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Field Test Best Practices: A Resource for Practical Residential Building Science

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Current Transformer

Current transformers (CTs) allow measurement of current flow without interrupting the circuit of interest. They are commonly used in buildings field testing where inserting an ammeter in series with the circuit is impractical. A CT that measures alternating current (AC) is made of a toroidal (or similar) magnetic core with a length of wire wrapped around the torus to form a secondary winding. When a current-carrying conductor is passed through the CT ring along its cylindrical axis a magnetic field is induced in the CT core, which in turn induces a current in the secondary coil that is proportional to the current through the ring.

There are three (3) main varieties of CTs that we encounter in residential field testing:

  • Solid-core CTs
  • Split-core CTs
  • Clamp-on CTs to be used with digital multimeters (DMMs)

Solid-Core Current Transformer

Solid-core current transformers (CTs) are approximately toroidal in shape and require the circuit being measured to be disconnected during installation. They are more accurate than split-core CTs, but are often not a practical solution for residential circuit breaker panels because it is necessary to physically disconnect each current line to install the CT. Solid-core CTs can be a good choice for permanent installation if you have the opportunity to install the CTs while the circuits are being wired up for the first time.

Split-Core Current Transformer

Split-core CTs are very common in residential current monitoring because they can be installed without disconnecting the circuits. Though convenient, they are generally more expensive and less accurate than solid-core CTs. Split-core CTs either have a section of the CT that is removed for installation (left in figure below) or have a section that opens on a hinge (right in figure).

Current Transformer (CT) Installation

Current transformers are always marked to indicate which side must face the source and which side must face the load. The source direction is toward the utility power. The load is toward the appliance being powered or a circuit breaker. Since current is directional, polarity matters when making measurements; take care that the electrician installs the CT in the correct orientation.

Photo shows the source and load direction on a current transformer. Source is toward the utility power, load is toward the appliance being powered or a circuit breaker.

Orientation of source and load on a current transformer

Clamp-On Current Transformer Probe for Digital Multimeter

Clamp-on current transformer (CT) probes allow non-contact current measurements using a Digital Multimeter (DMM) or other instrument without interrupting the circuit under test. Not all CT probes work for both direct current (DC) and alternating current (AC). Clamp-on CTs are commonly used for one time measurements, rather than long term monitoring.

Watt (Power) Transducers

There are a variety of watt transducers available for residential energy monitoring. A watt transducer monitors the current, voltage, and the relative phase angle between the two in a circuit to determine true power delivered to a load. This information is reported as a direct current (DC) voltage or current signal proportional to the measured power. For residential (single phase, three-wire) circuits, the power in each of the two mains branches (black to neutral and red to neutral) must be monitored and summed to get the total power. Watt transducers can either use an external current transformer (CT) or an internal current transformer to measure current and have a input for sensing voltage.