Single Pair Ethernet for Building Automation: A Practical Primer

Single Pair Ethernet brings HVAC, sensors, and building-automation devices onto IP over one twisted pair. What IEEE 802.3cg, PoDL, and ANSI/TIA-568.5 mean for designers and installers.
By Todd Taskerud, AWS CCP, RCDD/NTS/OSP/WD, LEED GA
4 min read
Rows of building and industrial control cabinets

Why Building Automation Is Moving to Ethernet

For decades, building automation ran on its own wiring and its own protocols. HVAC controllers, sensors, and actuators communicated over fieldbus networks such as BACnet MS/TP or Modbus RTU on serial two-wire cabling, and that traffic reached the IP network only through gateways. The approach works, but it creates islands: separate cable plants, separate troubleshooting tools, and limited bandwidth at the edge.

Single Pair Ethernet (SPE) is designed to close that gap. It carries Ethernet over one twisted pair instead of four, at distances and costs that make sense for sensors and field devices. It is now a regular topic at industry events—BICSI's 2026 program includes a dedicated session on SPE in building automation—and it is worth understanding before your next intelligent-building project.

The Standards Behind SPE

  • IEEE 802.3cg defines two 10 Mb/s single-pair physical layers. 10BASE-T1L ("long reach") supports point-to-point links of up to 1,000 meters—well beyond the 100-meter limit of conventional four-pair Ethernet. 10BASE-T1S ("short reach") supports shorter links and a multidrop mode where several devices share one segment.
  • Power over Data Lines (PoDL), defined in IEEE 802.3bu and extended by 802.3cg, delivers DC power over the same single pair, so many field devices need no separate power wiring.
  • ANSI/TIA-568.5 covers balanced single twisted-pair cabling and components for commercial premises, giving designers a standards basis for SPE cabling just as TIA-568.2-D does for four-pair copper.
  • Connectors: the IEC 63171 family defines SPE connector interfaces, including variants intended for commercial building and industrial environments. Confirm the interface your device manufacturers have chosen before specifying patch cords and outlets.

Where SPE Makes Sense

  • HVAC and BMS field devices — sensors, VAV controllers, valves, and actuators that need modest bandwidth but are scattered across large floor plates or long runs.
  • Retrofits of fieldbus networks — in some cases the existing two-wire cabling can be evaluated for reuse with 10BASE-T1L, which can reduce the cost of moving a legacy system onto IP. Assess cable type, condition, and length with the device manufacturer before planning on reuse.
  • Lighting, access control, and occupancy sensing — devices that benefit from end-to-end IP addressing and centralized management.
  • Industrial and process environments — 10BASE-T1L was developed with process automation in mind, including long runs to instrumentation.

SPE is not a replacement for four-pair Category 6A to workstations, access points, or cameras. Those devices need far more bandwidth and power than 10 Mb/s SPE provides. Think of SPE as the layer below the conventional LAN, reaching the thousands of small devices that four-pair cabling was never economical for.

Design Considerations

Topology

10BASE-T1L links are point-to-point, which suits a star or a "home run to a field switch" design. 10BASE-T1S multidrop allows several devices on one segment, which resembles a traditional fieldbus daisy chain. Choose based on device count, distances, and how the owner wants to isolate faults.

Pathways and Separation

SPE cabling should follow the same pathway, separation, and fill discipline as the rest of the telecommunications infrastructure under ANSI/TIA-569. Long 10BASE-T1L runs often travel through mechanical spaces and alongside power, so plan separation from electrical conductors and noise sources early. Fire-rating requirements (plenum, riser, general purpose) under the NEC apply to SPE cable exactly as they do to other communications cable—see our guide to plenum vs. riser ratings.

Power

PoDL power classes vary. Confirm the power each device needs and the power the field switch can supply per port, and account for voltage drop on long runs in the same way you would for long PoE runs.

Labeling and Administration

A building may have thousands of SPE endpoints. Apply ANSI/TIA-606-C labeling from day one so that a technician can trace a sensor to a switch port without guesswork. This is often the difference between an intelligent building that is maintainable and one that is not.

Coordination Between Trades

SPE sits at the boundary between the controls contractor and the low-voltage contractor. Decide early who designs, installs, terminates, tests, and documents the SPE cabling. Ambiguity here is the most common cause of finger-pointing at commissioning.

Testing

Field testers with SPE support can certify single-pair links against the applicable performance requirements. Specify certification testing in the contract rather than relying on device-level link lights, and require test reports as part of the closeout package.

Closing Thoughts

Single Pair Ethernet brings building automation onto the same IP infrastructure as everything else, without forcing four-pair cable to every sensor. For owners, it means one network to manage; for designers, it means a standards-based cabling layer for the edge of the building. Heather Technologies can help you source SPE-ready cable, connectivity, and test equipment—reach out to discuss your project.

Todd Taskerud, RCDD/NTS/OSP/WD, is a structured cabling and data-center infrastructure specialist at Heather Technologies.