DRY CONTACTS VS WET CONTACTS: The Critical Wiring Distinction for Building Automation Systems (BAS)

If you are reviewing control submittals, designing a building automation system (BAS) architecture, or troubleshooting an interlock on-site, you will constantly encounter the terms "Dry Contact" and "Wet Contact." Getting this distinction wrong is one of the quickest ways to fry a brand-new DDC controller input channel or cause an expensive field equipment malfunction. Yet, project specifications often gloss over this detail, leaving design and estimation engineers to figure it out during the submittal phase. Let’s demystify exactly what these terms mean, why they matter, and how to apply them safely in your control circuits.

Getting this distinction wrong is one of the quickest ways to fry a brand-new DDC controller input channel or cause an expensive field equipment malfunction. Yet, project specifications often gloss over this detail, leaving design and estimation engineers to figure it out during the submittal phase.

Let’s demystify exactly what these terms mean, why they matter, and how to apply them safely in your control circuits.

What is a Dry Contact? (Potential-Free)

A dry contact—often referred to in specifications as a "potential-free" or "volt-free" contact—is a set of physical relay contacts that do not inherently provide any voltage or power.

Think of a dry contact purely as an isolated mechanical switch.

How it works: When the relay changes state, it simply opens or closes the physical gap between two terminals.
Who provides the power? The controller or device reading the status must supply the sensing voltage (usually a low-voltage DC wetting current from a DDC Digital Input).
Common BAS Example: The auxiliary status switch on a motor starter or the run-status contact on a Variable Frequency Drive (VFD). The VFD provides an isolated terminal block; your BAS controller sends its own 24 VDC/VAC signal down one wire, through their switch, and back to the Digital Input channel to sense if the circuit is closed.

What is a Wet Contact? (Powered)

A wet contact is a switch that is already energized by a power source native to the host equipment. When the contact closes, it actively delivers voltage to the output terminal.

How it works: Instead of acting as an isolated loop, the internal circuit connects one side of the switch directly to a hot power rail (e.g., 24 VAC, 120 VAC, or 24 VDC).
Who provides the power? The equipment hosting the contact provides the voltage.
Common BAS Example: A fire alarm control panel (FACP) relay that outputs an active 24 VDC signal during an alarm state to trigger duct dampers, or a packaged chiller control panel that outputs a 120 VAC signal to command an external condenser water pump starter.

The Danger Zone: Why Getting it Wrong Breaks Hardware

The most common field failure happens when a BAS engineer assumes a field device is providing a dry contact, but it is actually a wet contact.

Imagine you are wiring a Chilled Water Pump status into a DDC controller's Digital Input (DI). Your controller is designed to apply its own internal 15 VDC sensing voltage across a dry contact. If the pump control panel actually delivers a wet 120 VAC signal into that DI terminal because the field jumper wasn't removed, the input channel's solid-state electronics will instantly blow out.

Conversely, if you wire a true dry contact to an input that expects a powered voltage signal, the DDC controller will simply register "open" forever because no electrical current is moving across the loop.

How to Approach This in Specs and Submittals

As a BAS design or estimation engineer, you cannot leave this to chance on the field drawings. Take these steps during submittal reviews:

Verify the Chiller/VFD/Boiler Cutsheets: Look deeply at the manufacturer's terminal wiring diagrams. If it says "Isolated Contact Rating: 2A at 24VAC," it is usually dry. If it shows an internal connection to a transformer rail, it is wet.
Specify Isolation Relays for Voltage Mismatches: If a packaged piece of equipment forces you to accept a wet 120 VAC output for system status, do not wire it directly to your BAS panel. Pass that wet 120 VAC through the coil of a small, inexpensive interposing pilot relay. Use the dry auxiliary contacts of that pilot relay to safely interface with your low-voltage DDC controller.
Check the Graphics Logic: Ensure that your hardware submittal clearly flags whether a digital input point is configured for "dry contact loop" or "external voltage sensing" to save your startup technicians hours of troubleshooting.