BACnet ARC156: Low Cap vs. Mid Cap Cable
As commercial buildings become smarter and more connected, reliable communication between HVAC controllers, sensors, and building automation systems (BAS) has become critical. One of the most common communication protocols used in modern HVAC and building controls is BACnet, and within certain building automation platforms, ARC156 has become a preferred high-speed communication network.
Choosing the correct cable for a BACnet ARC156 network is just as important as selecting the controllers themselves. Terms such as low capacitance and mid capacitance frequently appear on cable specifications, but many contractors and engineers are unsure of what they mean or why they matter.
This guide explains BACnet, ARC156, low-capacitance cable, and mid-capacitance cable—and why selecting the proper cable can make the difference between a reliable building automation system and one plagued by communication errors.
What Is BACnet?
BACnet (Building Automation and Control Network) is an open communication protocol developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). It is standardized under ANSI/ASHRAE Standard 135 and allows devices from different manufacturers to communicate on the same building automation network.
BACnet is commonly used to connect:
- HVAC equipment
- Air handling units (AHUs)
- Variable air volume (VAV) boxes
- Chillers
- Boilers
- Lighting controls
- Access control systems
- Energy management systems
Unlike proprietary protocols, BACnet enables interoperability between equipment from multiple manufacturers, reducing system complexity and increasing flexibility during future upgrades.
What Is ARC156?
ARC156 is a 156 kbps ARCNET-based communication network used by several building automation manufacturers as a BACnet transport. Compared to BACnet MS/TP, ARC156 offers significantly higher communication speed and deterministic token passing, meaning every controller gets a predictable opportunity to communicate on the network.
Some important specifications include:
- Communication speed: 156 kbps (156,000 bits per second)
- Typical cable size: 22 or 24 AWG stranded, twisted, shielded copper
- Maximum network segment length: 2,000 feet (610 meters)
- Daisy-chain topology (no star wiring)
- Shielded twisted pair construction recommended
Although 156 kbps may seem slow compared to Ethernet, it is more than sufficient for transmitting sensor readings, temperature values, occupancy data, alarms, and controller commands throughout commercial buildings.
Why Cable Capacitance Matters
One of the most overlooked specifications for BACnet cable is capacitance.
Capacitance is measured in picofarads per foot (pF/ft) and represents the cable’s ability to store electrical charge between conductors.
The lower the capacitance, the less electrical energy is stored in the cable. This allows communication signals to transition more quickly between high and low states, helping preserve signal quality over long cable runs.
As cable capacitance increases:
- Signal edges become slower.
- Digital pulses become rounded.
- Communication errors increase.
- Network reliability decreases.
- Troubleshooting becomes more difficult.
Since BACnet ARC156 uses high-speed digital communication, maintaining clean signal transitions is essential for dependable network performance. Excessive capacitance can distort these signals, especially on longer runs.
What Is Low Capacitance Cable?
Low-capacitance cable is specifically engineered for high-speed communication networks.
Typical characteristics include:
- Capacitance: approximately 12–16 pF/ft
- Twisted pair conductors
- Overall foil shield
- Drain wire
- 100–120 Ω characteristic impedance
- Stranded copper conductors for easier installation
Many BACnet manufacturers specifically recommend low-capacitance, twisted, shielded copper cable for both MS/TP and ARC156 networks.
Benefits
Low-capacitance cable provides:
- Longer communication distances
- Fewer transmission errors
- Faster signal rise times
- Better noise immunity
- More stable controller communications
- Reduced maintenance costs
For commercial campuses containing hundreds of HVAC controllers, these advantages translate directly into increased system reliability.
What Is Mid Capacitance Cable?
Mid-capacitance cable typically has:
- Capacitance: approximately 20–30 pF/ft
Although still suitable for many control applications, mid-capacitance cable stores more electrical energy than low-capacitance cable.
That extra capacitance increases the time required for digital signals to charge and discharge the cable, making it less desirable for long, high-speed communication networks.
Mid-cap cable may perform adequately on:
- Shorter controller runs
- Smaller HVAC systems
- Lower-speed communication protocols
However, on long BACnet ARC156 segments approaching the 2,000-foot maximum, lower capacitance generally provides a greater performance margin.
Numerical Example
Consider two 2,000-foot cable installations.
Low Capacitance Cable
- Capacitance = 13 pF/ft
- Total capacitance: 13 × 2,000 = 26,000 pF = 26 nF
Mid Capacitance Cable
- Capacitance = 25 pF/ft
- Total capacitance: 25 × 2,000 = 50,000 pF = 50 nF
The mid-capacitance installation has nearly twice the total capacitance, meaning the network drivers must charge and discharge almost twice as much stored electrical energy during every signal transition. That extra loading can reduce signal quality, particularly as network length increases.
Why Contractors Should Care
Communication issues often stem from installation practices rather than controller failures.
Common causes include:
- Using the wrong cable type
- Excessive cable capacitance
- Improper grounding
- Star network wiring
- Missing end-of-line termination
- Excessive cable length
Selecting the proper cable during installation can prevent hours of troubleshooting later.
Typical Applications
Low-capacitance BACnet cable is commonly used in:
- Hospitals
- Universities
- Airports
- Office buildings
- Manufacturing facilities
- Data centers
- Hotels
- Schools
These facilities often contain hundreds of BACnet devices that must communicate continuously for HVAC, lighting, energy management, and life-safety systems.
Final Thoughts
BACnet has become the backbone of modern building automation, enabling equipment from different manufacturers to communicate seamlessly. Within many HVAC control systems, ARC156 provides a fast, deterministic 156 kbps network that supports reliable controller-to-controller communication.
While network hardware often receives the most attention, the communication cable is equally important. Selecting a low-capacitance, twisted, shielded cable helps maintain signal integrity, reduces communication errors, and supports dependable performance over runs of up to 2,000 feet (610 meters). Mid-capacitance cable can still be suitable for certain shorter or less demanding applications, but for larger ARC156 installations, lower capacitance provides additional reliability and performance headroom.
For contractors, engineers, and facility managers, understanding the relationship between BACnet, ARC156, and cable capacitance leads to smarter cable selection, fewer commissioning issues, and building automation systems that continue to perform reliably for years to come.
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