You've hit on a common point of confusion and a practical challenge with using portable cords in a way that complies with the NEC. Here's the breakdown

Understanding the Issue
110.14(C) is Crucial

This section of the NEC mandates that the ampacity of conductors must be selected and coordinated based on the lowest temperature rating of any termination point, conductor insulation, or connected device. It prevents overheating and potential failures.

THHN/THWN and Tables

For typical building wiring, tables 310.15(B)(16) through (19) provide ampacities for conductors (like THHN) at various temperature ratings (60°C, 75°C, 90°C). This allows you to easily select the appropriate wire size for your application while respecting the termination temperature limits.

Portable Cord's Dilemma (Table 400.5(A)(1))

As you correctly noted, Table 400.5(A)(1) for flexible cords doesn't have temperature rating columns like the building wire tables. This gives the impression that all cords covered by the table are 90C.

SO and SOW Type Cord This is very commonly 90C rated insulation (as well as being oil and water resistant.) So in the absence of any other information, assuming this insulation temperature rating is a safe bet.

Addressing the Compliance Challenge1. Verify the Insulation Temperature Rating

The first and most important step is to

look at the actual cable jacket markings. The manufacturer should have printed the insulation temperature rating directly on the cable. Look for something like "90°C" or "105°C". If it's not explicitly marked, contact the manufacturer and get it in writing. Do not assume a temperature rating based solely on the cord type (SOOW, etc.).2. Terminate within Rating

If the cord is 90C, you need to ensure that your connection points are also rated for 90C. If they aren't, you need to follow 110.14(C), and use a lower ampacity.
3.

Derating if Necessary

If the temperature rating on the jacket is lower than 90C, then use the lower temperature rating to determine the final ampacity.4.

Derating for Ambient Temperature

Do not forget that if the temperature in the environment exceeds 30 degrees C, you must apply a correction factor. Use Table 310.15(B)(2)(a) for the adjustment.
5.

Derating for Bundling

If the cords are bundled or run in a conduit, you may need to apply ampacity adjustment factors as outlined in the NEC.

Practical Example

Let's say you have a SOOW cord marked "90°C". You are connecting it to a device that is only rated for 75°C terminations. Even though the cord can handle higher temperatures, you must use the 75°C column in the wiring ampacity tables (310.15(B)(16) or similar) to determine the allowable ampacity for that circuit. Select the conductor size based on the 75°C rating. The cord itself may have a higher ampacity listing, but the termination limits it.

The "Temperature Measurement" Thought

You're correct that measuring the temperature under load would provide real-world data. However, this is generally not a practical or acceptable method for initial design and code compliance. The NEC intends for you to design the system to be compliant, not test it and hope it passes. Temperature measurement is more for troubleshooting or research/development purposes.

Important Considerations



Equipment Listing

Always ensure that the devices you are connecting the cord to are listed and rated for the intended application. This includes the voltage, current, and temperature ratings.

Overcurrent Protection

Proper overcurrent protection (fuses or circuit breakers) is crucial to protect the cord and the connected equipment from overloads and short circuits. The overcurrent protection should be sized appropriately based on the conductor ampacity after all derating factors have been applied.

Local Amendments

Always check with your local electrical inspector or authority having jurisdiction (AHJ) for any local amendments to the NEC. They may have specific requirements or interpretations regarding portable cord usage.

In summary: You're right to be concerned about temperature ratings and portable cords. The key is to always* verify the insulation temperature rating marked on the cord itself, and then apply 110.14(C) diligently, using the lowest temperature rating between the conductor, terminations, and connected equipment to determine the allowable ampacity. Careful planning and attention to detail will ensure a safe and code-compliant installation. Flag for review