Safety and Warning Notes from CT

Safety Warning!

Current Transformers (CTs) are typically installed on electrical equipment operating at potentially lethal high-voltage levels. Please ensure to read the CT installation safety page before proceeding with the installation.

Key Points

Install the CT on the phase conductor corresponding to the meter’s voltage input phase.
Position the CT with its 'towards the source' arrow or label facing towards the breaker supplying power to the load.
Connect the white and black CT leads to the corresponding CT input terminals marked with white and black dots.

Opening and Closing CTs

Split-core CTs open by pulling the latch upwards or downwards, depending on the product's design. Ensure the mating surfaces are clean for accurate measurements; debris can cause gaps, reducing accuracy. Place the CT around the conductor and close it firmly until you hear a click.

Solid-core CTs require the phase conductor to pass through the CT opening. This can be straightforward for smaller wire gauges but may not be practical for larger gauges or multiple parallel conductors.

Phase Conductors

For accurate measurements, install the CT on the phase conductor corresponding to its respective voltage input connection. For instance, a CT1 or ØA CT should be installed on the same phase conductor connected to the VA or ØA voltage input, and so on. Labeling wires with colored tape or labels can be helpful for identification.

To reduce magnetic interference between CTs on adjacent phases and prevent flashover arcs caused by debris across conductor terminals or bus bars, it's advisable to space CTs about 25mm (1 inch) apart.

For optimal accuracy, the CT aperture should not be more than 50% larger than the phase conductor. If the CT aperture is significantly larger, center the conductor in the CT aperture or, if that's not possible, place the conductor on the bottom of the 'U' shape of the CT, away from the open end where flux leakage occurs.

Plastic cable ties can be used to secure the CT position on the phase conductor. Some CT models might also allow securing with cable ties to prevent accidental opening.

Connecting to the Meter

Since meter CT inputs are sensitive to Electrostatic Discharge (ESD), always ground yourself by touching an electrical enclosure or other grounded metal object before handling transformers. This is a best practice for all ESD-sensitive electronics.

To connect CT leads to the CT input terminals, strip about 1/4 inch (6mm) of insulation from one end of each wire, twist the strands together, insert into the terminal block, and tighten the screw. It's easier to connect the wires to the terminal block before attaching the block to the meter.

Unused CT inputs can capture electrical noise, so it's advisable to short-circuit the unused CT input terminals with a wire jumper about 1 inch long between the white and black CT terminals. This is generally not an issue if no line voltage is connected to the respective voltage input terminal.

Direction and Polarity

CTs come marked with symbols (arrows) or labels indicating the correct mechanical direction towards the current source (typically the utility meter or circuit breaker).
The electrical polarity, indicated by the white and black wires, must also be correct. Connect each pair of CT wires to their corresponding terminals on the black 6-position screw terminal block, marked with labels like CT1 or ØA, CT2 or ØB, CT3 or ØC. Ensure the white wire connects to the terminal with the white dot, and the black wire to the one with the black dot.

Remember, both the physical direction and electrical polarity of each phase must be correct for proper operation. If a phase is electrically or mechanically reversed, and the current flows in the opposite direction, the meter typically measures zero or negative energy for that phase.

CT Leads

If CT leads are excessively long, they can be shortened to minimize interference from electrical noise. While CT wires can be extended if necessary, it's usually better to position the meter close to the conductor being measured.

However, if extension is needed, CT wires can be extended up to 30m (100 feet) using shielded wire. The National Electrical Code (NEC) requires wiring CT wires in conduits. Metal conduits provide shielding, but if using PVC, shielded wire is recommended. For shorter distances, the meter's voltage input power conductors can run in the same conduit, but for longer distances, to minimize power line noise interference, they should not be in the same conduit as the CT extension wires.

CT Installation Safety


For maximum safety, our manual recommends disconnecting the power of active circuits before installing a Current Transformer (CT). However, this is not always necessary.

Qualified electricians should install CTs and instruments in all cases, adhering to all NFPA and NEC rules.

The following discussion lists and summarizes some relevant NFPA articles. However, other relevant articles not mentioned may exist, and interpretations of these articles can be inaccurate or incomplete. Before installing a current transformer without disconnecting circuit power, consult an NFPA expert!


  • According to the National Fire Protection Association (NFPA), 70E: Electrical Safety in the Workplace (2009):
    Article 130.1 Work Justification: Before working on a circuit, exposed conductors and circuit parts must be put into a safe state (power disconnected).

  • Article 100 Definitions:
    Exposed (applies to live electrical conductors or circuit parts). Susceptible to inadvertent contact or approach closer than a safe distance. This applies to electrical conductors or circuit parts that are not suitably guarded, isolated, or insulated.

Therefore, if the conductor to be monitored by the CT is insulated, it is not "exposed." The CT is fully insulated except for the ends of its output wires, which produce only milliamperes of current and are nominally 0.333Vac (below the 50V limit of 130.1.(A).(3)), making it incapable of producing an arc.

Thus, the NFPA seems to permit the installation of CTs on insulated live conductors, provided all safety rules in NFPA 70 and 70E are followed.

Special Situations

Additionally, Article 130.1 allows for work on exposed live circuits where turning off the power would introduce "greater hazards" or is infeasible. Refer to the standard for the exact rules that permit this. This allows for the installation of CTs on live, un-insulated conductors like busbars, requiring written work permits. J&D emphasizes the importance of proper training, arc flash safety equipment, and insulated gloves for these types of installations, necessary and permissible under NFPA rules.

UL 2808

As specified in the UL 2808 Energy Monitoring Current Transformers Investigation, Issue Date: June 10, 2016, Section 3, “9.3 Field wiring leads of indoor CTs must be suitable for contact with exposed live parts of Class 1 circuits and/or routing through the same conduit as other Class 1 circuit conductors. The conductors are composed of Class 1 wiring…"