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Control Wiring: A Primer

Control wiring provides a path for command and status communication between control devices in a lighting and control system and, in many cases, power to the devices as well.

“Control wiring is the medium by which a user communicates a desired level to the lighting device,” says Thomas Hinds, Product Manager-Fluorescent Dimming Ballasts for Lutron Electronics Co., Inc. “The choice of control wiring is important to ensure signal quality, minimize noise and interference, and meet electrical codes.”

Wiring is interrelated with overall lighting control system considerations—such as selection of control system and components, layout and installation practices—and is therefore an important consideration when choosing a lighting control solution.

Line voltage wiring

Line-voltage (Class 1) wiring provides power (120-277VAC) and load connectivity for lighting equipment as well as, optionally, ground, neutral and travelers for multiway applications. Conventionally, line-voltage wiring has provided the means for grouping light fixtures by circuit/switch-leg to create control zones. The lights are switched ON/OFF using a controller that closes or opens the circuit to provide or deny electric power. The wiring can be used for powerline carrier communication of raise/lower (dimming) as well as switching signals.

Pros: Line-voltage wiring is very familiar to electrical workers and therefore less prone to installation error, can provide both power and communication, can be run with other line-voltage wiring in the same conduit, and can be installed in long runs. It is particularly suitable for retrofit applications if the existing line-voltage wiring is already properly located and is being used.

Cons: Line-voltage wiring offers limited control options and flexibility, as circuiting/zones are relatively fixed. This type of wiring is usually required to be installed in some type of protected run, such as conduit. This can increase its installed cost relative to low voltage, with another cost consideration being copper prices. An electrician is also required to install it.

Low voltage wiring

Low-voltage (Class 2) wiring provides a pathway for power (10-24VDC) and communication/feedback for low-voltage control devices.

Pros: Most codes allow low-voltage wiring to be run without conduit and junction boxes, allowing it to be installed independent of the power wiring (e.g., laid on top of suspended ceiling tiles using plenum-rated conductors), resulting in dramatic gains in flexibility and allowing construction of sophisticated control systems involving layered lighting control as granular as individual light fixtures. It is safer to install and, in some cases, may not require an electrical trade to install it. And it is relatively easy to reconfigure based on future changes.

Cons: Unshielded low-voltage conductors can pick up electrical contamination in the environment—e.g., arc welding machines—or adjacent power conductors, particularly over longer runs. There may also be limits to the number of control devices that can be connected to the network. The potential for miswiring is higher than with line-voltage wiring.

“Ultimately, the decision on type of wiring should be a consequence of understanding the control requirements, space layout and end-user control needs,” says Eric Fournier, Director of Product Marketing for WattStopper. “For example, if the building owner is renting his office space and changing tenants on a regular basis with frequent modifications in furniture layout—i.e., conference room transformed into open office with cubicles—you will more than likely opt for low-voltage devices instead of line-voltage. But if the building owner needs automatic shutoff for a small private office—i.e., 12 ft x 12 ft—and line-voltage wiring is already present at the switch box location, you will more than likely use a stand-alone line-voltage occupancy sensor.”

Low voltage control wiring: analog versus digital

Dedicated low-voltage control wiring enables advanced flexibility and functionality in today’s lighting control systems. The most common types are analog 0-10VDC and digital.

Analog 0-10VDC: 0-10VDC wiring uses two wires with a 1-10V potential between them.

“Analog offers good signal resolution, enabling closer identification of actual conditions,” says Ronald Bryce, National Sales Manager for PLC-Multipoint, Inc. “For example, consider a photosensor. If a standard 0-10VDC photosensor is scaled to represent 0-250 footcandles (fc), each volt of the signal from the sensor—called the return signal—represents 25 fc. To deactivate the lights in a shopping mall, the control system normally turns them OFF at 7 fc. The return signal in this case would be 0.28VDC.”

However, Bryce adds, transferability of the signal is a disadvantage with analog. Because of the reference to 0V, the distance an analog signal can be sent is limited. This can be mitigated by using a 4-20mA signal instead of a 0-10VDC signal, although 4-20mA devices are generally more expensive and not all control systems can operate with these signals.

