BACnet MS/TP Communication Data Bus Fundamentals
Network Guide 41
Power Supply Requirements for 24V Controllers
BACnet MS/TP is a Three-Wire Data Bus
Even though data is transmitted over a 2-wire twisted pair, all EIA-485 transceivers interpret
the voltage levels of the transmitted differential signals with respect to a third voltage reference
common to all devices connected to the data bus (signal reference). In practice, this common
signal reference is provided by the building's electrical system grounding wires that are
required by electrical safety codes worldwide. Without this signal reference, transceivers may
interpret the voltage levels of the differential data signals incorrectly, and this may result in
data transmission errors.
Avoid Ground Lift
24V Power wiring runs should not be too long, nor have too many devices connected to it. Wir-
ing used to supply power to devices has a resistance that is proportional to the length of the
wiring run (see Table 9).
If the power run from the power supply is relatively long and it supplies power to many
devices, a voltage will develop over the length of wire. For example, a 1000 ft of 18 AWG cop-
per wire has a resistance of 6.4 Ohms. If this wire is supplying 1 Ampere of current to con-
nected devices (as shown in Figure 25), the voltage developed across it will be 6.4 volts. This
effect is called ground lift.
Note: ECB/ECL-VAV controllers are insulated devices and therefore may not be
grounded. When not grounded, the reference for the BACnet MS/TP data bus is
made by connecting the shield of the BACnet MS/TP data bus to the controller's
COM terminal to provide a signal reference. This shield is grounded at one point
only - see Figure 13.
ECB-PTU line-powered controllers are double-insulated devices and therefore are
not grounded. The reference for the BACnet MS/TP data bus is made by connecting
the shield of the BACnet MS/TP data bus to the controller's COM terminal to provide
a signal reference. This shield is grounded at one point only - see ECB-PTU Line-
Powered Data Bus Controller Shield Grounding Requirements on page 25.
Table 9: Resistance of Common Copper Wire Sizes
AWG Diameter (Ø) Area Copper wire resistance
(inch) (mm) (kcmil)
(mm
2
)
(Ω/km) (Ω/1000ft)
14 0.0641 1.628 4.11 2.08 8.286 2.525
16 0.0508 1.291 2.58 1.31 13.17 4.016
18 0.0403 1.024 1.62 0.823 20.95 6.385
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