You've highlighted a common point of confusion regarding service conductor sizing in the NEC. Let's break down how 500 kcmil can be used on a 400A service based on the 2017 NEC (and it largely applies to current editions as well).

Understanding the NEC Sections You Cited



230.42(A) Minimum Size and Rating

This is paramount. The service conductors

MUST have an ampacity sufficient to carry the calculated load as determined by Article 220. This is where you start. You calculate the load (demand factors, etc.) and determine the minimum ampacity the service conductors need. 230.42(B) Rating of Ungrounded Conductors. For installations where conductors are installed in a raceway or cable, the rating of the ungrounded conductors shall not be less than the rating of the service disconnect. 230.90(A) Rating of Overcurrent Protection

The overcurrent protection device (breaker or fuses) must have an ampacity rating not less than the ampacity of the service-entrance conductors. This section is about protecting the conductors from overcurrent.

240.4(B) Protection of Conductors

This allows you to round up to the next standard overcurrent protection device size IF the conductor ampacity doesn't perfectly match a standard breaker/fuse size and certain conditions are met.

How 500 kcmil Works with a 400A Service

The key is to understand the ampacity of 500 kcmil copper conductors based on the insulation type and termination temperature rating.1.

Ampacity of 500 kcmil Copper

According to Table 310.15(B)(16) (formerly Table 310.16) in the 2017 NEC, 500 kcmil copper conductors (THHN, XHHW, etc.) typically have the following ampacities based on their temperature rating: 60°C: 310 amps 75°C: 380 amps 90°C: 430 amps2.

Temperature Rating and Terminations

This is CRITICAL. You must use the correct ampacity based on the

lowest temperature rating of any termination, conductor, or connected equipment. If your service disconnect is only rated for 75°C terminations, you can only use the 75°C ampacity of the 500 kcmil conductor (380 amps).3. Applying the NEC Rules



230.42(A)

Let's say your calculated load per Article 220 is 320 amps. The 500 kcmil copper conductors (rated at 380 amps at 75°C) are adequately sized to carry this calculated load.

230.42(B)

In this case, since the load is 320 amps and the conductor is rated to 380 amps, we have to select a 400 amp service disconnect as the conductor rating is not less than that.

230.90(A) & 240.4(B)

Since the 500 kcmil conductors are rated for 380 amps (at 75°C terminations) and your calculated load is 320 amps, you need a 400 amp breaker. Using 240.4(B), you can round up to the next standard size, which is 400 amps. The 400 amp breaker protects the conductors from overcurrent, and meets the requirements of 230.90(A)

Why the apparent contradiction doesn't exist (with this example)



230.42(A) is Primary

The primary goal is to size the conductors to handle the load as calculated by Article 220.

Overcurrent Protection is for Protection

The overcurrent protection (the 400A breaker) is there to protect the conductors from excessive current that could cause overheating and fire, not to limit the current to exactly the conductor ampacity.

In summary

Using 500 kcmil conductors on a 400A service is permissible if:1. The calculated load, per Article 220, is less than the ampacity of the 500 kcmil conductor.
2. The 500 kcmil conductor ampacity, based on its temperature rating and the temperature rating of all terminations, is not less than the rating of the service disconnect3. The overcurrent protection is not less than the ampacity of the service-entrance conductors.

Important Considerations



Local Codes and AHJ

Always check with your local electrical inspector (AHJ - Authority Having Jurisdiction) for any local amendments or interpretations of the NEC.

Voltage Drop

Don't forget to calculate voltage drop. While the 500 kcmil might be sufficient for ampacity, it might not be the optimal size to minimize voltage drop over long distances.

Neutral Conductor Sizing

Remember to size the neutral conductor according to the requirements of 220.61, which may allow for reduced sizing.

Aluminum Conductors: The same principles apply to aluminum conductors, but the ampacity values in Table 310.15(B)(16) are different for aluminum. Flag for review