The term submeter refers to measuring the electrical load of an appliance, circuit, or sub-panel while measuring the total load of the parent panel. This technique provides an accurate measurement of the amount of power used by the appliance, the energy it consumes and the percent of total load it represents.
To achieve this, an energy monitor must have extra channels to measure the appliance load(s) in addition to monitoring the main feed coming into the home.
At a minimum one will need a multi-point monitor but a circuit level monitor will work best. These links will take you to product specification tables for multi-point and circuit level monitors. The number of channels associated with each model can be found in the third row of each table.
Measuring appliance circuit loads requires a separate current transformer, or CT to be placed around the hot wire exiting each pole on the load side of the circuit breaker. 240 volt double pole breakers require two CT's while 120 volt breakers only need one CT. These CT's measure the current going to each circuit.
CT's are sized by the maximum amount of amperage they are designed to measure. The best way to size a CT is to match its amperage rating to the size of the breaker protecting the circuit it measures. This way, the breaker will most certainly protect the CT as the CT usually has a 20-30% safety margin built in. It is OK to oversize a CT but be prepared for a slight decrease in accuracy.
CT's are configured as either solid core or split core. Solid core CT's are shaped like a small donut. During installation the load wire must be disconnected, routed through the "donut hole" and reconnected. Solid core CT's are less expensive than split core CT's. They are well suited for smaller loads such as those coming off of individual circuit breakers.
The CT's you use should be those recommended by your home energy monitor manufacturer. The output signal from a CT can be in amps, volts or millivolts. Plus, the range of these outputs can also vary. If you do order CT's from another source be sure the output signal will match that which your home energy monitor system was designed to accept.
CT polarity must also be observed. A red dot, arrow or other symbol will be on one side of the CT. This mark normally goes toward the load side of the connection unless the installation manual recommends otherwise. If one of the CT's is reversed, negative current values may be sent to the home energy monitor and the data will not be correct. If this does happen, simply open, flip and reconnect the CT. Double check the polarity mark on the CT's before closing the circuit breaker panel.
Most average sized homes built in the United States today are served with a single 200-amp, 40 circuit breaker panel. Larger homes may have 400-amp service which consists of two 200-amp panels located next to each other near the service entrance. Prior to wiring any submeter loads, the home energy monitor system needs to set up to capture the entire load entering the home as shown here to the right.
If for any reason you are not entirely comfortable with working inside your electrical panel - by all means don't! Call a licensed electrician who can do this type of installation for you.
If you do chose to do the wiring yourself start with locating a good flashlight. Be sure to have it on when you turn off the main breaker. After the main breaker is off, remove the six screws that hold the front panel on the circuit breaker box and remove the panel. Locate the circuit breaker that serves the load you wish to submeter and shut it off.
If you have solid core CT's, remove the black wire from the circuit breaker, thread it through the CT and reconnect the wire. When you submeter a 240 volt circuit do the same with the red wire using a second CT. Do not route both the red and black wire through the same CT as they will cancel each other out. If you are using split core CT's simply clip them around the black (and red) wires. No wires will need to be disconnected from the circuit breaker.
A voltage connection will be required for the energy monitor to measure properly. Route voltage leads (1 black and 1 red wire) from a spare double-pole circuit breaker to the measurement unit or control module according to the manufacturer's instructions. Only one voltage connection is required for the entire panel
Replace the front panel on the circuit breaker box with the six screws you removed. Close the breaker for the submetered load, the energy monitor and finally the main breaker. Power will be restored and logging of the submetered load will begin.
A CAUTION ABOUT CT'S! If the wires coming from the secondary side of the CT are not grounded or terminated into the metering connection when the measured conductor is energized, excessive voltage may develop across the leads. This can cause arcing and possible shocks. Some CT's have voltage limiters built in but not all. The best practice is to always terminate the secondary leads before energizing the conductor.
Analyzing the Data
Review the finer points in your energy monitor's dashboard software to see how sub-metered circuits are tracked and displayed. Also check into the steps you will need to take to export .csv log files from the software into a spreadsheet program such as MS Excel.
After several days or weeks of data have been accumulated, export log files for the entire home as well as those for the appliance(s) that were submetered. Be sure the data logging intervals are the same for both files and align the time stamps by row on the spread sheet. Now you can draw some comparisons as to what percent of total load did the submeter load contribute - by hour, day, week or month depending upon the extent of your data.
If you are using our Power Panel Profiler, sum the kilowatt-hours of the submetered load by day and determine the average. Divide it by 24 and enter it into the proper circuit on the Profiler as an average wattage per hour. For a more realistic daily run time, divide the average number of kilowatt-hours per day by the measured kilowatts of load. If you enter this run time into the Profiler, be sure to use the measured kilowatts of load, not the average wattage per hour that you calculated above.
Graph the data. Look for the trends that tend to drive up electric usage. Consider the time this usage occurred. What caused it? Can it be reduced? How so? If a time-of use rate is available can this load be moved to a period when the rates are lower? These are the types of questions a good energy auditor must seek answers to if sustained energy savings are to be achieved.