Overview

The WattNode^® Modbus^® meter communicates using RS-485 (EIA-485) half-duplex, differential signaling at 9600, 19200, and 38400 baud. Under ideal conditions, cable lengths of 4000 feet (~1200 meters) or more are possible. However, the cable recommended for long lengths can be expensive, so it may be more affordable to convert from Modbus over RS-485 to Modbus TCP over Ethernet if there is an existing Ethernet infrastructure in place.

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 10mA and any gauge should work; we recommend 24 AWG to 18 AWG.
  • For terminated networks, the current can be 60mA or higher, so heavier gauge wire may be needed for very long runs.
    • We recommend 24 AWG to 18 AWG for runs up to 4000 feet (~1200 meters).
    • We recommend 20 AWG to 16 AWG for runs longer than 4000 feet.
• 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 300V rated cable. If you are monitoring a 277/480 VAC circuit, use 600V 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 150V or lower. Long runs of 300V or 600V 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 Modbus 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 Modbus cable.

Sample Cables

The following table lists some recommended 300V and 600V shielded cable.

Manufacturer	Part Num	Voltage Rating	Wire Pairs/ Conductors	AWG	Temperature Range	UL / NEC	Supplier	Price	Diameter
-	CAT-5, 5e, 6	???V	4 / 8	?? AWG	??	??	-	-	-
Carol	C2536A.41.10	600V	1 / 2	16 AWG	-20°C to 75°C	UL 2106 NEC CL2, CM	Digi-Key	$486 / 1000 ft	0.308"
Alpha Wire	3221 SL001	600V	1 / 2	22 AWG	-55°C to 105°C	-	Digi-Key	$789 / 1000 ft	0.181"
Alpha Wire	3231 SL001	600V	1 / 2	20 AWG	-55°C to 105°C	-	Digi-Key	$937 / 1000 ft	0.189"
Tyco / Raychem	44A1121-22-0/9-9	600V	1 / 2	22 AWG	-65°C to 105°C	-	Newark	$470 / 100 ft	0.132"
Tyco / Raychem	44A1121-20-0/9-9	600V	1 / 2	20 AWG	-65°C to 105°C	-	Newark	$538 / 328 ft	0.148"
Belden	9341 0101000	600V	1 / 2	18 AWG	-30°C to 90°C	UL 1277 NEC TC	Newark	$1,213 / 1000 ft	0.276"
Belden	9841 060500	300V	1 / 2	24 AWG	-30°C to 80°C	UL 2919 NEC CM	Allied	$451 / 500 ft	0.232"

See Also

• WattNode Wires and Cables

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Keywords: RS485, EIA485