CT Basics

What is a Current Transformer?

From Wikipedia:

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 eMon Energy.

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 eMon Energy 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.

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.

Split Core CT

Split Core CT



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 eMon Energy, 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.


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.

eMon Energy 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 CTs that eMon Energy 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.


Here you can plainly see "This side toward source"


Here the arrow points source(K)->load(L)


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.


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 method.