Remote Powering Methods Over Communications Cabling


Impact of the 2017 National Electrical Code on Remote Powering

Since the advent of the first Power over Ethernet (PoE) standard, IEEE 802.3af, in 2003, the information and communications technologies (ICT) industry has seen an array of PoE-enabled devices and applications proliferate commercial buildings, such as Wi-Fi, surveillance cameras, commercial lighting and sensors.  The success of the technology can be largely attributed to its advantage over traditional high-voltage powering methods due to deployment via the communications cabling system.  As many devices have become network- or IP-enabled, the communications cabling system that has historically been used to deliver data-only services can be used to deliver low-watt power as well.  While PoE garners much of the attention and market share, there are a number of other remote power methods available.

Class 2 Power

Class 2 power limits are defined within the United States National Electrical Code (NFPA 70), which states that Class 2 circuits have voltage limitations not exceeding 30VAC or 60VDC with a maximum power output of 100VA.  Due to its power limitations, a Class 2 circuit is considered safe from a fire initiation standpoint and provides acceptable protection from electrical shock.  For this reason, Class 2 systems do not require the same wiring methods as power, light and Class 1 systems defined within the code and can be delivered utilizing low-voltage communications cabling.  Examples of Class 2 power sources include the 24/28 VAC power supplies used in commercial video surveillance and access control systems.

SELV Power

Safety (or Separated) Extra Low Voltage Power (SELV) is defined within IEC 60950-1, which is an international standard for IT equipment. SELV circuits meet the following criteria:

  • Extra Low Voltage Circuit (< 35 VAC, <60VDC)
  • Low risk of accidental contact with a higher voltage primary circuit
  • The design of a SELV circuit typically involves an isolating transformer, guaranteed minimum distances between conductors and electrical insulation barriers.

Figure 1 – Isolating Transformer in SELV Circuit

Examples of SELV circuitry on IT equipment include the RJ45 network interface on an Ethernet switch or power over HDBaseT (PoH) appliances.

Isolating Transformer in SELV Circuit

Figure 1 - Isolating Transformer in SELV Circuit

IEEE 802.3 Power over Ethernet (PoE)

By definition, IEEE 802.3 PoE-enabled ports are SELV.  PoE is delivered via an endspan switch or midspan injector known as power sourcing equipment (PSE) over the communications cabling that provides connectivity to the powered device (PD).  Power is supplied in common mode over two or more of the differential pairs of wires of the balanced twisted-pair cabling system.

Endspan and midspan configurations for powering PoE-enabled cameras

Figure 2 – Endspan and Midspan Configurations for Powering PoE-enabled Cameras

The IEEE 802.3 LAN/MAN standards committee has approved a number of PoE standards:

  • IEEE 802.3af-2003 - Provides up to 15.4 watts of DC power (PSE), 12.95 watts at load (PD)
  • IEEE 802.3at-2009 - PoE standard also known as PoE+ provides up to 30 watts of DC power (PSE), 25.5 watts at load (PD)

Development is underway to increase the available power to the PD to 70 watts, which is being led by the IEEE P802.3bt DTE power via MDI over 4-pair task force.  The following table compares the different IEEE 802.3 PoE standards:

Current and proposed IEEE 802.3 PoE standards

Figure 3 – Current and Proposed IEEE 802.3 PoE Standards

Cabling and Infrastructure

Twisted-pair copper in the horizontal has an advantage over other media types such as fiber and wireless in its ability to provide a physical transport for both data communications and low-watt power delivery.  Due to the larger 23 AWG conductor size that is typical of many Category 6 and 6A cable constructions when compared with 24 AWG Category 5e cable, these cable types are better suited to dissipate heat due to remote powering applications over communications cabling such as Class 2 and PoE.

Whichever remote powering method you choose, we have the products, services and expert technical advice to get your project powered up. For more information on industrial communication and control, click here.