When you're specifying switching devices for a general purpose relay vs. definite purpose contactor decision, the terminology alone can create confusion. Both switch loads. Both use electromagnetic coils. And in certain current ranges, their specifications can look almost identical on paper. The difference comes down to how each device is designed, what it's rated for, and—critically—what the code actually requires for your application.
This guide is written for specifiers, controls engineers, and lighting reps who need to make the right call the first time, without a callback or a failed inspection down the road.
Defining General Purpose Relays
A general purpose (GP) relay is a switching device designed for versatility across a wide range of control applications. It can switch resistive loads, inductive loads, motor loads, and lighting loads—and it does so with both normally open and normally closed contact configurations, making it adaptable to virtually any control sequence.
In building automation, general purpose relays are the standard tool for interfacing low-voltage control signals with higher-voltage equipment loads. They provide electrical isolation between the control circuit and the load circuit, protect controller outputs from voltage spikes and inrush, and handle a wide variety of switching tasks from a single compact, prewired assembly.
RIB® (Relay-In-a-Box®) relays from Functional Devices, Inc. are general purpose relays purpose-built for BAS and HVAC applications. They ship as complete, enclosed, prewired assemblies—UL Listed as standalone field-ready devices—in a range of coil voltages and contact ratings. That UL Listing as a complete assembly is a meaningful distinction, as we'll cover below.
What Is a Definite Purpose Contactor?
A definite purpose (DP) contactor is a switching device engineered for a specific, well-defined application—most commonly HVAC compressors, condensing units, refrigeration equipment, and other high-cycle motor loads. The "definite purpose" designation means the device is designed and rated for one particular duty, not a broad range of applications.
DP contactors originated in the HVAC/R OEM market, where equipment manufacturers needed a compact, cost-effective switching device engineered for the start-stop duty cycle of compressors and fan motors in packaged HVAC equipment. They are smaller and less expensive than full NEMA general purpose contactors, with similar current ratings—but that cost and size reduction comes with important trade-offs in application flexibility and certification status.
Key characteristics of DP contactors:
- Rated for specific load types (HVAC motors, compressors, resistive heating)
- Compact and cost-effective compared to NEMA contactors
- Primarily normally open (NO) contacts
- Standard coil voltages: 24 VAC, 120 VAC, 240 VAC
- Current ratings typically from 15A to 90A
- UL Recognized (not UL Listed) in most cases—more on this below
Comparison Table: GP Relay vs. DP Contactor
BAS and HVAC Application Fit
Where General Purpose Relays Belong
In BAS and HVAC installation work, general purpose relays handle the vast majority of switching tasks. Any time you need to translate a low-voltage control signal into a switched circuit—for a fan coil unit, a damper actuator, a lighting zone, a make-up air unit, or a BAS controller output—a general purpose relay is the right tool.
The versatility of SPDT contacts is particularly valuable in BAS work. Many control sequences require both a normally open and normally closed contact from the same device—for example, switching one circuit on while simultaneously switching another off, or providing status feedback to a controller. A DP contactor, with its primarily NO contacts, can't do this without additional devices.
RIB® relays also offer a range of coil voltages that match virtually any BAS controller output: 24 VAC for standard BAS, 120 VAC for line-voltage control circuits, and 10–30 VAC/DC for universal applications. That flexibility, combined with a compact enclosed form factor and UL Listing, makes them the standard specification for BAS relay output points.
Where Definite Purpose Contactors Belong
DP contactors are most appropriately used inside OEM-manufactured HVAC equipment—packaged rooftop units, split systems, condensing units, and refrigeration equipment—where the OEM has engineered the device for a specific, known load and duty cycle. The OEM knows exactly how many start cycles per hour the compressor will make, what the inrush current looks like, and what ambient conditions the contactor will operate in. The DP contactor is spec'd and tested for that exact application.
In field installation work, DP contactors are sometimes used as direct replacement parts for failed OEM contactors in existing equipment. This is appropriate; you're replacing a DP contactor with an equivalent DP contactor in the same assembly it was designed for.
Where DP contactors are not appropriate: as general-purpose field switching devices outside of their intended OEM assembly, or in applications requiring NC contacts, multi-voltage coil options, or the installation flexibility of an enclosed general purpose relay.
Specifying the Right Device for Code Compliance
The UL Listing Difference
This is the most important compliance distinction between the two device types.
Most DP contactors carry UL Recognized status, meaning they are certified as components for use inside a listed assembly, not as standalone field devices. In practice, this means a DP contactor is certified for use inside the OEM's listed equipment, but may not be appropriate as a standalone field-installed device outside of a certified assembly.
RIB® general purpose relays are UL Listed as complete assemblies. An AHJ can accept a UL Listed RIB® relay for field installation without additional scrutiny. For field installers doing one-off switching jobs outside of an OEM assembly, this is the cleanest path to inspection approval.
