CT selection guide

In this article...

Introduction
Interpreting CT models and SKUs
Determining how many CTs to use
Selecting the mechanical dimension of the CT
Selecting the current rating of the CT
Rope CTs
Direct Current CTs
Extending CT leads
Three-phase inverter notes
CT dimensions and diagrams

Introduction

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

3-phase, 4-wire wye (with neutral) distribution panel feeds or individual loads require 3 CTs, one on each phase.
3-phase, 3-wire delta (without neutral) distribution panel feeds or individual loads require 2 CTs, on each phase not connected to the eGauge’s N terminal. See the eGauge delta system documentation for wiring diagrams.
Split-phase (phase-to-neutral) loads such as 120V residential outlets require a single CT.
Single-phase (phase-to-phase) loads such as 240V residential hot water heater require 2 CTs. If the measured load is purely single phase (no current on the neutral) such as many residential solar inverters, a single CT can be used. See configuration example 2.9 Appliances in the configuration guide for more details.
Revenue-grade accuracy monitoring requires a high-accuracy CT on each phase of the equipment to be monitored. See section 5 for more information.

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 (1.59")	23mm (0.91")	23.7mm (0.93")	26.6mm (1.05")	14.5mm (0.57")	10mm (0.39")
JD-SCT-016-xxxx	45mm (1.77")	26mm (1.02")	30mm (1.18")	31.6mm (1.24")	18.8mm (0.74")	16mm (0.63")
JD-SCT-024-xxxx	65mm (2.56")	37.5mm (1.48")	45mm (1.77")	33.7mm (1.33")	21.1mm (0.83")	24mm (0.04")
JD-SCT-036-xxxx	82.4mm (3.24")	48mm (1.89")	57.1mm (2.25")	40.2mm (1.58")	21.1mm (0.83")	36mm (1.42")

AE-RCT-xxx-xxxx Dimensions

SKU	Window Size	Coil Length	External Diameter
AE-RCT-106-XXXX	106mm (4.17")	400mm (15.75")	143mm (5.63")
AE-RCT-178-XXXX	178mm (7.01")	600mm (26.67")	207mm (8.13")
AE-RCT-271-XXXX	271mm (10.67")	900mm (35.43")	302mm (11.89")

JD-RCT-115-4085

ML-RCT-150-4800

ML-SCT-010-xxx

SKU	A	B	C	D	E
ML-SCT-010-XXX	39.6mm (1.56")	25.4mm (1.00")	10.2mm (0.40")	25.4mm (1.00")	26.7mm (1.05")

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

SKU	A	B	C	D	E
ML-SCT-019-XXX	50.8mm (2.00")	53.3mm (2.10")	17mm (0.67")	19.1mm (0.75")	19.1mm (0.75")
ML-SCT-032-XXX	82.6mm (3.25")	85.1mm (3.35")	26.9mm (1.06")	31.8mm (1.25")	31.8mm (1.25")
ML-SCT-050-XXXX	120.7mm (4.75")	127mm (5.00")	30.5mm (1.20")	50.8mm (2.00")	50.8mm (2.00")