Before new equipment is added to a plant, electricians or technicians determine whether an existing panel has the capacity to handle the power load drawn by the machine. The workers start by looking at the panel size—the number and size of circuit breakers installed vs. the number of empty circuit-breaker spaces.
Based on these observations, workers estimate how much power the panel is using. However, there are times when a panel that appears to be lightly loaded with several empty circuit-breaker spaces is actually overloaded because of the size of the loads on the other breakers. Other times, a panel that appears to be heavily loaded may be only partially loaded, offering ample spare capacity.
Logging is begun as part of a load study to determine panels' actual energy use and whether there is a specific need for additional power. This eliminates guesswork and saves the expense of adding power when it’s not needed. Electrical engineers perform similar load studies before large-scale expansions, and the purpose is the same: Measuring existing load levels (three-phase current draw) over a complete 30-day use cycle allows electricians and engineers to determine how much additional capacity is available in an electrical panel.
These studies also reveal opportunities to reduce energy consumption by turning off loads or adjusting the schedule of when they operate. Energy loggers create a chart of use patterns over time so that facility managers can analyze when and how energy is being used and determine where there is room for improvement. For example, an air handler running 24 hours a day may really need to operate only during the portion of a day when the space is occupied. In other instances, an energy-intensive process (e.g., operating an industrial electric oven) may be able to be shifted to the evening hours when electricity rates are lower.
To connect an energy logger, a technician must open and/or remove the covers of disconnects, motor control centers, panels, switchboards, and other types of cabinets that aren’t accessed very often because of their high voltage levels and the difficulty of powering down vital equipment. Thus, anyone doing this work must wear appropriate personal protective equipment (PPE) and follow all safety regulations when working on energized panels.
Experienced professionals have learned the hard way what not to do when conducting a load study. They know that mistakes during setup lead to erroneous and incomplete data, invalidating an entire 30-day study.
With that fact in mind, here are six common load-study mistakes and how to avoid them:
1. Not fully charging the logger prior to the session
Before performing a load study, confirm the battery in the energy logger is fully charged. As with any product, the battery will lose charge while it is stored—whether it is brand-new or simply hasn’t been used for a while. While the logger draws its power from a convenience outlet or the measurement line when recording, it still requires battery power for the purpose of reviewing settings and data before installation and for backup power in the event of an outage.
2. Not installing the logger at the correct disconnect or panel
Although it may sound obvious, the technician installing the logger must verify that he or she is installing the logger at the correct disconnect or panel. Many locations have multiple disconnects and panels, and it may not be clear which one is the target of the load study. When in doubt, contact the person requesting the study and verify the individual load or panel to be monitored.
3. Not verifying that the power source is live and that it is not a switched outlet
Load studies are performed at a wide variety of locations, including apartment buildings, commercial office buildings, industrial facilities, and retail stores. With older-model loggers, standard procedure is to plug the logger into a convenient outlet located near the panel under study. The technician installing the logger must verify that the power source is live and is not a switched outlet controlled by a switch, a time clock, or a photo cell. If the outlet is switched on and off on a regular basis, the logger’s batteries will be drained and the logger will stop recording.
4. Not labeling the power cord
The outlet that powers an energy logger may not be located immediately adjacent to the load being monitored; in this case, an extension cord from the outlet to the logger will be required. The extension cord must be installed so that it is not subject to physical hardship, does not present a hazard to personnel, and cannot be unplugged inadvertently. Placing a piece of adhesive tape (or posting a similar sign) on the wall near the outlet with DO NOT UNPLUG printed on it can prevent the unplugging of the extension cord by janitorial or maintenance staff.
5. Incorrectly setting up the logger
Always perform a simple checking routine before starting the logging session to ensure that all the voltage phase connections match. Confirm that phase A from the logger goes to the phase A conductor, and so on. Next, verify correct polarity for each current probe. The arrow on the current probe should point toward the load. Check every phase to make sure they all point in the same direction. Finally, verify the instrument is reading correctly: Power should be measuring positive (if the load is operating) and the power factor should show a reasonable value for the type of load. Using an instrument that automatically checks, highlights and corrects connection errors is a definite "nice to have."
6. Not verifying that recording has begun
Setting up an energy logger to perform a load study is a straightforward task, but the simplest mistake—not turning on the recording function—still can be made. After confirming that the appropriate current and voltage leads are connected to the load, select your recording parameters using the buttons and menus on the unit and push the “start logging” or “record” button. You should see both a message and an icon on the logger’s screen indicating that logging has begun. It is a good practice to wait through the first recording interval and verify the logger has recorded its first value. Then you can be absolutely certain recording has begun and the setup is correct.