ASHRAE Journal - November 2013 - B26

BACnet Today and the Smart Grid

Communicating
Submeter Data
Via BACnet
®

By David Bovankovich

lectric submeters provide a cost-effective way to identify large optional module and an IP address. Usamounts of energy savings (Table 1) that otherwise might be missed

by the master utility meter at the main service entrance. Submeters are
installed on the facility side of the main utility meter to shadow utilitymetered energy consumption at the enterprise level all the way down
to tracking a single device or circuit panel. In general, there are three
common types of submeters in commercial use today (Table 2).

The ﬁrst two types-feed-through and
current transformer (CT)-based-are
socket meters. CT-type socket meters
typically are used with loads of 400A and
above. In commercial applications, the
extra space required for CT cabinets and
meter bases diminishes available rental
space. Additionally, socket-type meters
are not UL listed. The third type is the solid-state electronic submeter, a non-socket
device that provides several operational
advantages, including space savings.1

Of Submeters and BACnet
It wasn't all that long ago that the only
way we could access information from
B26

an electric submeter was to walk up to it
and write down the reading. Now, the two
most common ways of talking to submeters are through proprietary energy
analysis software residing on the user's
PC tied into open-architecture protocols
or through a pulse output into an energy management system. The two basic
methods of communication are through
a "hard-wired" system or by means of a
phone modem.
The hard-wired system works through
dedicated EIA-485 (formerly called RS485) cabling or through an Ethernet connection that uses an existing network.
Ethernet communications do require an

B A C n e t ® t o d Ay A n d t h e S m A rt G r i d | A S u p p l e m e n t t o A S H R A E J o u R n A l

ing the EIA-485 approach allows up to
4,000 ft (1220 m) of cabling to be run
in the building. The manufacturer's software is happy to use all of these methods
simultaneously and is easily set up to do
this. One important thing to remember is
that pulse meters have to be used with an
IDR (Interval Data Recorder) to provide
communication.
When using a phone modem, the meters and IDRs can be "daisy-chained"
with the EIA-485 cabling and then converted to EIA-232 to connect with the
modem. This provides access to the entire network of meters using a single system with only one modem. Hundreds of
meters at a location can be read from a
single modem through IDRs and meters
from diverse equipment suppliers.
For communications, BACnet is useful
for talking with submeters, especially for
HVAC. BACnet-compatible meters can
communicate over either an EIA-485
cable system or use Ethernet cabling.
When used with EIA-485, the BACnet
About the Author
David Bovankovich is vice president of engineering for E-Mon in Langhorne, Pa.
n ovem Ber 2013

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