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.

Installing Multiple Conductors in a Single CT

Some consumption-monitoring products support the installation of multiple conductors within a single CT. If it is not possible to place the CT on a service panel’s main service conductors or busbars, then you can often run all of the individual circuit conductors through a single set of consumption-monitoring CTs. However, this approach presents some challenges. Notably, due to the number of conductors, it is easy to make a wiring error. This method may require extending some of the individual ac circuits, and in some cases, it may be difficult to fit the wiring and CTs within the enclosure. Finally, you must take care to ensure that all the conductors on Line 1 are installed in the Line 1 CT and all the conductors on Line 2 are installed in the Line 2 CT.

When running multiple conductors through a single CT, you must first determine if the sum of the conductors will physically fit within the CT while meeting NEC requirements. You can run a conductor-fill calculation to determine whether you can install a given number of conductors within a single CT. The NEC allows for a 60% fill calculation for conduit sections not exceeding 24 inches in length. A 60% CT fill calculation provides a conservative basis for calculating the number of conductors allowable within an individual CT (see Table 1 above). For example, based on a 60% fill calculation, you can install 36 12 AWG THWN conductors within a single Enphase consumption-monitoring CT.

You should consider several best practices when installing multiple conductors in a single CT: Always turn off the main breaker to reduce the risk of electrical shock, and be aware that the line side of the main breaker will remain energized. If the CT installation requires you to rewire or extend individual circuits, do so in an organized way to open up additional space within the service panel. Marking each conductor that passes through the Line 2 CT with red tape helps separate Line 1 and Line 2 circuits and ensures that all red-marked conductors are landed on Line 2. Using zip ties to bundle Line 1 and Line 2 conductors can organize the conductors and make it easier to install a CT around them.

If multiwire branch circuits are present, you should verify that the ungrounded conductors in these circuits are wired on opposite line conductors. Multiwire branch circuits use two hot conductors and a shared neutral to power 120 Vac loads. The neutral conductor of a multiwire branch circuit carries the unbalanced current of the two line conductors. You must always run multiwire branch circuits to a common-trip two-pole breaker. If you tie the red and black conductors of the multiwire branch circuit to the same line, then the neutral carries the full current of both circuits, which may overload the conductors and present a fire hazard.

Installing Parallel-Connected CTs

In many scenarios, installing multiple conductors through a single CT is difficult because some conductors may enter from the bottom and others from the top of the service panel. In addition, some service panels have 400 A services equipped with a set of two 200 A conductors and parallel-connected 200 A circuit breakers. In these cases, it is possible to use a set of parallel-connected consumption-monitoring CTs to monitor the home’s consumption. You can accomplish this by installing two consumption-monitoring CTs on each line and then parallel-connecting the CT wiring.

When you are installing parallel consumption-monitoring CTs, you must verify that all conductors passing through the Line 1 CT terminate at circuit breakers on Line 1, and that Line 2 conductors passing through the Line 2 CT terminate at circuit breakers on Line 2. Run a fill calculation to determine that conductors fill no more than 60% of the CT’s cross-sectional area. Finally, observe the safety requirements spelled out in the NEC, particularly with respect to multiwire branch circuits.

Retrofitting the Electrical Service

If the consumption-monitoring CTs do not fit, you cannot just quit. If all else fails, you may want to ask the utility for access to the service conductors to connect the CTs on the utility side of the service panel. Another option would be to coordinate with the utility to relocate the main service meter to an enclosure that is separate from the main service panel. Relocating the main meter to a separate enclosure makes the former utility side of the main service panel accessible for the installation of the consumption-monitoring CTs. If you relocate the service meter, you must cover the old service meter base with a service cover blank and install meter jumper bars in the old meter base.

Extending Consumption CT Conductors

You can often extend consumption CT wiring if the system-monitoring equipment is not located in close proximity to the service mains. Follow the recommendations provided in the manufacturer’s installation manuals. When extending the consumption-monitoring CT wiring, I recommend using Type CL3R, four-conductor, 14 AWG, 16 AWG or 18 AWG security cable. CL3R cable is readily available at many local hardware stores. The cable meets Code requirements for indoor and outdoor installations as detailed in Article 725 for Power Limited Circuits and Table 725.154 of the NEC.

Validating Monitoring System Accuracy

One good test to validate that the CTs and associated consumption monitoring are installed correctly is to turn off the PV system to verify that the monitor and the main service meter report the same consumption. If the data do not match, then troubleshoot using the following steps.

  • Confirm that the consumption meter is monitoring all circuits of the main service meter.
  • P Ensure that the CTs for each line (1 and 2) correspond to the power wiring of the consumption monitor. You can confirm this by testing the voltage between the line terminals at the monitor and the line terminals at the service.
  • Confirm that the consumption monitoring CTs are oriented correctly.
  • Confirm that the CT wires are not reversed at the wiring terminals.
  • If you are installing multiple conductors in a single CT, turn on the load breakers individually and confirm that each circuit increases the load reported by the consumption monitor.

If the consumption monitor is reporting a negative load, then one or both of the CTs likely are not oriented correctly on their corresponding line conductor or busbar.

Nick Soleil / Enphase Energy / Petaluma, CA / enphase.com

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