What’s the main question on the minds of electrical contractors? I’ll bet it is, “How do I grow my business?” or “What can I do to keep it from shrinking?”

Installing submeters is a sure-fire winner. As I stated in the October column, submeters can cut energy bills for users, improve the environment and provide business for contractors. Energy is one of the big costs in most businesses, so managing it is really important.

“Submetering is an excellent way for end-users to understand how and where energy is being consumed,” said Melissa Golden, market segment manager, Schneider Electric. She continued that, although it’s ideal for tenants, e.g., in a commercial office building, submetering can also be enacted in larger facilities and campuses to monitor how much energy different buildings or departments use.

What is a submeter?
Let’s start with a definition. The best one I came across is from Paul Golden, national business development manager, energy solutions, for Schneider Electric.

“It is important to define submetering here as either the metering of energy downstream from the master service entrance meter or as tenant submetering (also known as sub-billing) for the purposes of charging energy costs directly to the actual consumer,” he said.

What are watts?
It seems like a trivial question, but there’s a little more to it than first meets the eye. Some of the finer points are important in making a decision about which submeters to buy. If all of the devices running off utility power were resistive loads—such as incandescent lamps—the voltage and current would be in synch with each other, and watts would simply be volts amps. In our world, however, resistive loads are getting to be the exception. None of the “smart” products—computers, TVs, HVAC controls, electronic ballasts for lighting, or any digital devices—are resistive loads. This means that the current and voltage waveforms are not pure sine waves, so volts amps does not represent true power. Typically, current comes in spikes of shorter duration than the standard 60 Hz cycle, and the voltage waveform flattens during these current peaks. There are different ways this same effect is described: nonlinear loading, poor power factor, high odd harmonics. Real power is represented by “true rms” volts amps, which takes this distortion into account.

It is this real power that gives a valid picture of how much energy is actually being consumed. The misleading thing here is that some instruments claim to measure power, but their measurements are thrown off by waveform distortion.

“Meters that use microprocessors sample at 64 times per cycle in order to compute the true rms readings that they store and display,” said Steve DeBeradinis, vice president, sales, at National Meter Industries Inc., Bedford, N.H. That way, the meter won’t miss subcycle spikes or sags.

What is energy?
Energy is watts time. The standard unit billed by utilities is kilowatt-hours (kWh). This is the amount of energy consumed in one hour by a continuous 1-kilowatt load. So the only way to properly measure energy is to accurately measure power.

How to choose
The simplest meters display the kilowatt-hours downstream from where they have been installed. Even in the simplest installation, the readings’ accuracy is crucial. One of the reasons an owner will install submeters is to charge tenants for their portion of the building’s electric bill. If the sum of the individual readings doesn’t come close to the total utility bill, someone will be unhappy—either subtenants or landlord—depending on who is favored.

Accuracy
An energy meter measures current and voltage and multiplies the two. Current is sensed by placing the feed wire through a current transformer (CT).

There are two types of CTs: solid and split core. To install a solid core, you have to disconnect the feed wire. A split core is hinged so that you can just snap it around the wire without disconnecting it. Solid-core CTs are perfect for new construction because of their lower cost, and split cores are ideal for retrofits.

The accuracy of the reading depends on both the meter and the CT. It is critical that the CT accuracy meets the ANSI C-37.13 standard. All CTs must be labeled with accuracy, burden, volt-amperes and hertz for the ratio at which the CT is being used, DeBeradinis said.

“The most common mistakes during installation are reversed CTs [installing it backward] or placing the CTs on the wrong circuit, thus montitoring the wrong load. Use a licensed electrician for installation,” said Jim Sinopoli, managing principal, Smart Buildings LLC.

Fortunately, many modern microprocessor-based meters can display whether the CT is installed correctly.


BROWN is an electrical engineer, technical writer and editor. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at ebeditor@gmail.com or at www.writingengineer.com, an independent professional writing service.