CT FAQs and other information

All about CTs

CT selection guide

In this article...

  1. Introduction
  2. Interpreting CT models and SKUs
  3. Determining how many CTs to use
  4. Selecting the mechanical dimension of the CT
  5. Selecting the current rating of the CT
  6. Rope CTs
  7. Direct Current CTs
  8. Extending CT leads
  9. Three-phase inverter notes
  10. CT dimensions and diagrams
    1. JD-SCT-xxx-xxxx
    2. AE-RCT-xxx-xxxx
    3. JD-RCT-115-4085
    4. ML-RCT-150-4800
    5. ML-SCT-010-xxx
    6. ML-SCT-019-xxx to ML-SCT-050-xxx
    7. CC-ACT-020-xxxx
    8. CC-ACT-032-xxxx
    9. CR-Dxx-xxxx


A typical eGauge installation will measure the amperage of multiple conductors via multiple CT sensors. The CT sensors are installed around the current carrying conductors of the load or generation source you wish to monitor.

Picking the type of CT involves selecting the mechanical dimension and the amperage rating (in Amps). CTs can be used to monitor the main utility feed conductors for a facility or residence, conductors that feed sub panels, and conductors for individual circuits such as pumps, motors, air conditioning, car chargers, and lighting loads.

CTs can also be used to monitor conductors from generators as well as renewable energy systems such as solar PV systems, wind generators and hydro power systems.


Interpreting CT models and SKUs

The eGauge SKU identifies CTs based on 4 CT properties: Manufacturer (MFG), CT type (TYPE), inner diameter in millimeters (ID), and amperage rating (AMPS). The format of the SKU is MFG-TYPE-ID-AMPS.

MFG code Manufacturer
JD J&D Electronics
AE AccuEnergy
ML Magnelab
CC Continental Control Systems
CR CR Magnetics


TYPE code CT type description
SCT Split-core AC CT
RCT Rogowski Coil (Rope CT)
ACT High-accuracy split-core CT
DCT Solid-core DC (direct current) CT
DSC Split-core DC (direct current) CT


For example, the SKU JD-SCT-010-0075 means:

Manufacturer J&D Electronics
CT type Split-core AC
Inner diameter 10mm (0.39")
Amperage rating 75 Amps


Determining how many CTs to use

Some single-phase loads such as HVAC systems, dryers, and hot tubs are not balanced and will always require a CT on each phase. For example, a hot tub may have a 2-pole breaker, but use 240V for heating and 120V for lighting and pumps, which means current will be on the neutral. Facility main feeds and sub panel feeds are also not balanced.

eGauge offers these examples as a reference. The nature of the current on an appliance should be verified by a qualified electrician or via the appliance documentation before ordering.


Selecting the mechanical dimension of the CT

CTs are available in a number of different physical sizes. The inner diameter of the CT must be large enough to fit around the conductive wire you wish to monitor. The outer dimensions must be small enough to be installed inside the circuit panel or switch gear.

SKU Diameter Wire Size
JD-SCT-010-XXX 10mm (0.39") #14 to #2 AWG
JD-SCT-016-XXX 16mm (0.63") 1/0 to 3/0 AWG
JD-SCT-024-XXX 24mm (0.94") 4/0 AWG to 400 MCM
JD-SCT-036-XXX 36mm (1.42") 4/0 AWG to 400 MCM


Selecting the current rating of the CT

High-accuracy split-core CTs (AccuCTs) are available. High accuracy CTs can be identified by a CT type code of ACT.

AccuCTs are available in 0.79"and 1.25". All AccuCTs have an error of .2% from 1% to 100% of their rated current.

AccuCTs can be paired with any eGauge main unit. For revenue grade readings, all conductors must be monitored, including single-phase (phase-to-phase) balanced and unbalanced loads.

For best accuracy, pair the AccuCT with an eGauge device with the 0.5% certificate report. The total system (eGauge meter and CTs) will have an error no greater than 0.5%. This pairing meets the ANSI C12.20 0.5% accuracy class.

