You're asking excellent questions, and they aren't stupid at all! They highlight the importance of proper wiring and labeling for safety. The short answer is no, you can't leave pins unconnected on a receptacle while still connecting it to a different voltage than it was designed for. The issue revolves around code compliance, safety, and the intended use of listed equipment. Here's a breakdown

General Principles and Relevant Code Sections:
110.3(B) Installation and Use

"Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling." This is a foundational rule. The listing of a receptacle (e.g., by UL) is based on its intended use and the associated wiring configurations.

406.6 Receptacle Faceplates

"Receptacle faceplates shall be installed so as to completely cover the opening and seat against the mounting surface." While seemingly unrelated, this emphasizes that the physical installation must match the intended design.

406.9 Receptacles in Damp or Wet Locations

This highlights the importance of using receptacles suitable for the environment. Similarly, using a receptacle for a different purpose than it was designed for compromises its safety and listing.

Suitability [Article 100]

Equipment must be "Suitable" for the intended use. Using a receptacle in a way it wasn't designed for calls into question its suitability.

110.3 Examination, Identification, Installation, and Use of Equipment. This section has a broad range of conditions for using equipement that are generally accepted for safety.

Detailed Analysis of Your Scenarios


480V/3-Phase Pin and Sleeve for 480V Single-Phase



Violation

Yes, this is a violation. Pin and sleeve devices are designed to ensure proper phasing and voltage. Omitting a phase and using it for single-phase is a misuse of the device. Even with a label, it creates a potential hazard because someone might later assume it's a standard three-phase connection and attempt to connect a three-phase load. The lack of a third phase would damage the load.

Why

The interlock features of a pin and sleeve device are often tied to all phases being present. If it's intended for 3-phase, the absence of a phase could defeat the interlock, leading to unsafe connection/disconnection under load. The ground pin and proper phasing is critically important in these systems.

Better Solution

Use a properly rated single-phase receptacle and cord cap designed for 480V.2.

5-Wire 3-Phase NEMA L21-30 Receptacle for 208V Mixer (No Neutral)



Violation

Yes, this is a violation. Leaving the neutral pin unconnected is still wrong.

Why

The NEMA L21-30 is designed for 3-phase, 208V systems with a neutral. While the mixer doesn't use the neutral, the receptacle is designed for it to be present. There are other NEMA configurations such as L15 or L16 designed for three phase with ground but no neutral. If you have the proper voltage present and dont need a neutral you may use those instead.

Better Solution

Use a NEMA configuration appropriate for the voltage, phase, and absence of a neutral, or, if using the L21-30, properly connect the neutral conductor even if the mixer doesn't utilize it.3.

120/240V NEMA 14-50 with Neutral Omitted for 240V EV Load



Violation

Yes, this is a violation. Even with a label, leaving the neutral pin unconnected when using a 14-50 is incorrect.

Why

The NEMA 14-50 is designed for both 120V and 240V appliances and is wired with a neutral. Many EV chargers use the neutral for internal circuitry or pilot signals even in a primarily 240V application. Tampering with that configuration creates risk.

Better Solution

Use a NEMA 6-50 receptacle and plug, which is specifically designed for 240V, 50A circuits without a neutral. This is the standard solution for EV chargers that don't require a neutral. Or, if the EV charger manufacturer specifically allows the use of a 14-50 without a neutral connection and provides instructions, follow their guidelines precisely.

Why Labeling Isn't Enough

While a label is a good idea, it doesn't override the code requirements or the fundamental principle that equipment must be used in accordance with its listing. Labeling doesn't prevent someone unfamiliar with the modification from making a dangerous connection later. The next person may assume it's a standard receptacle and plug in the incorrect appliance.

In summary, using a receptacle in a way that deviates from its intended design and listing is a code violation and creates a potential safety hazard. Always use the correct receptacle type for the voltage, phase, and wiring configuration of the load. Always consult with a qualified electrician for any electrical work to ensure compliance with local codes and safe installation. Flag for review