A common 0-10VDC wiring type is stranded-copper twisted-pair 18AWG wiring. The wiring is stranded copper because it provides a more stable current path (as DC signals tend to be transferred by the outer edges of the conductor) while being relatively easy to work with; solid wire is usually acceptable in low-voltage systems that use AC control power. Twisted-pair offers some protection against noise while being economical and able to be transported in bulk and easily cut in the field to required length. Typical sizes range from 22AWG to 14AWG, with 18AWG being typical; size can be increased for longer runs where voltage drop is a concern.

The wiring may be specified with shielding or no shielding. As low-voltage wiring is sensitive to electrical noise and electromagnetic interference, shielding provides a degree of immunity, and can be specified for environments where these issues may be a concern.

Other wiring types include shielded or unshielded CAT5 and CAT6 twisted-pair wiring, which offers good immunity against interference in electrically noisy environments.

“Zip lines, arranged like lamp cord, can be very economical and can serve the purpose for short hauls in electrically clean environments,” says Greg Bennorth, Director of System Projects for Universal Lighting Technologies. “Twisted pair, with or without shielding, maintains its impedance over long distances and has a certain amount of noise immunity, but can be expensive. Coaxial cable, although having good noise immunity and being relatively inexpensive, is not generally used in lighting control.”

Digital: Digital wiring uses two wires with a maximum potential of 18V between them. Very little current is drawn through the wires, unlike analog wiring, which sends commands based on a voltage level and therefore draws current. Instead, command and status information is transmitted as digital binary messages.

“Digital networking technology has revolutionized lighting controls allowing multiple energy management strategies to be deployed at the individual fixture level,” says Andrew Parker, PE, LC, Director of Sales for Encelium Technologies. “This technology is further merging lighting controls with other building systems to enhance capabilities through sharing of data to complete the intelligent building infrastructure.”

This produces several benefits:

• lower susceptibility to electrical and radio frequency interference;
• ability to be installed as Class 1 or Class 2 wiring, depending on the manufacturer;
• a large number of devices can be connected to the network;
• network communications are constantly flowing through the network both ways, and can be monitored or recorded as desired, allowing real-time collection of performance data for energy analysis and maintenance;
• single wiring bus connecting all control devices;
• light fixtures can be grouped and regrouped (zoned and rezoned) using software, with no wiring changes; and economical integration of multiple control devices/strategies.

These advantages are delivered using advanced circuitry in intelligent control devices and are often made available with preterminated connectors, which will impose a cost premium.

For digital devices to be able to communicate using digital messages, they must be able to speak the same language—that is, use the same communication protocol—such as DALI, proprietary, BACnet, LON, RS485, etc.

RS485 uses a twisted pair of small-gauge wires with a shield conductor and can be run up to 4,000 ft., but is often polarity sensitive and requires a linear topology. LON-based communication often uses an unshielded twisted pair, allowing open topology and not being polarity sensitive, but is limited to 1,500 ft. before a repeater is required. DALI-based systems use a simple pair of wires without a requirement for twisting, specific topology or polarity, but is limited in the number of nodes that can be connected to the system, and is criticized for being very slow in transmission speed. Some manufacturers have created proprietary protocols based on DALI, offering its advantages but mitigating its disadvantages. However, if a component fails, it must be replaced by a compatible device from the same manufacturer. Additionally, proprietary systems may not wire the same even though their components may perform similar functions, requiring special attention to manufacturer installation instructions.

Low voltage control wiring: prefabricated option

Structured wiring featuring factory-tested and verified cable assemblies has emerged as a premium option providing a significant alternative to the traditional approach of making point-to-point connections using bulk wire. L ow-voltage analog and digital wiring may be available with preterminated connections such as RJ45 (computer), RJ11 (telephone) or some proprietary type of connector. These systems are often offered with plug and play controls that automatically configure upon installation, enabling immediate operation.

The advantage of this approach is it can simplify installation and reduce errors related to incorrect terminations. On the other hand, wire lengths must be predetermined, and while there may be installation labor savings, the cost of materials is generally higher.

General principles

Architecture: Some systems require separate input networks from output networks. For example, light fixtures may be grouped to respond to system commands coming from a separate network of input devices made up of occupancy and photosensors, manual controls, etc.