Pair the Accu-CT with a non-certificate, 1.0% eGauge, and the result will be a system with no greater than a 1% error. This pairing meets the ANSI C12.1 accuracy class.

Some reporting agencies may require certified units, so it is important to determine if you will require a certificate with the eGauge meter. Certified units must be specified during ordering; a certificate cannot be generated for a meter once it is purchased from eGauge.


Rope CTs

Rope CTs (Rogowski coils) are excellent for bus-bars and large switch-gear applications. Self-powered rope CTs are flexible and easy to install with a fixed current rating. Rope CTs require a minimum amperage and are not for use in residential or low-current scenarios.


Direct Current CTs

DC (direct current) CTs are available for use with the eGauge to monitor DC amperage sources. These CTs must be modified by eGauge in order to function properly and require an external 12Vdc power source. A power supply for the CT is included.

Solid-core DC CTs have a .79"window. Power on the conductor to monitor must be disabled, disconnected, and then threaded through the CT window.

Split-core DC CTs have a 1.2"window. It is not necessary to disconnect the monitored conductor when adding the CT.


Extending CT leads

Split core CTs can be extended with 600V rated twisted pair wire to cover much longer distances. STP (shielded twisted pair) is recommended, as it will reduce noise and interference. If you need to extend a CT wire farther than approximately 100 ft, contact eGauge support to see if a custom scaling factor will be necessary in your software settings. Be sure to use properly rated Ethernet cable for the voltage that will be present.

It is often not recommended to ground any part of the CT leads or shielding around the leads. Signal grounding is different than electrical grounding, and in some cases will cause problems that can invalidate readings. Consult with an electrical engineer if grounding is considered.


Three-phase inverter notes

For inverters with three-phase outputs, 3 CTs are recommended. When inverters are not actively producing (in standby-mode, such as during the evening), they consume a small amount of power. Three-phase inverters have been seen not to consume the same amount on each phase during this mode, so using a single CT may cause anomalous behavior in the graph which can show as night-time generation or over-consumption.

Further, some brands of three-phase inverters don’t produce on all 3 phases in some circumstances. In this case, a single CT would give erroneous readings and could be very inaccurate.


CT dimensions and diagrams

JD-SCT-xxx-xxxx dimensions


SKU A B C D E ød
JD-SCT-010-xxxx 40.5mm
JD-SCT-016-xxxx 45mm
JD-SCT-024-xxxx 65mm
JD-SCT-036-xxxx 82.4mm


AE-RCT-xxx-xxxx Dimensions

SKU Window Size Coil Length External Diameter
AE-RCT-106-XXXX 106mm
AE-RCT-178-XXXX 178mm
AE-RCT-271-XXXX 271mm




ML-SCT-010-XXX 39.6mm

ML-SCT-019-xxx to ML-SCT-050-xxx

ML-SCT-019-XXX 50.8mm
ML-SCT-032-XXX 82.6mm
ML-SCT-050-XXXX 120.7mm


Note: CTs are hinged, not full detachable.



Note: CTs are hinged, not full detachable.





Can the CTs be extended?

CT leads can be extended, please refer to this article for more information.

​Connecting unsupported sensors to the eGauge CT ports

The EG30xx and EG4xxx are only certified to be used with current transducers (CTs) as described in the section B.6 of the EG30xx Owner's Manual or section B.6 of the EG4xxx Owner's Manual, respectively. Connecting any other sensors can be dangerous, and will void the eGauge hardware warranty.

CTs that are not officially supported but output 333mV from 0-100% of their maximum current rating should function as intended with the eGauge. However, eGauge cannot make any guarantee that these readings will be accurate. These CTs may still cause damage, which would not be covered under warranty.

CTs that output any other type of output signal (including 500mV) will damage the eGauge permanently. This damage is not covered under warranty.

