A current sensor detects the current flowing through a conductor and reports it to a building automation system (BAS), controller, or meter, either as a simple on/off status or a proportional analog signal. Most are built around a current transformer (CT), the element that generates a signal proportional to the current passing through it. This guide covers the Functional Devices, Inc. current sensor selection choices that matter for HVAC, BAS, and energy monitoring work, to help you specify the correct model for your application.
What a Current Sensor Does: Switches vs. Transducers
Two output types cover most building automation work:
- Current switches determine whether a load is running. When current through the conductor crosses a set point, a solid-state or relay output changes state, and the BAS reads it as on/off status. Use them for fan proving, motor status, and confirming that a stage is energized.
- Current transducers report how much current is flowing, as a proportional analog signal (commonly 4–20 mA or 0–10 Vdc). Use them for applications where you need a specific measurement.
Functional Devices also builds combination units under its RIB® (Relay-In-a-Box®) brand that pair a sensor with a control relay in one enclosure. This saves panel space when the sensed current also needs to switch a load.
Split-Core vs. Solid-Core Current Sensors
Another distinction is how the sensor goes around the conductor:
- Split-core sensors open on a hinge and clamp around a wire that's already in place. Nothing gets disconnected, so they are the default for retrofits and energy audits where shutting down the circuit isn't an option.
- Solid-core sensors are a closed ring that the conductor threads through during installation. They often cost less and can read slightly more accurately, which makes them a good fit when you're wiring a new panel.
Match the core to the access you have. Split-core is usually best for live, installed wiring, whereas solid-core is generally the right choice when the conductor is open during a new install.
How to Size a Current Sensor
Sizing is mostly a question of range and threshold.
First, confirm the current type and range. Most building loads are AC, but verify this, and size the sensor's range based on the motor's full-load amps (FLA) or continuous running current. While a motor draws a brief, heavy inrush current at startup, solid-state current sensors are built to tolerate these transient spikes without damage. Sizing a sensor to the running current ensures it remains highly accurate during normal operation.
Second, choose the threshold behavior. A fixed threshold (an "F" in the model number) works when the load draws no current until it's commanded on and has no meaningful standby draw. An adjustable threshold (an "A" in the model number) lets you set the trip point above any potential leakage or standby current, which matters for electronically commutated motors (ECMs) and other low-draw loads that sit just below a fixed trip point.
Low current is its own sizing question, as a standard switch may never see enough current to actuate. The RIBXGTA-ECM is built for that case, with an adjustable threshold down to 0.25 A.
HVAC Monitoring Applications
On HVAC equipment, current sensing is a simple, direct way to confirm that a device is doing what it's been told to do.
- Fan Proving: A current switch on the fan motor confirms it's running by sensing its current draw, which holds up better over time than a sail switch in many applications.
- Compressor and Pump Status: Sensing motor current tells the BAS whether the compressor or pump has started, so a failed start raises an alarm instead of going unnoticed.
- Motor Health Trending: A transducer that trends running current can flag a motor pulling more than it should, which often precedes a mechanical failure.
BAS Integration With Current Sensors
Getting a current sensor into a building automation system is usually straightforward, as long as the output matches the input.
For status points, a current switch with a solid-state switch output wires directly to a binary input on the controller. Because these outputs are non-polarized and power-limited, the BAS controller treats them like a standard digital contact closure. However, ensuring the output type matches the expected controller configuration is critical for the BAS to read the on/off status correctly.
For measured values, a transducer's 4–20 mA or 0–10 Vdc output lands on an analog input, where the controller scales it to display actual amps.
Energy Submetering Use Cases
For energy work, a transducer is the correct tool, because submetering needs a measured value rather than a status bit.
Clamp split-core transducers around feeders or branch circuits to break consumption down by load, tenant, or system without taking anything offline. That data feeds energy audits and fault detection, cost allocation, and the baselining that shows whether an efficiency project is having its intended effect.
Recommended Functional Devices Current Sensors
Functional Devices offers current sensors across the full range of outputs, core styles, and thresholds. A few common starting points:
- For on/off status over a dry contact to a BAS, the RIBXGTA (split-core, adjustable, 0.75–150 A) is a standard choice.
- For low-current loads like ECMs, the RIBXGTA-ECM drops the adjustable threshold to 0.25 A.
- Where the sensed current also needs to switch a load, an enclosed combo such as the RIBHX24BA pairs a 20 A SPST relay with an internal, adjustable current sensor (.25–20 A) to monitor the exact branch circuit it is switching
Frequently Asked Questions
What's the difference between a current switch and a current transducer?
A current switch gives an on/off signal when current crosses a set point, telling the BAS whether a load is running. A current transducer outputs a proportional analog signal—4–20 mA or 0–10 Vdc—that reports how much current is flowing, for monitoring and metering.
When should I use a split-core current sensor instead of solid-core?
Use split-core when the conductor is already installed and can't be taken offline, which covers most retrofits and energy audits. Solid-core suits new installations, where it usually costs less and reads slightly more accurately.
How do I size a current sensor?
Make sure the load's normal running current plus startup inrush falls within the sensor's rated range, then pick a fixed or adjustable threshold depending on whether the load has standby or leakage current to look past.
Can a current sensor detect a low-current ECM?
A standard fixed-threshold switch often can't. For low-draw loads, choose an adjustable model such as the RIBXGTA-ECM, which trips as low as 0.25 A.
Get the Right Current Sensor for the Job
Need help matching a sensor to your application? Explore the current sensor range from Functional Devices, find an authorized Functional Devices distributor in your region, or call the technical team at 800-888-5538.