RS-485 Cables for BACnet and Modbus

Overview

The WattNode^® BACnet and Modbus^® meters communicate using RS-485 (TIA/EIA-485-A) half-duplex, differential signaling at 9600, 19200, 38400, and 76,800 baud. Over short distances <100 ft. (30 m), in electrically quiet environments, the type of cable used is usually not a concern, but when you need to run long distances, in noisy environments, at higher baud rates, the type of cable used becomes more critical.

Under ideal conditions, cable lengths of 4000 ft. (~1200 m) or more are possible. However, the cable recommended for long lengths can be expensive, so if there is an existing Ethernet infrastructure in place it may be more affordable to convert from BACnet MS/TP or Modbus RTU over RS-485 to BACnet TCP/IP or Modbus TCP/IP over Ethernet.

Cable Selection

You should consider the following:

• How many conductors do you need?

  A minimum of three conductors, but the shield may be used as the common conductor, so shielded two conductor cable may be used. If you do not use shielded cable, then at least three conductors are required. Some RS-485 devices do not use a common connection, but we recommend always connecting common for reliable performance and to avoid damage due to surges.

• What wire gauge do you need?
  • For unterminated networks, the current will generally be less than 10 mA and any gauge should work; we recommend #24 AWG to 18 AWG.
  • For terminated networks, the current can be 60 mA or higher, so heavier gauge wire may be needed for very long runs.
    • We recommend #22 to #20 AWG for runs up to 1000 ft. (~300 m).
    • We recommend #20 to #16 AWG for runs up to 4000 ft.(~1200 m).
• What should the cable impedance and capacitance be?
  • Cables suitable for use in an RS-485 network should have an impedance of between 100 and 130 ohms, a capacitance between conductors of less than 30 pF per foot (100 pF per meter), and a capacitance between conductors and shield less than 60 pF per foot (200 pF per meter).
• Do you need shielding?

  Because RS-485 is differential, it is less susceptible to interference, so shielding is not always necessary. However, we recommend shielding for long runs and if there is electrically noisy equipment nearby like variable speed drives. If you use shielded cable, connect the shield to earth ground at one end (generally the PC or RS-485 master).

• Do you need twisted wires?

  Yes, especially for non-shielded cable.

• What voltage rating do you need?

  We recommend wire or cable rated for the highest voltage present. So if you are monitoring a 120/208 Vac panel, you should use 300 V rated cable. If you are monitoring a 480Y/277 volt circuit, use 600 V rated cable. If you have the WattNode in a separate enclosure and there is no way the mains wires can contact the Modbus output cable, then you could safely use lower voltage rated cable, such as 150 V or lower. Long runs of 300 V or 600 V rated cable may be expensive, so it may be more economical to use lower voltage rated cable and use a protective jacket in the regions where the cable is in the vicinity of dangerous voltages.

• Can you run the RS-485 network cable adjacent to or in the same conduit with mains wires?

  We strongly recommend against this. There may be interference from the high voltages and currents present on the mains wires, and if there is any insulation fault, arcing, etc. on the mains wires, it could put dangerous voltages on the low-voltage RS-485 network cable.

Typical Cable Part Numbers

The following table lists shielded twisted pair cables for use in BACnet and Modbus RS-485 networks. Some of the cables listed do not meet the RS-485 specifications for impedance and/or capacitance, so you should limit the length and/or baud rate used.

A typical RS485 cable specification might include the following:

• • Network wiring to be installed using 24 AWG 7/26 Bare Copper – 2 conductor stranded, 7/32 drain wire, characteristic impedance (Zo): 100 to 200 ohms, mutual capacitance <= 12.5 pF/foot @ 1 KHz nominal, capacitance to shield <= 60 pF/foot, propagation velocity >= 60%, UL Type CMP/CL3P 24 AWG.
  • Maximum length is 4000 ft. End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. All communication cabling to be installed in a daisy chain configuration, no T’s are permitted. Shield on communication cable is to remain continuous and ground only at the controller. Polarity of the wires must be observed. Splicing of communications cable is not permitted.

*Cost data current as of 2015-04-06

Category 5 Ethernet cable

Category 5 (Cat 5, Cat 5e, and Cat 6) cables are the most common Ethernet cables in use today. Because they are produced in such large quantities, Cat 5 cables are relatively inexpensive, often less than half the price of specialty RS-485 cabling.

With a typical capacitance of 15 pF/ft. and a characteristic impedance of 100 ohms, Cat 5 cables almost meets the electrical requirements for RS-485 cables. The capacitance is close to 12.5 pF/foot but there is a small impedance mismatch (100 ohms for Cat 5, 120 ohms for RS-485). In practice, Cat 5 cables have been used successfully in many installations, but there are some concerns.

The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels. Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. Another concern is that the small #24 AWG solid wires, which are typically used in Cat 5 cables, can break easily when connected to a screw terminal block, which does not provide any strain relief.

See Also

• WattNode Wires and Cables

Keywords: RS485, EIA485