Enabling Smart Buildings through Evolutions in DC Power
Authors: Gale Moericke (CRUX), Isaac Sachse (Belden), Miguelangel Ochoa (ITTERA), Sekhar Kondepudi (Cisco)
As smart buildings and smart infrastructure continue to evolve, the need for efficient and reliable in-building power distribution is increasing. Driven by a focus on sustainability, IoT sensor proliferation, and the convergence of Information Technology (IT) and Operational Technology (OT) systems, direct current (DC) power emerges as a compelling alternative to traditional alternating current (AC) power.
Innovative DC power technologies, such as Class 2 power (including PoE) and newer Class 4 Fault Managed Power (FMP), present distinct advantages and limitations. When used correctly, these technologies can significantly revolutionize in-building power distribution.
This is the first blog in a series exploring in-building DC power technologies, starting with defining key terms.
What is limited DC power?
In the US, the National Electrical Code (NEC) is the foundational rulebook governing the design and installation of electrical distribution systems. It outlines wiring methods for distributing AC and DC power throughout a building. It includes provisions for special power circumstances with limited power and/or voltage, referred to as “class-rated” power. The NEC currently outlines four classes of power, as shown below:
| Type of Circuit | Main Features | Common Applications |
| Class 1 |
Contains two subgroups. One is power circuits limited to 30 V (Volts) and 1000 VA (volt-ampere) power. The other is remote control and signaling circuits limited to 600 V. |
Industrial controls, remote device control, signaling, and power-limited applications. |
| Class 2 | Contains power systems limited to 60 V and 100 VA (100 W if DC), which is low enough to be considered safe from a fire initiation standpoint and provides acceptable protection from electric shock. PoE power is a Class 2 power. | Low-voltage, power-limited for safety. Thermostats, security systems, doorbells, and PoE-powered Ethernet network devices such as IP phones, wireless access points, and security cameras. |
| Class 3 | Contains power systems limited to 100 V and 100 VA, which is considered safe from a fire initiation standpoint but does not provide acceptable protection from electric shock. Additional overcurrent protection is required. | Audio systems, signaling systems, fire alarm systems, and nurse call systems
|
| Class 4 | Contains power systems up to 450 V, requiring the use of a listed transmitter and receiver to limit the energy transmitted into the circuit and monitor the circuit for faults. This class is commonly referred to as fault-managed power (FMP), with integrated fault detection and isolation to prevent fire initiation and provide acceptable protection from electric shock. | Remote devices in smart buildings, hotels, smart poles, agriculture applications, and other Internet of Things (IoT) systems |
This introductory blog post focuses on Class 2 and Class 4 power systems, which are particularly relevant to the evolving smart building industry. Future posts will delve deeper into these systems, exploring specific use cases and their benefits for in-building power distribution.
Class 2 Power-Limited Circuits
NEC Class 2 outlines specific rules and requirements for low-voltage DC power systems. This classification, which has been part of the NEC for over 30 years, imposes a 100 VA limit on power delivered over a given DC circuit. Due to this limitation, these circuits are considered safe from fire initiation and provide acceptable protection from electric shock. In many jurisdictions, installation of Class 2 circuits does not require a licensed electrician. However, local regulations may vary, so checking with your local jurisdiction is recommended. Additionally, these circuits often do not necessitate mechanical protection, meaning conduit is not required in most environments. There are two primary alternatives under the Class 2 umbrella:
a) Power over Ethernet (PoE) is a special type of Class 2 power technology based on IEEE 802.3 standards, enabling transmission of 15 to 90 Watts (W) of DC power from power sourcing equipment (PSE) to power devices (PDs). PoE is delivered simultaneously with data over balanced twisted-pair cabling or category cables that comprise in-building network infrastructure, simplifying installation and reducing the need for separate AC power outlets for networked devices. It is widely used around the world to power devices such as IP telephones, wireless access points, surveillance cameras, smart lighting, shades, and more. PoE power is commonly delivered over two or four pairs of a four-pair cable but can also be delivered over a single pair (SPoE) for building automation system devices. Since PoE systems transmit power and data concurrently, they are commonly deployed and managed by IT professionals. For the purposes of this blog series, we will focus on four-pair PoE configurations.
b) Direct DC Power includes all other Class 2 limited-power circuits except PoE. These systems generate Watt-limited power from a power source delivered over 18 AWG or larger copper conductors directly to the device(s) being powered. These conductors may be deployed in tandem with fiber optic cabling that delivers data, bundled with fibers in a single hybrid copper-fiber cable.
Class 2 circuits have some limitations. PoE is limited to a maximum distance of 100 meters (m) over twisted-pair cabling per IEEE 802.3 standards. Direct DC power distances are electrically limited depending on the size of the conductor and voltage drop. For instance, a 16 AWG copper conductor pair can supply 75 W up to about 182 m, while a 12 AWG copper conductor pair can deliver the same power over a longer distance of about 457 m. Class 2 power is also code-limited to 100W, enhancing safety but restricting its suitability for powering many devices.
Class 4 DC Power
NEC Class 4 was adopted as Article 726 into the 2023 NEC, making it a brand-new way to deliver more DC power over longer distances. As an innovative response to Class 2 limitations, a Class 4 system can deliver up to 450 V but with the same level of fire safety and shock protection as Class 2 systems. Rather than being power-limited (e.g., low voltage), a Class 4 system is defined by its ability to limit the amount of energy that can go into a fault, which is why it is called Fault Managed Power (FMP). A Class 4 circuit requires a Class 4 transmitter (source) and a Class 4 receiver (load). Together, these devices actively monitor the circuit and immediately stop power in the event of a fault. While FMP systems based on this technology have been available for several years, NEC adopted the entirely new Class 4 definition for these circuits and components in response to marketplace early adopters and to avoid confusion among code enforcement officials.
Class 4 could have as transformative an impact as PoE. Multiple vendors have now introduced Class 4 systems to the marketplace. While still considered a new technology deployed by early adopters, interest is growing.
Subsequent posts in this blog series will delve more into the details of Class 2 and Class 4 circuits, explore specific use cases, and discuss the value propositions for choosing these circuits for in-building power distribution.
Reference
For further details, refer to NFPA 70 National Electrical Code 2023 edition and the relevant Articles, as follows:
- Article 724 – Class 1 Power-Limited Circuits and Class 1 Power-Limited Remote-Control and Signaling Circuits
- Article 725 – Class 2 and Class 3 Power-Limited Circuits
- Article 726 – Class 4 Fault-Managed Power Systems
