You're on the right track, but there are several crucial points to consider before concluding that your plan is safe and compliant with the NEC. Here's a breakdown

Understanding the Goal:

You're aiming to install a non-rated device in a Class I, Division 2 location by only using its 4-20mA output and ensuring any unused dry contacts are rendered non-incendive, thereby avoiding explosion-proof or purged enclosures.

Analysis of NEC 501.105(B)(2) and Non-Incendive Circuits



NEC 501.105(B)(2)

This section covers wiring methods for Class I, Division 2 locations. It generally requires wiring methods suitable for Division 1, but provides exceptions.

Exception 2

This is the key. It allows general-purpose enclosures (like your junction box) to contain devices with contacts if the circuits are proven to be non-incendive.

Non-Incendive Circuit

This is a circuit where, under normal operating conditions (and a specified set of fault conditions), it will not cause ignition of a specific flammable atmosphere. It's defined in Article 100.

Crucial Considerations and Requirements


"Normal Operating Conditions and Specified Fault Conditions"

This is where it gets tricky. Simply leaving the contacts unconnected may not be enough. You MUST demonstrate that, even with the contacts unconnected, the circuit meets the definition of non-incendive under both normal conditions and credible fault conditions. Ask yourself:

Induced Voltage/Current

Even with the contacts disconnected, could stray capacitance or induction from other wiring in the junction box induce a voltage or current in the open circuit that could cause ignition? This is more likely if the wiring runs parallel to other powered conductors.

Internal Device Failure

If the device internally malfunctions, could the circuitry connected to the dry contacts (even though unused) create an incendive spark?

Contact Resistance

Even if disconnected, the contact points themselves within the device's internals could still present a minimal resistance. A voltage surge could potentially cause sparking, especially in a corrosive or contaminated atmosphere.

Static Discharge

Depending on the materials and environment, static discharge to the disconnected wires could potentially be an ignition source.2.

Demonstrating Non-Incendivity



Testing and Documentation

You must have documentation that proves the circuit is non-incendive according to a recognized standard (e.g., ISA 12.12.01, IEC 60079-11 [for intrinsically safe], or other relevant standard). This often requires testing by a qualified testing laboratory (e.g., UL, CSA, FM).

The manufacturer of the instrument is the most likely source for this data. It's unlikely that the manufacturer will have tested the device under the specific conditions of open dry contacts. Intrinsic Safety (IS)

If the device is designed with intrinsically safe barriers to protect the 4-20mA loop it's more likely to be compliant. However, you still need documentation verifying the non-incendive nature of the open contact circuit. IS standards are very rigorous and require specific barriers and wiring methods.3.

4-20mA Output

You're focusing on the 4-20mA signal which is a good start, but that signal must be powered by and connected to an intrinsically safe or non-incendive barrier that is approved for use in Class 1, Division 2.4.

Junction Box

The junction box itself must be appropriate for the wiring methods used. Even though you're aiming for non-incendive, you still need a suitably rated enclosure. The enclosure must be properly grounded.5.

Documentation and Signage

You must clearly document your design, including the justification for claiming the circuit is non-incendive, the testing data, and the wiring diagram. Consider labeling the junction box with a clear statement like: "WARNING: Device utilizes Non-Incendive Circuits per NEC 501.105(B)(2) Exception 2. Dry Contacts MUST remain unconnected."

What to Do


Contact the Instrument Manufacturer

This is the most important step. Ask them directly if they have documentation to support the claim that the unused dry contact circuit is non-incendive under normal and fault conditions when the contacts are left open. If they can't provide this, you cannot proceed safely and legally with your plan.2.

Consider Alternatives



Intrinsically Safe (IS) Barrier

The safest and most common solution is to use an IS barrier for the 4-20mA signal and choose a device with IS rated contacts or outputs, if a device with dry contacts in class 1 div 2 is absolutely needed.

Explosion-Proof Enclosure

If you absolutely must use the dry contacts, and you cannot prove non-incendivity, the device must be installed in an explosion-proof enclosure.

Purged Enclosure

A purged enclosure is another option but requires more maintenance and monitoring.

In summary:

While your idea is conceptually correct, the burden of proof is on you to demonstrate that the dry contacts are truly non-incendive under all credible conditions. Simply disconnecting them is not sufficient. Without proper testing and documentation from the manufacturer or a qualified testing laboratory, you cannot safely or legally use this device in a Class I, Division 2 location using the exception you cited. Pursue the intrinsically safe solution with the dry contacts, an explosion-proof enclosure, or a purged enclosure. Flag for review