You're on the right track with your questions. Let's break down UL508A Sections 30 and 32, "switch units," and the relevance of NEC 670.4(B).

Understanding Section 30 (Disconnect Switches) vs. Section 32 (Overcurrent Protection of Feeder) Section 30: Disconnect Switches deals with disconnects that are used to isolate the entire control panel (or specific sections within it) from the power source. The primary purpose is safe isolation for maintenance and servicing. Section 30 covers things like: Type of disconnect (fused or non-fused) Ampere rating and short-circuit current rating (SCCR) Marking requirements Operating mechanisms and access (e.g., lockout/tagout) Section 32: Overcurrent Protection of Feeder focuses on protecting the control panel from overcurrents (overloads and short circuits) originating in the supply side. This section deals with the selection and placement of fuses or circuit breakers to prevent damage to the panel components and wiring due to faults in the incoming power.

Which Section Applies to YOUR Main Disconnect? Often, BOTH Sections Apply! Your main disconnect needs to be a properly rated disconnect switch (Section 30), AND the feeder to the panel needs to be protected by overcurrent protective devices (OCPDs, such as fuses or a circuit breaker) per Section 32. Think of them as separate but related requirements. Scenario 1: Fused Disconnect Switch

If your main disconnect is a fused disconnect switch, it fulfills both the disconnect switch requirements of Section 30 and provides overcurrent protection as part of Section 32. The fuses within the disconnect provide the OCPD. This is a very common and often preferred solution.

Scenario 2: Non-Fused Disconnect Switch and Separate OCPD

If your main disconnect is a non-fused disconnect switch, you MUST have a separate overcurrent protective device (circuit breaker or fuses) located

upstream of the disconnect switch to comply with Section 32. This OCPD is typically located in the building's electrical distribution panel.

What is a "Switch Unit?" Think of a "switch unit" as a generic term for a disconnect switch. It encompasses both fused and non-fused types. Yes, a non-fusible switch like the Allen-Bradley 194R-NU line is a "switch unit."NEC Article 670.4(B) and Your Control Panel 670.4(B) is about MACHINE TOOLS. While you're building a control panel that might control a machine tool, 670 is NOT directly applicable unless you are incorporating the control panel in a machine tool. The statement "The disconnecting means shall not be required to incorporate overcurrent protection" in 670.4(B) specifically refers to MACHINE TOOL control panels. The NEC allows the overcurrent protection for the machine tool's control panel to be provided elsewhere, typically upstream. This is because machine tool disconnects often need to be readily accessible to the operator, even if the OCPD is located remotely. UL508A is generally more strict. Even if your equipment were a machine tool, and even if the NEC would allow the disconnect and the overcurrent device to be separate, a UL508A listed enclosure with a load of motors will generally need to include an overcurrent device in the panel.

Key Considerations and Common Practices


SCCR (Short-Circuit Current Rating)

Critically important! Your panel's SCCR must be clearly marked on the panel and must be higher than the available fault current at the panel's supply terminals. The SCCR of the main disconnect, along with all other components, contributes to the overall panel SCCR. A fused disconnect switch with appropriately sized fuses is a simple way to achieve a high SCCR rating.2.

Location of OCPD

Think about where the overcurrent protection is located. If it's inside the control panel (e.g., a fused disconnect switch), you've handled both Section 30 and Section 32 within the panel itself. If it's outside the panel (e.g., a circuit breaker in the building's panelboard), you still need a properly rated non-fused disconnect switch and to ensure the upstream OCPD provides adequate protection for the panel's components and wiring.3.

Motor Loads

UL508A has specific requirements for motor overload protection. This protection is separate from the overcurrent protection discussed in Section 32. You'll need to use appropriately sized overload relays or other approved devices to protect the motors from overloads.
4.

Service Entrance

If your panel is used as a service entrance, then special requirements would apply. This is usually only true for a panel that's physically outdoors.5.

UL508A Supplement SB

Consider including Supplement SB. This supplement requires equipment to withstand a short-circuit current to the marked SCCR for the equipment with minimal damage (if any). Supplement SB testing is very valuable for compliance.

Practical Advice



Fuses are generally preferred because they provide very high SCCR ratings. If you use fuses, be sure to properly calculate the size of the fuses according to NEC 430, as well as per the requirements of UL508A, which will be slightly different, but generally more conservative than the NEC. Carefully select your disconnect switch. Make sure the disconnect switch's voltage and current ratings are appropriate for your application. Also, be sure that the short-circuit current rating of the switch is adequate. Document Everything. Keep detailed records of all component selections, calculations, and wiring diagrams. This documentation will be essential for UL listing and inspection.

In Summary:

Your main disconnect switch needs to meet the requirements of Section 30 of UL508A. The feeder to the panel needs to be protected by overcurrent protective devices according to Section 32 of UL508A. You can achieve both of these requirements either with a fused disconnect switch or a non-fused disconnect switch combined with a separate OCPD upstream. Careful planning and documentation are essential for UL508A compliance.Consult with a qualified electrical engineer or a UL certification expert for guidance on specific applications and to ensure full compliance with UL508A. Flag for review