Let's break down this scenario and the relevant rules in the National Electrical Code (NEC) to see if it makes sense to have a tap conductor larger than the feeder.

Understanding the Setup 500 kcmil Feeder

This is your main feeder carrying power. It's protected by a 400A circuit breaker.

Feeder Tap

You're tapping off this feeder to supply a load.

400A OCPD (Overcurrent Protective Device) at the Tap

The tap conductor is protected by a 400A circuit breaker.

The Issue

Your concern is that the ampacity of the tap conductor (requiring 600 kcmil to handle 400A) might be larger than the original feeder (500 kcmil). This seems counterintuitive.

NEC Rules and Considerations

The NEC allows feeder taps under specific conditions outlined in NEC Article 240.21(B) (Feeder Taps). The reason they allow it is that the overcurrent protection at the feeder protects the entire circuit, including the tap. The key is the tap conductors are still considered to be protected by the main OCPD on the feeder.There are several types of tap rules, but let's focus on a common one: the 10-foot Tap Rule. To qualify for the 10-foot tap rule, all of the following conditions must be met. (1) Tap Conductor Ampacity

The tap conductor must have an ampacity of at least

1/3 the rating of the overcurrent device protecting the feeder. In your case, 400A (feeder breaker) / 3 = 133.3A. So, the tap conductor must be rated for at least 133.3A. (2) Tap Length

The tap conductor cannot be longer than 10 feet.

(3) Enclosure

The tap conductor must be enclosed in a raceway.

(4) Tap Termination

The tap conductor must terminate in a circuit breaker or fuses.

(5) Ampacity of Tap OCPD

The rating of the overcurrent device protecting the tap conductors shall not exceed the ampacity of the tap conductors.

Analysis and Ampacity Requirements1. Original Feeder

The 500 kcmil conductor is protected by a 400A breaker. This implies that the 500 kcmil has an ampacity of at least 400A. According to NEC Table 310.16, a 500 kcmil copper conductor, rated for 75 degrees C, has an ampacity of 380A. A 500 kcmil aluminum conductor rated for 75 degrees C has an ampacity of 320A. Based on the NEC, the terminals on the equipment need to be rated for 75 degrees C unless specifically tested otherwise, which is why it is typically used.
2.

Tap Conductor (Using 10-foot Tap Rule Example)

To use the 10-foot tap rule, the tap conductor ampacity must be at least 1/3 of the feeder OCPD. So, in this case, the tap conductor ampacity must be a minimum of 133.3A. This means you do not need 600 kcmil for the tap. In fact, you can use 2/0 AWG (ampacity 175A @ 75 degree C) or 3/0 AWG aluminum (ampacity 145A @ 75 degree C) and the circuit is code compliant if the tap is a 10-foot tap.

Does a Larger Tap Ever Make Sense?

While it's possible to have a tap conductor with a higher ampacity than strictly required by the tap rules, it's usually not economically justifiable. You'd be spending extra money on a larger conductor for no practical benefit, unless there were very specific voltage drop or future load growth considerations that heavily outweighed the cost.

In Summary The tap rules in the NEC are designed to allow smaller tap conductors under specific conditions because the main feeder overcurrent device still provides protection. You don't necessarily need a larger conductor (600 kcmil) for the tap than the original feeder (500 kcmil) if you meet the tap rules. In this specific scenario, you do NOT need to increase the conductor to 600 kcmils. Always carefully consider all the requirements of the chosen tap rule to ensure code compliance.

Disclaimer: I am an AI Chatbot. Consult a qualified electrical engineer or electrician for any electrical design or installation work. The NEC is a complex document, and local codes and interpretations can vary. This information is for educational purposes and not a substitute for professional advice. Flag for review