Let's break down NEC 240.21(B)(1) and apply it to your specific situation.

NEC 240.21(B)(1) - 10-Foot Tap Rule

This rule allows you to tap a feeder without immediate overcurrent protection at the tap point, provided certain conditions are met. Here's the relevant part you mentioned and its explanation: "The length of the tap conductor in no case shall exceed 10 ft." (You meet this) "The ampacity of the tap conductor shall not be less than(1) 240.21(B)(1)(a) - Not less than the calculated load of the circuit supplied by the tap conductors, and(2) 240.21(B)(1)(b) - Not less than the rating of the overcurrent protective device at the termination of the tap conductors, and(3) 240.21(B)(1)(c) - For field installations only, not less than 1/10 of the rating of the overcurrent device protecting the feeder conductors."

Applying it to your scenario


Feeder Breaker

400A, 80% rated. This means the continuous load on the feeder should not exceed 320A (400A 0.80).
2.

Tap Length

Less than 10 feet (you meet this).3.

Ampacity Requirements (240.21(B)(1)(a), (b), and (c))

You MUST meet ALL THREE ampacity requirements below

(a) Load

You need to know the calculated load served by your tap conductors. Let's call this `Load_Amps`. This could be based on connected equipment, a panelboard downstream, etc. We need this number.

(b) Termination OCPD

You mentioned the "rating of the overcurrent protective device at the termination of the tap conductors." This means the breaker protecting the tap conductors at their load end. Do you have a breaker there, a fuse, or something else? If you have a 200A breaker at the end of your tap, the ampacity has to be at least 200A. Lets Call this `Termination_OCPD_Amps`.

(c) 1/10 of Feeder OCPD

1/10 of 400A = 40A. This is the minimum ampacity your tap conductors can have, even if the actual load is lower.

Determining the Conductor Size


Determine Ampacity

Compare `Load_Amps`, `Termination_OCPD_Amps`, and 40A. The highest of these three values is the minimum ampacity your tap conductors need. Let's call that `Required_Amps`.2.

Select Conductor

Using NEC Table 310.15(B)(16) (or the equivalent for your specific installation conditions) find the smallest conductor that has an ampacity equal to or greater than `Required_Amps`. Remember to consider the conductor insulation type (THHN, XHHW, etc.) and termination temperature ratings (60°C, 75°C, 90°C) and any derating factors for ambient temperature or number of conductors in a raceway.

Example Scenario (Illustrative)

Let's assume the following: `Load_Amps` = 150A (the actual calculated load on the circuit being fed by the tap is 150 amps). `Termination_OCPD_Amps` = 200A (you have a 200A breaker at the end of your tap)Then:1. Comparing 150A, 200A, and 40A, the highest value is 200A. So `Required_Amps` = 200A.
2. Now, go to NEC Table 310.15(B)(16). Assuming copper conductors, termination rated for at least 75°C, and normal installation conditions, you'll likely find that a

3/0 AWG copper conductor with THHN insulation is sufficient, as it's typically rated for at least 200A (check the exact table for your insulation type).

Important Considerations



Terminations

Make sure the conductor size is compatible with the terminals on the 400A breaker and the downstream overcurrent protection device.

Voltage Drop

While the 10-foot tap rule may allow a smaller conductor based on ampacity, always check for voltage drop, especially if the downstream load is sensitive to voltage variations.

Local Codes

Always consult local amendments to the NEC. Local jurisdictions may have stricter requirements.

Qualified Electrician

This is complex electrical work. I strongly recommend consulting with a qualified and licensed electrician. They can perform the load calculations, select the correct conductor size based on your specific installation, and ensure the work is done safely and in accordance with all applicable codes.

In summary, I cannot definitively say whether 600kcmil or 500kcmil is correct without knowing the actual load and the termination OCPD. You need to determine `Required_Amps` as explained above, and then consult the NEC ampacity tables.Disclaimer: This information is for general guidance only. Consult with a qualified and licensed electrician for all electrical work.* Flag for review