Installing Consumption-Monitoring CTs: Page 2 of 2

Quality Assurance

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 /

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