of NEMA`s guidelines on metal trays, VE 2, is designed to help installers properly handle, store and install cable trays. Support spans – The support span is the distance between the medias. NEMA standard support ranges are based on 8`, 12`, 16` and 20`. NEMA classes: The following table summarizes NEMA classes based on the cables/workloads and support scopes described earlier. In cases where cable loads cannot be determined prior to specification or purchase, an estimate of cable weight may need to be made. The following table shows the maximum weight of insulated copper conductors that can be contained within a linear foot of the tank of the specified load widths and depths. The National Electrical Code (NEC) severely limits the filling area of cables and actual loads will be lower. For example, the weight of multiwire control and/or signalling cables is close to that of the switchboard; However, Article 318-8(3)(b) limits filling to 50 % of the cross-section of the plate, where 6 `is the maximum depth that may be used for the calculation. A cross section 6″ deep x 36″ wide can only be loaded at 130 pounds per linear foot, according to the table below. As the cables grow and the spaces between the cables increase, the overall weight decreases.
The total weight of the cable is rarely greater than the NEMA categories. These requirements can be adequately met even if there are installation conditions where the cable tray is mechanically discontinuous, e.g. penetration of the firewall, expansion space in a long straight cable tray where there is a change in height of a few feet between two horizontal cable tray sections of the same series. or when cables fall from an overhead line chute to enter the equipment. Understanding NEMA`s policies and how to apply them is essential to ensure that the right product is selected and used correctly to avoid unnecessary installation costs. This summary is designed to help you decipher these NEMA guidelines and give you confidence in running your next cable tray project. For end users, the most important standard within NEMA VE 1 is load testing. Understanding NEMA tray testing and load classes is important to select the right cable tray for the application.
The National Electrical Code (NEC®) Guide, Section 392, highlights instructions and specifications for installation.
