Power Metering: When, Why, How?

In existing buildings we often need to meter the current load to know how much new power we can add to the existing electrical equipment. In some cases we can use the available record drawings and/or review the installed conditions to verify if additional load can be supported, but that involves verifying the connected load of every circuit which can be very time consuming and costly. A better way to determine the existing load on a feeder, panelboard or switchboard is through the use of demand power metering.

For example, schools looking to add additional plug loads or a building with evaporative cooling looking to upgrade to DX cooling will need to know how much new load the existing electrical system can support. In order to do so, the electrical engineer must check a myriad of component capacities including fuses, disconnects, switchboards, transformers and the like and review against the current usage.

Load Studies

For all new electrical loads that are placed on an existing electrical distribution system, the NEC states that a load verification must be provided to determine if the new load will exceed the rating of the existing equipment. The NEC defines the requirements which we must operate within when using a power metering study to determine existing loads. This also depends on the authority having jurisdiction. Some AHJ’s like OSHPD only require a 3 day load study. Most, however, require a 30 day load study to be confident that the peak load is captured. It may also be important to capture the data in the warmer portions of the year if the panel has a significant HVAC load.

12,000 Volt Service Under a Load Study

12,000 Volt Service Under a Load Study

Our example shows a 12,000 Volt service being monitored by a specialty electrical contractor so we could determine if an existing 1,000 ton chiller could be increased to a 1,250 ton chiller. You can see the meter on the floor of the cabinet. The red hoop around one of the feeders is the current transducer (CT’s) which will measure the amperage draw of the line.

The meter data captures volts, amps, kVAR, kW and various other points we use to assess the status and capacity of the existing equipment. Once the data is available, the electrical engineer analyzes the amp draw at max load. The electrical engineer, per NEC requirements, must then add 25% to the average max peak over a 15 minute period to determine the remaining capacity.

Load studies can be performed for single panelboards or distribution switchboards where new circuits are added, or even on medium voltage transformers as shown above.

Alternate Methods

It is important to keep in mind that there is another method of determining existing loads through demand meter history. For electrical services with demand meters, we can take the peak demand from 12 months of utility bills to find out what the peak electrical load is.

Making a Decision

Once we have reviewed the current power usage combined with proposed additions, we can determine what, if any, electrical upgrades will be necessary to move forward with the project.

So that is when, why and how power metering is required. If you have any questions about the best approach to power metering for your projects just ask. We are happy to help!

Justin Pardee