Load Balancing: Why and How?
The importance of balance comes up in many contexts these days. Behavioral professionals speak of work-life balance. Mechanical engineers employ counter balancing shafts in motorcycle engines to reduce vibration. Airlines carefully fill seats to assure that the airplane’s balance point is preserved.
In poly-phase electrical systems, balance is also important. Ideally, each phase should be carrying identical loads. This helps reduce current on the neutral line and excess heat on individual phases. It also helps optimize the utility meter readings for lower bills.
The largest impact on your bills occurs when demand spikes are occurring on the high load phase, which they usually are. The higher base load on that phase increases the amplitude of the demand spikes, which are billed at a significantly higher rate than simple kWh consumption.
Of course, in the real world at any point in time, phase loads are rarely identical. Lights are turned on and off. HVAC systems cycle on and off. Computer printers cycle through print jobs. In other words, loads are always changing and affecting phase-to-phase balance. Constant, perfect balance is unattainable within reasonable budgetary constraints.
Our challenge, therefore, is to achieve an average balance over time. This will optimize the efficiency of your electrical infrastructure and minimize your power bills.
This means that we cannot simply take a brief snapshot of phase loads to see if they are out of balance. Using a logging meter, we have to meter each phase over a sufficiently long period of time to take in all of the load fluctuations. We would recommend logging a full billing cycle before attempting any changes.
Ideally, at the end of that time, total kWh per phase will be very close to the same. If there is more than a 5% difference, it could be worthwhile to move one or more circuits from the high load phase to the low load phase. Metering individual circuits can help you determine which circuits are best candidates for relocation in the panel.
To illustrate, let’s look at an example. A small manufacturing plant with a 3-phase, 480 volt service is billed for 500,000 kWh per month. This is a 24/7 operation so their average base load is just under 700 kW. After metering the system for a month, we see 185,000 kWh on phase A, 165,000 on phase B, and 150,000 on phase C. If they were balanced, we should see about 167,000 kWh per phase. With this information, we see that if we could move a 15-20,000 kWh circuit (or combination of circuits) from phase A to phase C, we could balance out the loads. This will decrease the base demand by (on average) 25 kW. Assuming a demand charge of $15/kW, that amounts to $375/month. The ROI is likely less than one month.
I realize that this is a bit of an oversimplification and I reiterate that I’m talking about averages. A number of factors could impact these numbers in either direction, but the basic concept is sound.
In general, going through the trouble of monitoring phase balance and making corrections will save you money on your bills and on equipment maintenance. In many cases, this will add up to substantial amounts over time, and once you understand the concept, it really isn’t that difficult to implement.