=========
CT Basics
=========
What is a Current Transformer?
------------------------------
`From Wikipedia <https://en.wikipedia.org/wiki/Current_transformer>`_:
*A current transformer (CT) is a type of transformer that
is used to measure alternating current (AC).
It produces a current in its secondary which is
proportional to the current in its primary.*
In our case, the primary is *one* of the conductors of the circuit
that we want to measure, and the secondary is the output jack
that plugs into the IoTaWatt.
So without connecting anything directly to any high-voltage
wiring, it's possible to get a scaled down measure of the
primary current that can be used to passively measure power
(Watts) of a circuit.
Types of CTs
------------
CTs come in various types, sizes, and capacities,
and are made for a variety of end uses. This tutorial
doesn't try to address all aspects. That's what
Wikipedia does well. Here we'll try to focus on the
CTs that are suited to use with the IoTaWatt in typical
scenarios.
Physically, a CT needs to have an iron core through
which one or more primary conductors pass. The most basic
type is a solid-core CT, where an iron doughnut is wrapped
with turns of wire. This type of CT is relatively inexpensive,
typically very accurate, but requires that the primary
conductor be disconnected and reconnected to install, thus
exposing the installer to high-voltage and disrupting the
primary circuit.
.. figure:: pics/CTbasics/solidCoreCT.jpg
:scale: 30 %
:align: center
:alt: Solid Core CT
Solid Core CT
Split-core CTs also require an iron core around the primary,
but do so using two hinged halves that mate to form the continuous
iron loop. This type of CT can be installed by simply snapping
the two halves over an active primary conductor.
.. figure:: pics/CTbasics/splitCoreCT.jpg
:scale: 30 %
:align: center
:alt: Split Core CT
Split Core CT
Installation
------------
.. image:: pics/warningSymbol.png
:scale: 30 %
:align: left
:alt: **Warning!**
*The installation of CTs can be dangerous and/or
cause hazardous situations resulting serious injury or death.
Worldwide, there are a variety of electrical conventions, regulations
and standards. It is the user's responsibility to insure that the
installer is qualified and all local codes and regulations are followed.*
To measure the current in a circuit, a CT is installed on one, and
only one, of the conductors in a circuit. Either the conductor is
passed through the solid-core, or the split-core is clamped over it.
CTs must have a load. Without a load, they will develop very
high voltages that can damage the core windings and/or
create a safety hazard. When plugged into the IoTaWatt, the
secondary windings are loaded by a *burden resistor*.
Some CTs have protective diodes, called TVS diodes, that will
protect against damage when unplugged for short periods. Even if
a diode protected CT is to be unplugged for an extended length
of time where the primary is energized, the CT should be removed
or shorted. Shorting will not damage the CT.
Polarity
--------
CTs are manufactured to produce a secondary current that is in
phase with the primary current when installed with a particular
orientation. In single and split-phase installations it is
important to observe polarity in certain situations. In
three-phase installations, it is imperitive that a polarity
convention be observed.
IoTaWatt will accept many different CTs from different manufacturers.
While most have some type of markings that can be used as a
reference for polarity, there is no universal standard. Typically,
CTs from the same manufacturer will be consistent with respect to source
and load indicators.
And so it is for the Echun and AccuEnergy CTs that IoTaWatt, Inc makes available.
When installing, we use the notion of a *source* and a *load*.
The source can be conceptualized as *where the power comes from*
and the load as *where the power goes*.
So for the mains, or incoming power to a service, the *source* would
be the meter side, or incoming power feed. The *load* would be the
main circuit breaker or fuse side.
For branch circuits, it would be just the opposite. The *source* would
be the circuit-breaker side, and the *load* would be the appliance side.
For a solar inverter connection, the *source* would be the inverter side,
and the *load* would be the circuit-breaker or other point of
interconnect.
.. figure:: pics/CTbasics/orientationECOL09.jpg
:scale: 30 %
:alt: ECOL09
Here you can plainly see "This side toward source"
.. figure:: pics/CTbasics/orientationECS1050.jpg
:scale: 30 %
:alt: ECS1050
Here the arrow points source(K)->load(L)
.. figure:: pics/CTbasics/orientationECS25200.jpg
:scale: 30 %
:alt: ECS25200
This is an Echun ECS25200 clamp type CT
used for 200A mains. Both sides are shown.
Note the arrows just under the opening.
