Installing Consumption-Monitoring CTs

Quality Assurance

Changes to utility rate structures and net metering rules are forcing solar companies to develop new strategies so they can continue to grow in today’s evolving markets. As state public utility commissions from Nevada to Hawaii modify the rules for PV installations, solar companies are developing and deploying systems that include consumption monitoring, load management, power ramping and battery storage. Technologies such as these can enable the sale of PV systems in markets that limit or prohibit the export of solar energy to the grid.

With the development of high-efficiency lithium-ion batteries, there are new opportunities to add storage to a PV system for meeting zero-export requirements, for demand management and for lowering utility distribution charges. New solar-plus-storage technologies are particularly valuable in zero-export markets such as Hawaii, where, from a regulatory standpoint, it is much easier to install a PV system that does not export power to the grid.

Engineering effective self-consumption and solar-plus-storage systems requires the measurement and tracking of how much energy a residence consumes, as well as how much energy the PV system produces.  Most of the systems available today rely on the installation of current transformers (CTs) for monitoring a home’s solar production and energy consumption. When required, the installation of a production monitoring CT is straightforward for most PV systems. The associated wiring is generally accessible since the crew often installs it at the same time as the PV system. However, the installation of CTs for consumption monitoring is not always as straightforward.

In residential applications, consumption monitoring requires the installation of CTs on the home’s main service conductors. CTs used for consumption monitoring are almost always split-core CTs, which you can open and close around the existing service entrance conductors or busbars. At some sites, the conductors or bussing that feed the main breaker of the service panel are not accessible or do not have adequate space around them for the installation of CTs. In this article, I provide guidance for site assessors who are evaluating a residence to determine whether it can easily accommodate consumption monitoring. I present some tips and tricks for sites that may not appear to support consumption monitoring at first glance. I also suggest potential modifications to service wiring that can enable the installation of consumption-monitoring CTs.

Assessing a Site for Consumption Monitoring

The CTs used for monitoring home consumption vary in their physical dimensions. They may be calibrated for use with the system monitor that they are paired with, but be aware that not all CTs are certified as revenue grade. For example, Enphase Energy offers the Envoy-S Metered system monitor, which enables both revenue-grade production monitoring and home energy consumption monitoring. The 200 A consumption-monitoring CTs provided with the Enphase Envoy-S Metered monitoring system can accommodate conductors up to 4/0 RHW, 350 MCM THWN, and 350 MCM XHHW, and can also accommodate 0.75-inch busbars.

When installing consumption-monitoring CTs, you must determine whether you can readily install the split-core CTs around the service conductors or busbars. You typically encounter two service configurations: separate units for the utility meter and service panel, or panels that combine the utility meter and service panel in one enclosure. The salesperson or technician who is evaluating the site should record the service configuration (separate or combined meter and service panel), as well as answer and document the following three questions:

  • Can you access at least  2 inches of both of the Line-1 and Line-2 conductors or busbars in the main service panel?
  • Can the core of the CTs fit around each busbar or conductor?
  • Is the clearance around each busbar or conductor adequate to accommodate a CT?

The physical design and construction of combined meter and service panel products may present unique challenges. For instance, with many Eaton service panels, including Eaton’s Solar Power Center panels, it may initially appear that the consumption-monitoring CTs fit on one of the service entrance conductors but not both of the service entrance conductors or busbars. However, if you test the voltage across the main breaker, you will find that the line conductors are rotated inside the circuit breaker. This allows you to install one CT on the conductor above the main breaker and one on the bussing below the main breaker. You can install the CTs on the right conductor and right busbar in the Eaton Solar Power Centers. You must place the main breaker in the open position when attaching the CT to the busbar. Note that it may help to loosen the bolt that holds the busbar when attaching the CT.

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