Simple steps to perform a load study

To determine present loading, it is necessary either to precisely calculate the existing loads, or measure them. How these calculations are performed across the globe varies but typically some measurement of the consumed current and energy will be used in developing a safe, reliable result to the problem.

In much of North America, the primary method (Article 220 of the 2014 National Electrical Code) is to find the maximum demand during a one-year period. But this only works if a year of demand data is available. The alternative is to record the demand over a 30-day period to find the maximum typical demand, known as a load study.

The NEC specifies that:
- The highest demand is the highest demand among all feeders
- You should take measurements when the building is occupied
- Include heating and cooling loads, whichever is larger, or correct to account for these loads
- Add in any other periodic loads.

Performing a load study with a logger such as the Fluke 173x series takes just five steps.

1 Hook up to the feeders or service

Connect the Fluke Logger to line power and secure the area. Ensure the instrument is powered to avoid an instrument that has used up all its battery power after a short time. For a three-phase wye system there will be seven or eight connections (in some cases neutral current is not considered):
- Three phase voltages
- Neutral voltage
- Three phase currents
- Neutral current.

2 Set power system parameters

Set the Network Topology to match the system being recorded. Verify the nominal voltage (mains voltage) and line frequency are correct. The Fluke Logger includes displays that make it easy to check everything is connected correctly; in some instruments there’s even an automatic configuration and correction tool to ensure that the system is connected and set up correctly.

3 Set the recording time

Set the Fluke Logger to 15-minute averaging intervals and a 30-day recording duration. The 15-minute average time is specified in the NEC 220 as the designated period.

4 Record the data

On the Power display the Fluke Logger will display a min, max, and average of these values every 15 minutes:
- Power in Watts for each phase and total
- Reactive Power in vars for each phase and total
- Apparent Power in VAs for each phase and total
- Power Factor for each phase and average
- Averages of Energy in kWh and Reactive Energy in kvarh

A live trend screen will appear when selected and plot a new minimum, maximum and average on the display every 15 minutes, moving from left to right.

The Fluke Logger can also be set to monitor the 15-minute period in terms of power demand. This demand period is often used by utilities to charge industrial and commercial consumers variable rates. Minimising this demand can save users money based on their electricity tariff agreement.

5 Download and review the measurements

For 30 days of recording, with a measurement every 15 minutes, there will be 2880 sets of measurements.

Application software packages usually have a built-in report generator that includes graphs of current and real power, as well as maximum average current on a bar chart. The report can range from a single current or power number to a full-blown document with graphs and tables. But the ultimate goal is still the same: to get an accurate picture of the system load, help design a safe upgraded system and satisfy the electrical authorities.