The advice about always using the 60°C column for Romex (NM cable) is generally correct and a very safe practice. However, the NEC (National Electrical Code) and how you apply it in practice for SER cable is more nuanced.Here's the breakdown

Romex (NM Cable): Generally, yes, use the 60°C column.
Reasoning

The terminals on most common residential breakers and receptacles are only rated for 60°C. Even though some NM cable may be rated for 90°C, you are limited by the weakest link in the system, which are typically the termination points (breakers/receptacles).

Practical Application

Always using the 60°C column for ampacity calculations on Romex is a standard, accepted, and very safe practice. It ensures you never exceed the temperature rating of the terminations. This practice is very safe and I highly recommend it for Romex

Exceptions (Rare)

Very rare circumstances might allow you to use the 90°C rating only for

derating (adjusting the ampacity downwards due to multiple conductors in a conduit or high ambient temperatures) after you have already determined the initial ampacity from the 60°C column. Even in these cases, the final ampacity must still never exceed the 60°C value. This is an advanced concept, and you should be very familiar with NEC Article 310 before applying it.

SER Cable (Service Entrance Cable): It depends, but 75°C is most common. The Importance of Terminal Ratings

The key to SER (and all wiring) is the temperature rating of the

terminals where the wire connects (breakers, meter sockets, distribution blocks, etc.). You MUST use the lowest temperature rating of any termination point involved in the circuit. Common Scenarios



75°C Terminals

Most main breakers in residential panels and many meter sockets are rated for 75°C. In this case, you MUST use the 75°C ampacity column for SER cable. This is very common, especially with newer panels.

60°C Terminals

Some older panels and less expensive breakers may have 60°C terminals. In this case, you'd need to use the 60°C column.

90°C Terminals

Some high-end or specialized equipment might have 90°C terminals. If and only if all termination points in the circuit are rated for 90°C can you use the 90°C column for the initial ampacity.

NEC Article 310.15(B)(16) covers ampacity. You need to look at the conductor insulation type and the temperature rating of the termination points. NEC Article 110.14(C) deals specifically with the temperature limitations of terminations. Read this carefully!In Summary: SER Cable1. Check the Terminal Ratings

The first and most crucial step is to

verify the temperature rating of all terminations (breakers, meter socket, etc.). These ratings are typically printed on the device.2. Use the Lowest Rating

Use the ampacity column that corresponds to the lowest temperature rating of any termination point in the circuit.3.

75°C is Common

In many (probably most) residential installations, you'll find that the 75°C column is appropriate for SER cable. This is especially true with modern equipment.4.

When in Doubt, Derate

If you are unsure, use the 60°C column. It's always safer to be conservative.5.

Derating After Ampacity

The 90°C rating might be usable solely for derating purposes (adjusting downwards due to ambient temperature or multiple conductors), but only after you've determined the initial ampacity based on the appropriate termination temperature (60°C or 75°C). The final calculated ampacity must NEVER exceed the ampacity allowed by the lowest temperature-rated termination.

Important Safety Notes



Always Consult the NEC

The National Electrical Code (NEC) is the final authority.

Local Codes Matter

Your local electrical codes may have additional requirements.

Qualified Electrician

If you are not a qualified and licensed electrician,

do not attempt electrical work. Electrical work can be dangerous and even deadly if not done correctly. Always hire a professional. Inspections

Electrical work typically requires inspections by a qualified electrical inspector. Make sure you follow the proper permitting process.*

Oversizing

Many electricians will "oversize" the wire gauge to reduce voltage drop over long runs, even if the ampacity is sufficient. This is a good practice for efficiency and reliability.

Example

Let's say you're running SER cable to a 100A main breaker panel.1. You check the main breaker and it's rated for 75°C.
2. You check the meter socket and it's also rated for 75°C.3. In this case, you MUST use the 75°C ampacity column to determine the correct wire gauge for the 100A load.

Disclaimer: This information is for general guidance only. Electrical codes and practices can be complex and vary by location. Always consult the latest edition of the NEC, local codes, and a qualified electrician before performing any electrical work. This is not a substitute for professional advice. Flag for review