Modern home rewiring typically uses cable whose conductors are rated for 90 degrees C, such as the common NM-B cable used throughout residential wiring, though circuits are usually sized using a lower temperature column for safety. The temperature rating refers to the maximum temperature the wire’s insulation is designed to handle, and understanding it, along with why the sizing uses a lower rating, helps explain how wire is selected and protected. This is a technical area governed in detail by code, so the practical specifics are set by a licensed electrician.

Here is what to know about wire temperature rating in home rewiring.

Wire sizing, temperature ratings, and ampacity are defined in detail by code and depend on many factors, so always confirm with a licensed electrician and your local code edition.

What the temperature rating means

A wire’s temperature rating is the maximum operating temperature its insulation is designed to withstand. Common ratings are 60 degrees C, 75 degrees C, and 90 degrees C. The rating is tied to the insulation type, and the marking on the cable indicates it. A higher temperature rating means the insulation can tolerate more heat, which relates to how much current the wire can safely carry.

Modern cable is typically 90C rated

The standard cable used in modern residential rewiring, NM-B cable (often called Romex), has conductors rated at 90 degrees C. This is an improvement over older cable with lower-rated insulation. So the wire going into a modern rewire generally carries a 90 degrees C conductor rating, which is why you will see that marking on the cable.

Why circuits are sized at a lower rating

Even though the conductor insulation is rated 90 degrees C, circuits are generally sized and protected using a lower temperature column, often the 60 degrees C column for typical residential NM cable, rather than the full 90 degrees C ampacity. This is a conservative, code-driven approach:

• The terminations, devices, and connection points may be rated for a lower temperature
• Sizing to the lower rating keeps the whole circuit, including its weakest-rated point, within safe limits
• The higher 90 degrees C rating is often used for derating calculations, not for pushing more current through the wire.

In other words, the 90 degrees C rating does not mean the wire is loaded to its 90 degrees C capacity.

Why this matters

The temperature rating and how it is applied matter because they ensure the wire does not overheat under its load. Using the correct wire and sizing it conservatively keeps the insulation well within its limits, which is central to preventing the overheating that can damage wiring or cause a fire. This is one reason proper wire selection and sizing is work for a licensed electrician.

It is set by code and the electrician

Because wire sizing, ampacity, temperature ratings, and derating are governed in detail by code and depend on many factors, the practical decisions are made by a licensed electrician following the applicable code. They select the correct cable and size each circuit properly for its load and conditions. Homeowners do not need to calculate this themselves, but it helps to know that modern cable is 90 degrees C rated and sized conservatively for safety.

Modern home rewiring typically uses cable with 90 degrees C rated conductors, such as standard NM-B cable, but circuits are generally sized using a lower temperature column, often 60 degrees C for residential NM cable, so the whole circuit stays within safe limits. The temperature rating is about the insulation’s heat tolerance, and the conservative sizing prevents overheating. A licensed electrician selects and sizes wire according to the applicable code.

Stella Electric rewires homes with correctly rated and properly sized wiring for homeowners across the Baltimore area and throughout Maryland, with flat-rate pricing and no overtime charges. If you are planning a rewire, we handle the technical details. Financing is available on qualifying projects.