NEC Considerations
NEC Article 110.3(B) requires that listed equipment be installed and used in accordance with its listing and manufacturer's instructions. For a DP contactor, that means using it in the application it was designed and listed for. Using a DP contactor as a general-purpose field switching device—particularly outside of a certified OEM assembly—may not satisfy this requirement, depending on AHJ interpretation.
NEC Article 110.2 gives the AHJ authority to approve equipment, but approval is generally based on listing status. A UL Listed general purpose relay gives the AHJ a clear basis for approval. A UL Recognized DP contactor in a field application may require additional justification.
For lighting control applications specifically, some AHJs require switching devices that are specifically listed for the intended lighting load. A general purpose relay with ballast ratings (such as the RIB2401B with its 20A ballast rating at 277 VAC) satisfies this requirement. A DP contactor without explicit ballast ratings may not.
The Practical Takeaway for Specifiers
For field-installed BAS, HVAC, and lighting control applications: specify a UL Listed general purpose relay. The RIB® lineup covers virtually every application in this space and gives you clean AHJ acceptance, SPDT contact flexibility, and a compact prewired form factor.
For OEM-assembled HVAC equipment where you're specifying or replacing the internal switching device for a compressor or condenser fan motor in a packaged unit: a DP contactor is the appropriate and code-correct choice, as long as it's used in the assembly it was designed for.
RIB® GP Relay Product Line
For BAS and HVAC field applications, the following RIB® models cover the majority of general purpose relay needs:
RIBU1C — 10A SPDT, dual coil voltage (10–30 VAC/DC and 120 VAC). Light-duty BAS switching, damper actuators, and pilot duty applications. UL Listed, UL916, UL864.
RIB2401B — 20A SPDT, 24 VAC/DC or 120 VAC coil. The most widely used general purpose relay in the RIB® lineup. Fan coil units, lighting, make-up air, and general HVAC switching. Ballast rated at 277 VAC. UL Listed, UL916, UL864.
RIB24P30 — 30A DPDT, 24 VAC/DC coil. Higher-current general use loads and applications requiring two independently switched circuits from a single coil.
For the full RIB® relay lineup by coil voltage, contact rating, and configuration, browse RIB® Relay Products at Functional Devices.
Frequently Asked Questions
What is the difference between a general purpose relay and a definite purpose contactor?
A general purpose relay is designed for versatility across a wide range of switching applications and carries both NO and NC contacts. A definite purpose contactor is engineered for a specific application—typically HVAC compressors and motors in packaged OEM equipment—and is primarily NO only. Most DP contactors are UL Recognized components, not UL Listed standalone devices.
Can I use a definite purpose contactor as a general purpose relay?
Not reliably. DP contactors are designed and certified for specific applications inside OEM-assembled equipment. They typically lack NC contacts, have limited coil voltage options, and in most cases carry UL Recognized status rather than UL Listing—which means they are not certified for standalone field installation. For general BAS and HVAC field switching, a UL Listed general purpose relay is the correct specification.
Are RIB® relays general purpose relays?
Yes. RIB® relays from Functional Devices are general purpose relays with SPDT or DPDT contact configurations, multiple coil voltage options, and a range of contact ratings from 10A to 30A. They are UL Listed as complete, field-ready assemblies, making them suitable for direct field installation in BAS, HVAC, and lighting control applications.
What does "definite purpose" mean in a contactor?
"Definite purpose" means the device is designed and rated for one specific application—most commonly HVAC/R compressor and motor switching. The rating is matched to that application's duty cycle, inrush current, and load type. Using a DP contactor outside of its intended application may void its certification and may not satisfy NEC or AHJ requirements.
Why does UL Listing matter when specifying relays and contactors?
UL Listed devices have been evaluated as complete, field-ready products and can be accepted by AHJs for field installation without additional scrutiny. UL Recognized components are certified only for use inside a listed assembly. For field-installed switching devices in BAS and HVAC applications, UL Listing provides the cleanest path to inspection approval. See our full guide: UL Listed vs. UL Recognized: What HVAC and BAS Contractors Need to Know.
Can I use a general purpose relay instead of a definite purpose contactor in an HVAC unit?
As a direct replacement for an OEM contactor inside a packaged unit, it's not recommended—the original DP contactor was specified for that equipment's exact duty cycle and load. For field-installed switching outside of OEM equipment, a UL Listed general purpose relay is the appropriate choice.
Bottom Line for Specifiers
For field-installed BAS, HVAC, and lighting control applications, a UL Listed general purpose relay—specifically the RIB® lineup from Functional Devices—gives you the contact flexibility, coil voltage options, compact form factor, and clean AHJ acceptance that a definite purpose contactor can't match in that context.
Browse the full RIB® relay lineup or find an authorized Functional Devices distributor to source the right device for your next project.