Communicating Submeter Data via BACnet- ASHRAE Journal

Electric submeters provide a cost-effective way to identify large amounts 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 utility-metered 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

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 sol¬id-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 an electric submeter was to walk up to it and write down the reading. Now, the two most common ways of talking to sub¬meters are through proprietary energy analysis software residing on the user’s PC tied into open-architecture protocols or through a pulse output into an ener¬gy 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 RS- 485) cabling or through an Ethernet con¬nection that uses an existing network. Ethernet communications do require an optional module and an IP address. Us¬ing the EIA-485 approach allows up to 4,000 ft (1220 m) of cabling to be run in the building. The manufacturer’s soft¬ware 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 me¬ters and IDRs can be “daisy-chained” with the EIA-485 cabling and then con-verted to EIA-232 to connect with the modem. This provides access to the en¬tire network of meters using a single sys¬tem 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

To download a copy of this article click here.