Current Transformer (CT) Selection Guide
Article ESA-1008D December 5, 2014Contents
1 Introduction . . . 3 1.1 Extending CT leads . . . 32 Selecting the mechanical dimension of the CT . . . 4
3 Selecting the current rating of the CT . . . 4
4 Revenue Grade CTs . . . 5
5 Direct Current CTs . . . 5
7 Appendix . . . 6
7.1 Standard 0.4" CT dimensions. . . 6
7.2 Standard 0.75" to 2.00" CT dimensions . . . 7
7.3 High Accuracy 0.75" CT dimensions. . . 8
7.4 DC CT dimensions . . . 9
1
Introduction
This guide is intended to help you select the right quantity and type of current transformers (CTs) needed for an eGauge installation.
A typical eGauge installation will measure multiple amperage inputs from CT sensors. The CT sensors are installed around the current carrying conductors of the load or generation source you wish to monitor. CTs can be used to monitor the main utility feed conductors for a facility or residence, conductors that feed sub panels, and individual circuit conductors 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.
• Split-phase loads (eg, 120V residential outlet, phase-to-neutral) require a single CT • 2 phase loads (”single phase”) require 2 CTs. See footnote1
• 3 phase loads require 3 CTs
Example: a split phase 240V electric water heater: one CT can be used on the phase, that amperage reading can be applied to the other voltage in the configuration section of eGauge installation. Most residential 240V PV inverters are also balanced, but not 3 phase inverters.
Some loads such as HVAC systems and dryers are not balanced and will always require a CT on each phase. Facility main feeds and sub panel feed are also not balanced. CTs are available in a number of different physical sizes. The inner diameter ID 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 work inside the circuit panel or switch gear. eGauge offers these examples as a reference. The nature of the current in an appliance should be verified by a qualified electrician or via the appliance documentation before ordering.
Picking the type of CT involves selecting the mechanical dimension and the current rating (in Amps).
1.1
Extending CT leads
Split core CTs can be extended with appropriately rated2 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 factor will be necessary in your software settings.
Appropriately rated CATV Ethernet cable can be used to extend up to 4 CTs.
1If a load is balanced (meaning the same current is flowing across all phases), then a single CT can be used to measure one
amperage. eGauge can be configured to apply the amperage reading to the second phase, as long as eGauge has a voltage reference to each phase in the system. See configuration example 2.9 Appliances in the configuration guide for more details.
2
Selecting the mechanical dimension of the CT
CTs are available in a number of different physical sizes. The inner diameter ID 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 work inside the circuit panel or switch gear.
CT diameter: Wire Size:
0.4" #14 to #2 AWG 0.75" 1/0 to 4/0 AWG 1.25" 250 to 400 MCM 2.0" 400 to 750 MCM Table 1: General sizing guidlines
6" Rogowski coil “rope” CTs are good for bus bar and parallel feed conductors (or any installation with limited space, assuming the current does not drop below 25A or go above 4800A and there is adequate space for the rope in the panel).
3
Selecting the current rating of the CT
CTs are available with in a wide amperage range from 20A to 4800A. An easy way to select the amperage needed is to match the breaker or disconnect rating that you or measuring with the CT you choose. For example if you are monitoring a 50A circuit, then a 50A CT would be appropriate. If you have a 15Amp breaker, then the closest larger rated CT, a 20A CT, should be used. The CTs and eGauge will not be damaged if the amperage on the measured conductor exceeds the rating of the CT, within reasonable limits.
If the circuit or service is determined to have a maximum amperage that is well below the rated breaker ampacity, you may choose a lower amperage CT.
Example: a typical residential service is rated for 200A with 2 phases feeding the home. Although the service is rated to have up to 200A on a single phase at any given moment, the reality may be that neither phase will pull over 100A ever. In this situation 2 100A CTs would work.
eGauge offers this examples as a reference. The nature of the current should be verified by a qualified electrician before ordering.
Split core CTs are available with set amperage ratings. The load you wish to monitor should fall within the range from 10% to 100% of the CT being used to monitor that current. CTs can not be used to monitor currents above their rating.
Example: a 50A CT would be a good choice to monitor a load that is typically between 5 and 50A. Accuracy begins to diminish when a current is below 10% of the CT rating.
4
Revenue Grade CTs
High-accuracy split-core CTs are available.
The Accu-CTs come in 50A, 100A, and 200A. All are 0.75" Accu-CTs have an error of .2% from 1% to 100% of their rated current.
Accu-CTs 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 Accu-CT 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.
5
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 to be supported 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.
6
3-phase inverter notes
For PV inverters with 3 phase outputs, 3 CTs are recommended. When inverters are not producing (in standby-mode, such as during night), they consume a small amount of power. 3-phase inverters have been seen not to consume the same amount on each phase during this mode, so using a single CT will cause anomalous behavior in the graph which can show as night time generation or over-consumption.
Further, some brands of 3-phase inverters don’t produce on all 3 phases in certain circumstances. In this case, a single CT would give erroneous readings and would not be accurate.
7
Appendix
7.1
Standard 0.4" CT dimensions
Figure 1: SCT0400-xxx dimensions Part Number A B C D E SCT0400-xxx 1.56" 1.00" 0.40" 1.00" 1.05" Table 2: SCT0400-xxx dimensions7.2
Standard 0.75" to 2.00" CT dimensions
Figure 2: CT dimensions between 0.75" and 2.00"
Part Number A B C D E
SCT0750-xxx 2.00" 2.10" 0.67" 0.75" 0.75" SCT1250-xxx 3.25" 3.35" 1.06" 1.25" 1.25" SCT2000-xxx 4.75" 5.00" 1.20" 2.00" 2.00"
7.3
High Accuracy 0.75" CT dimensions
7.4
DC CT dimensions
7.5
CT part numbers
Standard Split-core CTs: Current Rating 0.40" ID 0.75" ID 1.25" ID 2.00" ID 5A SCT0400-005 10A SCT0400-010 SCT0750-010 20A SCT0400-020 SCT0750-020 30A SCT0400-030 SCT0750-030 50A SCT0400-050 SCT0750-050 75A SCT0400-075 100A SCT0750-100 SCT1250-100 200A SCT0750-200 SCT1250-200 SCT2000-200 400A SCT1250-400 SCT2000-400 600A SCT1250-600 SCT2000-600 800A SCT2000-800 1500A SCT2000-1500Rope CTs (8 foot leads):
Current Rating 6" Rope ID:
50–4800A RCT-1800-COIL High Accuracy CTs: Current Rating 0.75" ID 50A ACT-0750-050 100A ACT-0750-100 200A ACT-0750-200
