Today's industrial facilities use a host of control circuits for motor control, instrumentation signals, and plant shutdown schemes. Correctly applying the Articles of the Code that govern their use can be difficult without knowing how to classify the circuits with which they're associated and which cables to use for their installation.
How does the NEC control your signaling and control wiring requirements?
Today's industrial facilities use a host of control circuits for motor control, instrumentation signals, and plant shutdown schemes. Motors, valves, thermocouples, transmitters, emergency shutdown systems, and just about every process in a modern plant interacts with a highly intricate control system or device like a distributed control system (DCS) or a programmable logic controller (PLC). Correctly applying the Articles of the Code that govern their use (Art. 430, Part VI; Art. 725; or Art. 727) can be difficult without knowing how to classify the circuits with which they're associated and which cables to use for their installation. But once you have such a knowledge base from which to start, it will be easier to know which Article to apply and when.
A DCS supplies power to, or receives power from, the circuits which it operates in several ways. A power supply within the DCS typically provides 24VDC or 120VAC power to the electronic cards within the system. In some installations, the DCS power supply may directly power solenoids, limit switches, or other field instruments. This arrangement means the devices are loop powered. In other cases, a field instrument may supply a powered output, often referred to as field-powered circuit. In either arrangement, the circuit is typically considered low-energy.
Although it hasn't been used since the 1971 NEC, the term low-energy best describes the types of circuits associated with DCSs and PLCs. Voltages in these systems are typically 24VDC and 120VAC, although the voltage can be in the mV range, as is the case with thermocouples. They may operate proximity switches, transmitters, or other low-voltage devices. The current is usually in the mA range.
Class 1, 2, or 3 circuits and cable insulation ratings.
Since few circuits associated with a DCS use a listed Class 2 or 3 power supply, they can't be classified as Class 2 or 3, even though they may really be low-energy circuits. Therefore, if you're designing and installing in accordance with the requirements of Art. 725, these circuits are classified as Class 1 by default. As a result, Class 1 circuits are by far the most common type of remote-control and signaling circuit used in today's industrial facilities.
All Class 1 circuit conductors must have insulation suitable for 600V, but Class 2 and 3 circuits may use conductors with insulation rated for only 300V. As a general rule, you can't install Class 1 circuits in the same cable, wireway, or enclosure as Class 2 or 3 circuits. Thus, it's very important to research the classification of all power supplies and the outputs of electronic cards used for instrumentation before determining whether you can, or should, use 300V cable in your design.
As previously stated, if a remote-control and signaling circuit isn't derived from a listed Class 2 or 3 power supply, or isn't installed in accordance with the requirements of Art. 727, the circuit has to be classified as a Class 1 circuit. However, there are exceptions to this rule.
The Code also implies in 430.72(A) that motor control circuits tapped from a motor branch-circuit, short-circuit, and ground-fault protective device (usually fuses or a breaker) need not be protected per the requirements of 725.23 because these circuits aren't considered to be Class 1 circuits and the requirements of Art. 725 don't apply (Fig 1.). Conversely, motor control circuits served by an external power source and those derived from the secondary side of a control power transformer are classified as either Class 1 remote-control and signaling circuits or power-limited Class 1 circuits, and the requirements of Art. 725 apply.
A Class 2 or Class 3 circuit can also be classified as a Class 1 circuit if its failure can introduce a direct fire or life hazard. In this case, you must reclassify and install it as a Class 1 circuit, even if it has a listed Class 2 or 3 power supply. Once the circuit is reclassified it can be installed alongside other Class 1 circuits.
One common misconception among many installers is that it's acceptable to route Class 1 circuits with other power circuits. After all, this basic concept is outlined in the requirements of 300.3(C)(1). However, the requirements of 725.26 don't allow you to install Class 1 circuit conductors in the same raceway, cable, or enclosure with power circuits unless they're functionally associated (Fig. 2). A change in the 2002 NEC (725.26) also prohibits the placement of Class 1 circuits in a cable tray with power conductors that aren't functionally associated. You should only combine systems that are functionally associated in their application in a common cable tray.
This change to the Code may create problems for industrial users. For example, new installations now require a separate tray system or a barrier for Class 1 circuits. It's no longer acceptable to install one common tray for all of the 600V insulated conductors. Design engineers now need to separate all Class 1 remote-control and signaling circuits from all unassociated power and lighting circuits. Separating Class 1 remote-control and signaling circuits from power circuits may also present new challenges when using composite power and control cables.
Choosing the right cable.
Cable rated with 300V insulation is a common type of wire used in industrial plants for instrumentation, but power limited tray cable (PLTC) can't be used on Class 1 circuits. The use of PLTC for most instrumentation and DCS circuits is in conflict with the requirements of the NEC. These low-energy circuits created a demand for a type of 300V cable that could go where 600V or PLTC couldn't — 600V insulated cable can be very bulky and many enclosures in instruments, instrument-marshaling cabinets, and in terminal boxes lack adequate space for it.
Instrumentation tray cable (ITC) was introduced in the 1996 NEC. ITC doesn't create the conflicts with the Code that PLTC can, and it isn't as bulky as 600V cable. It can be installed under raised floors in control rooms and rack rooms without using cable tray or raceway, as long as it's protected from physical damage. And possibly its greatest benefit when it's used is that the NEC no longer considers the circuit to be a Class 1, 2 or 3 circuit, and the requirements of Art. 725 no longer apply. ITC offers relief for low-energy instrumentation circuits because the Code classifies it as an unidentified type of circuit. Its thinner insulation also makes it much easier to work with.
One limitation of ITC is the circuit must operate at 150V or less and 5A or less, which is common with instrumentation circuits (Fig. 3 below). The Code also restricts you from installing ITC with power, lighting, Class 1, or non power-limited circuits.
Points to remember.
If a circuit has any type of signaling function, the Code considers it to be a remote-control and signaling circuit. If the power source isn't listed as a Class 2 or 3 supply, the circuit is probably a Class 1 circuit by default, requiring 600V insulation. If the circuit is an instrumentation circuit and meets the requirements of Art. 727.5, you can use ITC (300V insulation).
Some cable manufacturers make cables with a dual PLTC/ITC rating. However, you can use cables marked as PLTC only on Class 2 and 3 circuits. You can only use cables marked as ITC on instrument circuits operating at not more than 150V or 5A. If your instrument cable is dual labeled ITC/PLTC, you can use it on Class 2 and 3 circuits and instrument control circuits that comply with the limits of Art. 727.
Where the motor branch-circuit, short-circuit, and ground fault protective device (i.e. without a control power transformer) supply a motor control circuit, you must follow the requirements of Table 430.72(B). In this case, the requirements of Art. 725 don't apply. If a motor control circuit of 600V or less is derived from a control power transformer or a circuit breaker from another source, it will be classified as a Class 1 circuit and the requirements of Art. 725 apply.
When designing cable tray systems, always be aware of the classification of the circuit, the type of cable to be used, and the separation requirements set forth in the NEC.
Guidry is an electrical design supervisor for Fluor Enterprises, Inc. in Sugar Land, Texas, and a member of NEC Panel 11.