No other sensor type may be connected to a CT port - even if the output signal is 0-333mV, the eGauge has no way to reliably interpret this data. These types of sensors may cause damage not covered under warranty.

Supported CT types

The EG30xx and EG4xxx are Listed Devices, certified to be used with CTs that have the following specifications: UR and CSA, 0.333Vrms output at rated current, and 600V insulation. While any CT with these specifications can be safely attached, only CTs listed in section B.6 of the EG30xx Owner's Manual and section B.6 of the EG4xxx Owner's Manual have been evaluated by eGauge Systems to ensure correct performance and accuracy.

As a general rule, any CT used with an older model of eGauge should be automatically supported by a newer model. For example, the CTs used with an eGauge2 should be supported by an EG30xx or EG4xxx. Moving from a newer model of eGauge to an older model may require a firmware update to add support for certain CTs. This assumes that the CTs were originally purchased through eGauge Systems, and not a third party.

Connecting any other hardware to a CT port can damage the eGauge and will void the warranty.

Identifying Rope CTs

The eGauge supports Rope CTs from three different manufacturers. In order to determine which CT to select, refer to the tables below.

For JD CTs (colored red) there will be a label located on the CT clip itself:

CT Labeling CT Identifier on eGauge Installation Page
JRFS 115 JD JRFS 115mm/4.53" 4085A
JRFS 190 JD JRFS 190mm/7.48" 4085A


For AE Accuenergy CTs (colored orange), there is a label located on the CT lead:

CT Labeling CT Identifier on eGauge Installation Page
RCT16-1000 AE RCT 106mm/4.17" 2775A
RCT24-1000 AE RCT 178mm/7.01" 2775A
RCT24-2500 AE RCT 178mm/7.01" 6935A
RCT36-2500 AE RCT 271mm/10.67" 6935A


For the Magnelab CTs (colored green) there is a label located on the outside of the CT clip:

CT Labeling CT Identifier on eGauge Installation Page
RCT-1800-COIL ML RCT 152.4mm/6.00" 4800A
RCT-2400-COIL ML RCT 203.2mm/8.00" 4800A
RCT-3600-COIL ML RCT 279.4mm/11.00" 4800A

Note that eGauge no longer stocks Magnelab Rope CTs, but you may encounter them in the field (and the eGauge still supports them). They read very poorly below about 50A. If the CT features a separate calibration tag, this should be noted.

Additional CT types are supported - see our CT Selection Guide for a more comprehensive overview. Connecting CTs which do not meet requirements for supported CTs may result in reduced accuracy or damage to the eGauge.

Installing Continental Control Systems Accu CTs

Like most ferrite core CTs, Accu CTs from Continental Control Systems (CCS) are prone to damage if dropped, struck, or closed aggressively. In order to avoid damage during installation, the CT should not be forced closed. This can cause chips or cracks in the ferrite core, which reduces the accuracy of the CT.

To prevent damage to CCS CTs, the tabs on either side of the hinged portion of the CT should be squeezed together. The CT can then be closed as normal. The image below shows the tabs. When the tabs are squeezed, the CT should close without applying significant pressure. Failure to follow this step can cause damage to the CT. This damage may not be readily visible.

Face the stickered end of the CT towards the item being measured (e.g., for Grid the sticker faces the utility meter, for a hot water heater the sticker faces the hot water heater, not the breaker feeding it).

CT tabs with light pressure applied (under the thumb and index finger)

Rope CT Positioning Guide

Rope CTs should be installed properly to reduce the maximum error based on positioning. Ideally the conductor being monitored will be perpendicular to the rope CT loop, and pass through the center of the rope CT as shown below. Tie-wraps or other non-conductive fasteners can be used to center the rope CT around the conductor.



Position Number Position Description Error
1 Centered < 0.4%
2, 3, 4 Near outer edge < 2.0%
5 Near connector < 4.0%




Conductor through center of rope CT (ideal placement)


Conductor touching points 4 and 2 (less than ideal placement)