Yes, that's absolutely true and a critical point to understand for safe and compliant electrical installations. Here's a breakdown

Understanding the 90°C vs. 75°C Rating Issue
Lug Ratings

Lugs are often rated for 90°C. This means they can handle the heat generated by a conductor operating at 90°C without the lug itself overheating or failing. However, this doesn't automatically mean you can use the lug at its full 90°C potential.

Equipment Ratings

Electrical equipment (circuit breakers, panels, transformers, disconnects, etc.) often has a lower temperature rating, typically 75°C. This is the maximum temperature that the equipment's terminals are designed to withstand. Exceeding this temperature can damage the equipment, cause premature failure, and create a fire hazard.

Why the Difference Matters

The NEC (National Electrical Code) has specific rules about matching conductor ampacity to the lowest temperature rating in the circuit path. Even if your wire is rated for 90°C and your lug is rated for 90°C, if the equipment terminal is only rated for 75°C, you must use the 75°C ampacity of the conductor.

Where to Find Equipment Temperature Ratings1. Equipment Label

The most reliable place is the equipment's nameplate or label. Look for information such as "Suitable for Use With 75°C Conductors," "Terminals Rated 75°C," or similar wording. The temperature rating might be right near the voltage and amperage ratings.2.

Manufacturer's Instructions

If the equipment label is unclear, consult the manufacturer's installation instructions. These documents often provide more detailed information on acceptable conductor types, temperature ratings, and other installation requirements.3.

UL Listing Information

If you know the UL listing number for the equipment, you can often search for it on the UL website. The product certification information may specify the terminal temperature rating.

Key NEC Sections

Several NEC sections address this issue. Key sections include: NEC 110.14(C): Electrical Connections (This is the big one!) This section deals with the temperature limitations for conductors and terminals. NEC 310.15: Ampacities for Conductors Rated 0–2000 Volts (This is where you find the ampacity tables, and you'll need to use the correct column based on the temperature rating).

Example Scenario

Let's say you have a circuit with: #6 AWG THHN copper conductors (rated 90°C) Lugs rated for 90°C A circuit breaker with terminals rated for 75°CThe 90°C ampacity of #6 THHN is higher than the 75°C ampacity. However, because of the 75°C rated terminals on the breaker, you must use the 75°C ampacity for #6 copper, which is 55 amps. You cannot load the circuit to the higher ampacity that the 90C rating would suggest.

Important Considerations Upgrading Equipment

In some cases, it may be possible to upgrade equipment to a higher temperature rating (e.g., replacing a 75°C breaker with a 90°C rated breaker), but this must be done carefully and according to manufacturer's instructions and all applicable codes.

Derating

Remember to consider any derating factors (e.g., ambient temperature correction, multiple conductors in a raceway) in addition to the temperature limitation of the terminals. Derating can further reduce the allowable ampacity of a conductor.

Aluminum vs. Copper

This issue is particularly important with aluminum conductors. Aluminum has a lower ampacity than copper for the same gauge, and it's more susceptible to problems with overheating and corrosion at terminations if not properly installed and maintained.

In Summary

The 90°C vs. 75°C issue is a crucial safety concern. Always determine the temperature rating of all equipment terminals in a circuit and use the lowest temperature rating to determine the allowable ampacity of the conductors. This ensures that the entire system operates within safe temperature limits. When in doubt, consult a qualified electrician or electrical engineer. Flag for review