Okay, I understand your question and your background. You're looking for something in the NEC that specifically prohibits oversizing the cord and plug combination for a given appliance's load.The short answer is yes, there are sections in the NEC that indirectly address this issue, although it doesn't state it explicitly as "you can't use a 5-30P with an 18/3 cord on a 2A appliance." The code relies on a combination of factors to discourage this practice and ensure safety

Here's a breakdown of why your example is a problem and the relevant NEC sections:

1. Ampacity of Conductors (Overcurrent Protection)



NEC 240.4(D) "Small Conductors"

This is a very important section. While it might seem like it's primarily about protecting building wiring, it indirectly applies to flexible cords. It states that conductors of certain sizes (including the 18 AWG in your SJOO cord) are generally protected against overcurrent by overcurrent devices rated no higher than the conductor's ampacity. 18 AWG is typically limited to 10 amps for power-carrying conductors. Your 30A breaker significantly exceeds this protection.

The reason the NEC doesn't explicitly say you can't do this is because the assumption is that the overcurrent protection (the breaker) will protect the conductors. However, with the 30A breaker and the 18 AWG cord, you have a situation where a fault within the appliance could easily cause the cord to overheat and potentially start a fire before the 30A breaker trips.2. Flexible Cords and Cables - Permitted Uses (400.7) and Prohibited Uses (400.8)

The permitted uses of flexible cords doesn't explicitly state restrictions, but the intended application of the cord is that the appliance would be used in applications for which the cord is rated. The prohibited uses can also be broadly interpreted. In your application, there may not be a specific prohibited use, but installing flexible cords in a manner not intended for that location is restricted.3.

Equipment Listing and Labeling (110.3(B)): This is a crucial section and a very strong argument against your example. Equipment must be installed and used in accordance with its listing and labeling. If the widget is UL listed (or listed by another NRTL), the listing almost certainly specifies the type of cord and the attachment plug to be used. Using a different cord and plug violates the listing. UL standards (like UL 817 for cord sets and power-supply cords) very carefully consider the relationship between the appliance's current draw, the cord size, and the attachment plug rating. They perform extensive testing to ensure that the cord will not overheat under normal and fault conditions. Your modification circumvents that safety evaluation.4. Receptacle Compatibility and Mismatch Prevention



NEC 406.3(F)

This section is focused on preventing the insertion of plugs into receptacles of different voltage ratings or configurations. While it doesn't directly forbid oversizing the plug relative to the cord, the spirit of this section is to prevent mismatches that could lead to unsafe conditions. Using a 5-30P on an appliance that draws only 2A creates a mismatch between the available fault current and the cord's ability to handle that current.5.

Good Workmanship (110.12)

This is a general rule, but it requires that electrical installations be done in a neat and workmanlike manner. Using an obviously oversized plug and cord for a small appliance would likely be considered a violation of this rule, as it's not a reasonable or professional practice. It creates a potential hazard.

Why the NEC Doesn't Directly Forbid This with a Simple Rule

The NEC is a minimum safety standard. It's impossible to anticipate every possible scenario. The code relies on a combination of specific rules, general principles, and the assumption that qualified individuals (electricians, engineers, inspectors) will apply good judgment. The combination of 110.3(B) [Listing and Labeling] and 240.4(D) [Small Conductors] combined with standard safety practice will be applied here.

Back to Your Example

Your example is a clear violation of 110.3(B) (assuming the widget is listed), and likely violates the spirit of several other NEC sections. A qualified inspector would almost certainly reject this installation.

In Summary:

While the NEC doesn't have a single sentence that says "you can't use a 5-30P on a 2A appliance with an 18/3 cord," the combined effect of the sections I've outlined, especially 110.3(B) and 240.4(D)*, makes it clear that your example is an unsafe and non-compliant practice. The core issue is that the overcurrent protection is inadequate for the conductor size. Flag for review