Hi Stephan, great to hear from you. This is a common challenge, and I'll break down the considerations based on NEC and NFPA 79.

Key Considerations and Regulations1. NEC and NFPA 79 Applicability



NFPA 79 (Electrical Standard for Industrial Machinery)

This standard directly applies to the electrical design and construction of your control cabinet and the machine. It references the NEC (NFPA 70) extensively. NFPA 79 is the primary document you need to comply with.

NEC (NFPA 70 - National Electrical Code)

While NFPA 79 is your main focus, the NEC underlies many of its requirements. The NEC provides general electrical safety requirements applicable to all installations.

UL 508A (Industrial Control Panels)

As you are in a PanelShopProgram from UL, you are obligated to build according to UL 508A.2.

Unlisted/Unrecognized Components (The Core Issue)

Both NFPA 79 and the NEC generally require that components used in electrical equipment (like your control panel and the machine itself) be listed or recognized by a Nationally Recognized Testing Laboratory (NRTL) like UL. "Listed" means the entire product has been tested and certified. "Recognized" often refers to components intended for use within larger equipment.3.

Tank Heater Requirements



NEC 427 (Fixed Electric Heating Equipment for Pipelines and Vessels)

This section of the NEC may apply depending on the exact application of the heater. It has requirements related to control, overcurrent protection, and grounding.

NEC 500-516 (Hazardous Locations)

Does the tank contain flammable liquids or gases? If so, the heater, wiring, and control systems must be suitable for the hazardous location and be appropriately rated.

NFPA 79 5.2 (Components)

Mandates listed or recognized components where available and suitable.

Thermal Overload Protection

A vital part of the design is ensuring adequate thermal overload protection for the heater element to prevent overheating and potential fires. The OL relay/protection should be placed on the secondary side of the transformer.

Possible Solutions and Exceptions (and their limitations)


Exception #1: Component Recognition (Potentially Tricky)



The Concept

While a listed heater is ideal, it may be possible to use a recognized component if it's specifically designed and intended for use within equipment. UL Component Recognition is signified by the "UR" mark (backwards UR).

The Challenge

You cannot simply use any unlisted heater and treat it as a recognized component. It must be specifically designed and marketed as a recognized component for use in equipment, and you must follow the manufacturer's instructions for its application. Obtain documentation from the heater manufacturer showing its intended use as a component within a larger system.

UL 508A Supplement SA (Special Purpose Applications)

It allows for the acceptance of unlisted components if they are acceptable to the AHJ (Authority Having Jurisdiction). In the case of UL PanelShopProgram this might be UL.2.

Exception #2: Field Evaluation (Most Common Approach - But Expensive)



The Concept

You can have the entire machine (including the tank heater) field-evaluated by a NRTL (like UL) after it's built. This is sometimes called a "one-time evaluation" or "special inspection."

The Process

You'd contact UL or another NRTL, and they would send an inspector to the machine's location to evaluate it against relevant standards. If it passes, they'll apply a field-evaluation label.

Considerations

This is usually the most straightforward way to deal with an unlisted component, but it can be expensive. It requires the machine to be fully assembled and ready for inspection, and any modifications required by the inspector will add to the cost.

Cost

Field evaluations can range from a few thousand dollars to tens of thousands, depending on the complexity of the equipment and the issues found.3.

Exception #3: "Mill Use" Equipment (Very Narrow Scope)



The Concept

In very specific circumstances, if the machine is for use within the same facility that manufactures it (i.e., "mill use"), there may be some flexibility. However, this is heavily scrutinized and depends on the local AHJ's interpretation. It's generally not a reliable exception.

Why It's Difficult

AHJs are increasingly strict about requiring listed equipment, even for in-house use.4.

Exception #4: Replacing the heater If possible, you could convince the customer to switch the heater to a UL-listed one. It might be the easiest option if feasible and available.

Specific Steps to Take


Communicate with the Customer

Explain the UL/NFPA requirements and the potential costs and delays associated with using an unlisted heater.2.

Heater Documentation

Get all available documentation for the heater, including specifications, installation instructions, and any internal testing data. The more information you have, the better.3.

Contact UL (or other NRTL)

Discuss the specific situation with a UL engineer. They can provide guidance on the best approach and the requirements for a field evaluation (if that's the chosen path). You can also check with them about component recognition possibilities if you have detailed heater documentation.4.

Hazardous Location Assessment

If the tank contains flammable materials, perform a thorough hazardous location classification and ensure all electrical components are suitable.5.

Detailed Design Review

Carefully review the design with respect to NEC Article 427 (if applicable), NFPA 79, and UL 508A. Pay close attention to grounding, bonding, overcurrent protection, and thermal overload protection.6.

Transformer Sizing and Protection

Ensure the transformer is adequately sized for the heater load (with a safety margin) and properly protected with overcurrent devices on both the primary and secondary sides.7.

AHJ Consultation (Recommended)

If possible, discuss the project with the local Authority Having Jurisdiction (AHJ). They have the final say on compliance.

In Summary

Using an unlisted heater in a UL-compliant control panel and machine is challenging but not always impossible. The most common path is a field evaluation, but it's important to understand the costs and requirements. Component recognition is a possibility but requires careful documentation and compliance with the manufacturer's instructions. Communication with the customer and consultation with UL (or another NRTL) are essential.Let me know if you have any more questions. Good luck! Flag for review