You're touching on a very important aspect of electrical installations: heat management. Let's break down why the NEC uses the "lowest" temperature rating, and why your suggestion, while seemingly logical at first glance, could lead to problems.

The NEC and Temperature Ratings

The NEC (National Electrical Code) prioritizes safety, and in the context of boxes and conduit bodies, that means preventing them from becoming fire hazards. The temperature rating requirement is primarily about preventing the plastic materials of the box/body from degrading due to prolonged exposure to heat.

Why "Lowest" Instead of "Highest"? Continuous Operation

Electrical systems are designed for continuous operation. Even if a conductor is capable of withstanding a high temperature, that doesn't mean it will be operating at that temperature all the time. The problem lies in the fact that the heat generated by the conductors can accumulate within the box. This heat buildup can exceed the box's temperature rating, even if the conductors are rated higher.

Protecting the Enclosure

The box or conduit body itself is a critical component. If it melts, deforms, or otherwise fails due to heat, it exposes the conductors, creates fire hazards, and can compromise the integrity of the electrical system. The "lowest" rating is about protecting the enclosure from damage.

Heat Dissipation

A box that encloses multiple conductors can become a "heat sink." Even if individual conductors aren't generating a ton of heat, the collective heat inside the box can be significant, especially in poorly ventilated locations.

Derating Considerations

Conductor ampacity (the amount of current a conductor can safely carry) is often reduced, or "derated," when multiple conductors are bundled together in a conduit or box. This derating is a direct consequence of the increased heat buildup.

Worst-Case Scenario

The NEC aims to address worst-case scenarios. The "lowest" temperature rating ensures that even under sustained loading and ambient temperature conditions, the box won't be exposed to temperatures exceeding its design limits.

Uniformity Considerations

While not the primary driver, requiring all conductors within a raceway to have the same voltage rating helps prevent confusion and potential overloads.

Simplification

Using the lowest temperature rating simplifies the installation process. Instead of having to analyze each conductor's individual operating temperature and then match the box accordingly, the electrician just needs to identify the conductor with the lowest temperature rating and select a box that meets or exceeds it.

Example

Imagine you have two conductors in a box: Conductor A: Rated for 90°C (high-temperature) Conductor B: Rated for 75°C (standard)If you used the highest rating (90°C), you might select a box rated for 90°C. However, if Conductor B is carrying a significant load, it could raise the temperature of the entire box above 75°C, potentially damaging the box material and creating a fire hazard, even though Conductor A is fine. By using the lowest rating (75°C), you ensure the box is rated to handle the heat generated by all conductors.

Your Suggestion and Its Potential Drawbacks

While your idea seems to make sense in terms of allowing the box to withstand the maximum possible temperature, it overlooks the fact that the continuous operating temperature inside the box is what's critical for the box's integrity. If the box is exposed to a temperature higher than its rating for prolonged periods, it will* degrade, regardless of the conductors' individual capabilities.

In summary: The NEC's "lowest temperature rating" rule is a conservative but effective way to prevent heat-related failures in electrical boxes and conduit bodies, ensuring a safer and more reliable electrical system. Flag for review