Archives December 2019

In applications where multiple motors are each powered by a separate VFD, care must be taken regarding the selection of the inverter to motor cables. Cable selection is even more critical if the cables are to be run any distance together in a raceway. Single conductor cables, while commonly used for some drive applications, can cause issues in such an installation. In addition to safety issues (see Southwire application note number 2012, VFD Cables – A Safe Bet), electromagnetic coupling can cause issues with drive performance.

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General
Tray cable, Type TC is an approved wiring method in the NEC found in article 336. It is an efficient method of installing feeders, branch circuits and control cable because multiple runs of tray cable can be installed in one cable support system (i.e. cable tray) rather than multiple conduit runs, which adds to labor and material cost.
Description
Tray cable is a factory assembly of two or more insulated conductors with or without an associated equipment ground conductor under a non-metallic jacket. For feeder and branch circuits, tray cable can be manufactured with any of the insulation types found in NEC 310.4 (A) or (B). Depending on the insulation used, tray cable will have either 600, 1000 or 2000 volt rating. Metallic shields are allowed over groups of conductors or under the outer jacket or both. Metallic sheaths or armor is not allowed under or over the non-metallic jacket, doing so would make the cable type MC cable.
Use/Locations
Type TC cable can be used for a variety of applications such as, power, lighting, control, signal circuits, class 1 circuits and non-power limited fire alarm circuits. Tray cable cannot be installed where it is subject to physical damage and must be installed in a cable tray with exceptions. Sections of the tray can have up to one foot breaks or separations without the need of adding protection to the cable in these areas.

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Background:
Color codes are used to identify conductors for point-to-point wiring and for circuit diagrams. The Insulated Cable Engineers Association outlines the color code in Standard S-73-532 in Annex E. The Standard breaks down the color code into methods of circuit identification. The most common methods used are Method 1, 3, and 4. After the Method is selected then the assembly of conductors must follow a color sequence from the tables with Table 1 and 2 being the most common.
National Electrical Code (NEC)
The NEC specifies that conductor colored white be used only as grounded conductors and that conductors colored green or green/yellow be used only as grounding conductors and that neither white nor green be used in any manner on ungrounded conductors. Tables 2 provide color sequences that do not include white or green conductors. If grounded or grounding conductors, or both, are used in the cable, they shall be colored white or green respectively, and inserted as the second or third, or both, designated conductor in the first sequence of circuit identification only. Where these conductors are required, they shall be specified.
Methods of Circuit Identification:
Method 1 - Colored Compounds
This method uses base color of insulation and uses tracers when needed, in accordance with Table 1 or 2. Base colors may be obtained by suitable color coatings applied to the insulation or jacket surface or by colored insulation or jacket compound. Tracers shall be colored stripes or bands marked on the surface of the insulation or jacket in such a manner as to afford distinctive circuit coding throughout the length of each wire. Tracers may be continuous or broken lines, such as a series of dots or dashes, and shall be applied longitudinally, annularly, spirally, or in other distinctive patterns.

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