The arrow pointing up to the opening
indicates the source side, and the down
arrow indicates the load side.
.. figure:: pics/CTbasics/orientationSCT013.jpg
:scale: 30 %
:alt: SCT013
This is the common SCT013 CT. If you are using
them exclusively, the arrow can be aligned consistently
as source to load. But note that if using with
the Echun CTs, they must be installed with the
arrow pointing from load to source. This isn't
a fault of either manufacturer. It just reflects
the lack of a standard for how to connect the
CT secondary to the 3.5mm jack used to connect.
Single and three-phase systems
------------------------------
All of the CTs in single or three-phase systems should
be installed identically with respect to load and
source. This is especially important when configuring
three-phase systems using the
`Derived Three-phase <threePhase.html>`_ method.
Split-phase systems
-------------------
Most of North America and some Asian countries use
a split-phase power system with dual voltage, typically
120/240V. With this power system, there are two mains
with exact opposite phase. The voltage between either
main and neutral is 120V, while the voltage between
the two mains is 240V. This provides an advantage of
the relative safety of lower voltage in small appliance
outlets, while still providing high voltage for
workhorse appliances like water-heaters, ranges,
and clothes dryers.
In these systems, while possible to use two voltage references,
typical IoTaWatt installations use a single reference
that reflects the phase and voltage of one of the sides,
or *legs* as they are commonly called. The result is that
CTs on the other leg should be oriented the opposite way to
be in phase with the opposite voltage reference. For simple singlephase 120V
(or 230V European type) circuits, IoTaWatt will automatically correct
reversed CTs and report positive power unless "Allow negative values" is
enabled in setup for that CT.
There is more to installing CTs on 240V circuits in split-phase
systems in the next chapter.
240V Split-phase circuits
-------------------------
As explained above, split-phase systems can provide
high-voltage for large appliances. These circuits are connected
to two adjascent CTs that are on different legs. The usual
convention is to use RED and BLACK wires or, as explained below,
BLACK and WHITE for *pure* 240V circuits.
240V only
^^^^^^^^^
When I say *pure* 240V circuits, I mean circuits that are
usually a single load, and do not have a third neutral wire
to use either leg independently for 120V. Examples of *pure* 240V
circuits would be a resistive water-heater, well-pump and
baseboard electric heater. What these circuits will have in common
is that they don't have a neutral wire, and usually use two
conductor with ground BLACK and WHITE leads.
With these circuits, you can place the CT on just one of the
conductors, and check the *double* box in input configuration,
directing IoTaWatt to double the voltage value to report
correct power and amperage.
120/240V circuits
^^^^^^^^^^^^^^^^^
Like the *pure* 240V circuits above, these circuits use two
adjascent circuit-breakers, but also have a neutral conductor.
They usually have RED and BLACK conductors on the circuit-breaker
and a white neutral conductor that connects to the neutral bussbar.
Typical appliances are ranges, ovens, and clothes-dryers. Circuits
feeding sub-panels are usually of this type as well.
For these circuits, the two legs must be measured individually
because the current in each is not always the same. There are a couple
of ways to do this.
The easiest way is to pass both the RED and BLACK conductors through
the CT. A CT will measure the total current of all of the conductors
that pass through the primary. But there is a twist. The phase of
the current in each conductor is exactly opposite the other, so they
will cancel each other out. Rather than get the sum of the two,
you get the difference between the two.
The solution is to pass one conductor through in the opposite direction
to the other. There is a common trick for this. In many panels,
the conductors are brought past the CT in a U shape so that there is
some excess wire in case the circuit needs to be moved within the panel.
You can use this U configuration to easily reverse one of the conductors.
In this case, the CT needs to handle the combined capacity of the two
circuit breakers when added together.
A 50A CT can probably be used up to about a 2x30A breaker.
.. figure:: pics/CTbasics/reverse240V.jpg
:scale: 30 %
:alt: Reversed CT conductors
The CT is clamped around the RED wire going down and
the BLACK wire going up.
An alternate method, and recommended with high amperage
sub-panel circuits, is to put a separate CT on each leg.
The CTs can be connected to two individual IoTaWatt inputs
and added together later for the total. With this method,
each of the two CTs only need match the capacity of one of
the circuit breakers.
Two individual CTs can also be combined with a common headphone
splitter and fed into a single IotaWatt input. When combining
this way, both CTs must be the same model with an individual
capacity sufficient to measure the combined capacity of the two
circuit breakers.