Wiring class: Line- and low-voltage wiring systems imply different requirements in the field. Class 1 wiring can be installed in the same conduit, while most codes allow Class 2 to be installed outside of conduit. For analog systems, the manufacturer is likely to recommend that Class 2 wiring not be installed with Class 1 wiring in Class 1 conduit, so as to prevent electrical interference with the analog signal voltage.

Termination: Bulk wiring can be cut and terminated in the field, or the wiring can be delivered in specified lengths with preterminated connections.

Shielding: The wire may be specified as shielded or unshielded; shielding provides more protection against electrical interference presented by adjacent electrical wire or equipment. It is generally recommended that low-voltage DC signals be carried by shielded conductors.

Speed: Depending on the speed of transmission, wiring run lengths may be capped.

Loading: For analog systems, the amount of loading (current requirement) must be observed.

Topology: Some communication protocols are limited in available topologies (linear/daisy chaining, star, hub and spoke, etc.).

Control protocol: Wiring requirements may be different based on the type of control protocol used. For example, a system using RS485-based communication will have different wiring requirements than a system using LON or Ethernet.

Voltage drop: In analog systems, voltage drop issues may limit length or require boosters. Wire gauge (size) may be increased to compensate. Consult with the manufacturer.

Polarity: Most low-voltage wiring requires that polarity be maintained and not be able to be reversed.

Stranded vs. solid conductors: It is generally recommended that low-voltage DC signals be carried by stranded conductors. Solid wire is usually acceptable in low-voltage systems that use AC control power.

Verification: Determine if and how the wiring installation can be tested.
Redundancy: When running conductor bundles/cables over long distances, consider running spare conductors to mitigate the possibility of conductor breakage during installation.

Questions to ask about control wiring

What local codes and standards—such as requirements for conduit or plenum-rated jackets— apply to the project? If I exceed these standards, does the potential payback justify any additional costs?

When the system is in full operation, how much control does the building owner want occupants to have over levels and schedules? What is the level of technical sophistication of these occupants?

What level of flexibility and functionality is required from the control system, which will determine what controls are desired?

What is the sequence of operations for the controls in the system?
Is the control wiring Class 1 or Class 2?

Is the control wiring independent of the power wiring?

Is a twisted-pair configuration required to maintain impedance?

Will the wiring require shielding to prevent electromagnetic and radio frequency interference?

How far is the lighting device from its control? What is the maximum run length?

Is there a device or power limit for the wiring run or control?

What control protocol will be used?

What topology is desired?

Are specific connectors required?

Are the wiring requirements a simple matter of voltage drop, wire gauge and number of conductors?

If the local code allows Class 2 wiring to be installed in plenums, is the selected wiring plenum-rated?

How will the wiring network be tested and troubleshot?

Can the manufacturer provide detailed point-to-point schematics? How rigid is the specification in the event of changes in the field?

Are different ballast voltages allowed?

Are different lighting types—fluorescent, HID, LED—allowed?

Special thanks to the following Lighting Controls Association member representatives for their valuable contributions to this whitepaper (listed alphabetically by company):

Andrew Parker, PE, LC, Director of Sales for Encelium Technologies
Thomas Hinds, Product Manager-Fluorescent Dimming Ballasts for Lutron Electronics Co., Inc.
Ronald Bryce, National Sales Manager for PLC-Multipoint, Inc.
Greg Bennorth, Director of System Projects for Universal Lighting Technologies
Pete Baselici, Senior Product Line Manager for WattStopper
Eric Fournier, Director of Product Marketing for WattStopper

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1 comment to Control Wiring: A Primer

  • Dick Brecher

    You have it exactly bass-ackwards: DC flows through the entire cross section of a conductor while AC flows along the skin of circular copper.

    This is called “skin effect.” The severity of it is proportional to frequency.

    We use stranded cable for DC LV lighting control because of its physical properties, not its electrical properties. It is easier to installed and train or pull into conduit.

    Solid #18 also has a nasty ability to work harden when bent. Then, the thin (50V – 300V) insulation can be compromised by a wire kink, producing a short to ground that may be very difficult to locate.

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